International Civil Aviation Organization
INFORMATION PAPER
31
August 2012
Compatibility between VDL Mode 4 and ICAO standard
systems operating in the frequency band 108 – 117.975 MHz
31 August 2012
Summary
This document contains the results of studies on the compatibility of the VHF air/ground communication
data link VDL Mode 4 with navigation systems operating in the VHF band 112 – 117.975 MHz. This
frequency band is used by VOR and GBAS. Adjacent band compatibility with the ILS Localizer which
operates below 112 MHz was also assessed. For the purpose of international coordination of frequency
assignments, the studies have been coordinated (and agreed) in the ICAO Navigation Systems Panel.
The approval of internationally agreed frequency assignment planning criteria for VDL Mode 4 in ICAO
(NSP) on the basis of the results of the studies presented in this document has been completed.
In addition, this document provides guidance material for using VDL Mode 4 in the band 112 – 117.975
MHz at the surface of airports and on the feasibility of implementing VDL Mode 4 on an aircraft.
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Table of Contents
EXECUTIVE SUMMARY .......................................................................... ERROR! BOOKMARK NOT DEFINED.
1
INTRODUCTION.................................................................................................................................................7
2
ORGANIZATION OF THE STUDIES ............................................................................................................ 10
2.1
GENERAL ..................................................................................................................................................... 10
2.1.1
VDL Mode 4 channel loading .......................................................................................................... 10
2.1.2
Polarization discrimination ............................................................................................................... 10
2.1.3
ILS/VOR signal generator noise ..................................................................................................... 10
2.1.4
Use of VDL Mode 4 at airports ........................................................................................................ 10
2.1.5
On-board compatibility of VDL Mode 4 airborne transmissions ................................................. 11
2.1.6
Frequency range ............................................................................................................................... 11
2.2
INTERFERENCE IN ILS AND VOR MONITOR RECEIVERS ............................................................................. 11
2.2.1
General aspects on monitor interference ...................................................................................... 11
2.2.2
ILS and VOR Monitor Interference scenarios ............................................................................... 12
2.3
EQUIPMENT USED IN THE TESTING .............................................................................................................. 14
3
COMPATIBILITY OF VDL MODE 4 WITH VOR ......................................................................................... 15
3.1
INTERFERENCE INTO VOR AIRBORNE RECEIVERS...................................................................................... 15
3.1.1
Test set-up ......................................................................................................................................... 15
3.1.2
Measurement results ........................................................................................................................ 16
3.1.3
Measurements with increased VOR signal level .......................................................................... 19
3.1.4
Measurements with increased channel loading............................................................................ 19
3.1.5
Appearance of flag ............................................................................................................................ 21
3.1.6
Audible interference .......................................................................................................................... 21
3.1.7
On-board compatibility of VDL Mode 4 transmissions and VOR receivers .............................. 21
3.1.8
Conclusions........................................................................................................................................ 22
3.2
INTERFERENCE INTO VOR MONITOR RECEIVERS ...................................................................................... 22
3.2.1
Test setup ........................................................................................................................................... 23
3.2.2
Results of measurements ................................................................................................................ 25
3.2.3
Separation distances ........................................................................................................................ 29
3.2.4
General observations ....................................................................................................................... 30
MONITOR RECEIVER BEHAVIOUR ............................................................................................................................. 30
IDENTIFICATION SIGNAL ........................................................................................................................................... 31
3.2.5
Summary of the VOR monitor measurement results ................................................................... 31
3.2.6
Conclusions........................................................................................................................................ 31
3.3
INTERFERENCE INTO VDL MODE 4 FROM VOR TRANSMISSIONS .............................................................. 31
3.3.1
Test setup ........................................................................................................................................... 31
3.3.2
Measurement results ........................................................................................................................ 32
3.3.3
Separation distances ........................................................................................................................ 33
3.3.4
Conclusions........................................................................................................................................ 35
4
COMPATIBILITY OF VDL MODE 4 WITH ILS ............................................................................................ 36
4.1
INTERFERENCE INTO ILS AIRBORNE RECEIVERS ........................................................................................ 36
4.1.1
Test set-up ......................................................................................................................................... 36
4.1.2
Separation distances ........................................................................................................................ 37
4.1.3
Conclusions........................................................................................................................................ 38
4.1.4
On-board compatibility of VDL Mode 4 transmissions and ILS receivers................................. 39
4.1.5
Appearance of flag ............................................................................................................................ 39
4.1.6
Increased level of wanted ILS-Localizer signal ............................................................................ 39
4.1.7
Audible interference .......................................................................................................................... 39
4.2
INTERFERENCE INTO ILS-LOCALIZER MONITOR RECEIVERS ...................................................................... 40
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4.2.1
Test setup ........................................................................................................................................... 41
4.2.2
Results of measurements ................................................................................................................ 42
4.2.3
Separation distances. ....................................................................................................................... 45
4.2.4
General observations ....................................................................................................................... 46
4.2.5
Conclusions on the ILS monitor measurement results ................................................................ 46
4.3
INTERFERENCE FROM THE ILS LOCALIZER INTO VDL MODE 4 .................................................................. 47
4.3.1
Test set-up ......................................................................................................................................... 47
4.3.2
Separation distances ........................................................................................................................ 49
4.3.3
Conclusions........................................................................................................................................ 50
5
COMPATIBILITY OF VDL MODE 4 WITH VDB GBAS ............................................................................. 51
5.1
INTERFERENCE FROM VDL MODE 4 INTO VDB GBAS .............................................................................. 51
5.1.1
Test set-up ......................................................................................................................................... 51
5.1.2
Separation distances ........................................................................................................................ 53
5.1.3
On board compatibility of VDL Mode 4 transmissions and GBAS receivers ............................ 54
5.1.4
Conclusions........................................................................................................................................ 54
5.2
INTERFERENCE FROM VDB GBAS INTO VDL MODE 4 .............................................................................. 55
5.2.1
Test set-up ......................................................................................................................................... 55
5.2.2
Separation distances ........................................................................................................................ 56
5.2.3
Conclusions........................................................................................................................................ 57
6
INTERFERENCE BETWEEN VOR AND GBAS ......................................................................................... 59
6.1
INTERFERENCE FROM VOR INTO GBAS .................................................................................................... 59
6.1.1
Measurement results ........................................................................................................................ 59
6.2
INTERFERENCE FROM GBAS INTO VOR .................................................................................................... 61
7
ERRONEOUS TUNING OF VDL MODE 4 ................................................................................................... 65
7.1
INTRODUCTION............................................................................................................................................. 65
7.2
VDL MODE 4 CHANNELS ............................................................................................................................. 65
7.3
MODES OF TUNING IN NORMAL OPERATIONS .............................................................................................. 65
7.3.1
The tuning function ........................................................................................................................... 65
7.3.2
Autonomous tuning using on-board sources ................................................................................ 66
7.3.3
On-board selection of services (by the flight crew) ...................................................................... 66
7.3.4
Directed tuning by ground stations (Autotune) ............................................................................. 67
7.4
CATEGORISATION OF FAILURE MODES ........................................................................................................ 67
7.5
MITIGATING FACTORS ADDRESSING MISTUNING IN NORMAL OPERATIONS ................................................. 68
7.5.1
Periodic repeat transmissions ......................................................................................................... 69
7.5.2
Quarantining of ground tuning instructions ................................................................................... 69
7.5.3
Error detection ................................................................................................................................... 70
7.5.4
Channel sensing................................................................................................................................ 70
7.5.5
System timers .................................................................................................................................... 70
7.5.6
Autotune ............................................................................................................................................. 71
7.6
CONCLUSIONS ............................................................................................................................................. 71
8 ON-BOARD ISOLATION BETWEEN VHF-COM (VDL MODE 4) ANTENNA AND ILS/VOR/GBAS
ANTENNA.................................................................................................................................................................. 72
8.1
INTRODUCTION............................................................................................................................................. 72
8.2
MEASUREMENTS.......................................................................................................................................... 72
8.3
TEST SETUP. ................................................................................................................................................ 73
8.4
MEASUREMENT RESULTS ............................................................................................................................ 75
8.4.1
Results with NAV antenna #9 (in the tail of the aircraft) .............................................................. 75
8.4.2
Results with NAV antenna #3 ( on the top of the fuselage of the aircraft) ................................ 75
8.5
CONCLUSIONS ............................................................................................................................................. 76
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9
SUMMARY OF CONCLUSIONS ................................................................................................................... 78
9.1
VOR AND VDL MODE 4 .............................................................................................................................. 78
9.1.1
Interference from VDL Mode 4 into VOR receivers ..................................................................... 78
9.1.2
Interference from VDL Mode 4 into VOR monitor systems ......................................................... 78
9.1.3
Interference from VOR transmitters into VDL Mode 4 ................................................................. 78
9.2
ILS AND VDL MODE 4 ................................................................................................................................. 78
9.2.1
Interference from VDL Mode 4 into ILS receivers ........................................................................ 78
9.2.2
Interference from VDL Mode 4 into ILS monitor systems ........................................................... 78
9.2.3
Interference from ILS transmitters into VDL Mode 4 ................................................................... 78
9.3
GBAS AND VDL MODE 4 ............................................................................................................................ 79
9.3.1
Interference from VDL Mode 4 into ILS receivers ........................................................................ 79
9.3.2
Interference from ILS transmitters into VDL Mode 4 ................................................................... 79
9.4
VOR AND GBAS ......................................................................................................................................... 79
9.5
ERRONEOUS TUNING OF VDL MODE 4 ....................................................................................................... 79
10
REFERENCES .............................................................................................................................................. 80
11
ACRONYMS .................................................................................................................................................. 81
11.1
ACRONYMS AND TERMINOLOGY .................................................................................................................. 81
APPENDIX A
- CALCULATION OF SEPARATION DISTANCES ............................................................ 82
A.1
INTERFERENCE MODEL ................................................................................................................................ 82
A.2
RE. PARAGRAPH 3.1 – SEPARATION DISTANCES FOR VDL MODE 4 (VICTIM) AND ILS LOCALIZER
(INTERFERER) .......................................................................................................................................................... 82
A.3
RE. PARAGRAPH 3.2 – SEPARATION DISTANCES FOR ILS LOCALIZER (VICTIM) AND VDL MODE 4
(INTERFERER) .......................................................................................................................................................... 83
A.4
RE. PARAGRAPH 4.1 – SEPARATION DISTANCES FOR VDL MODE 4 (VICTIM) AND VOR (INTERFERER) .. 83
A.5
RE. PARAGRAPH 3.1.1.1 – SEPARATION DISTANCES FOR VOR (VICTIM) AND VDL MODE 4 (INTERFERER)
83
A.6
RE. PARAGRAPH 5.1 – SEPARATION DISTANCES FOR VDL MODE 4 (VICTIM) AND GBAS (INTERFERER) 83
A.7
RE. PARAGRAPH 5.1 – SEPARATION DISTANCES FOR GBAS (VICTIM) AND VDL MODE 4 (INTERFERER) 83
A.8
RE. PARAGRAPH 4.4 – SEPARATION DISTANCES FOR GBAS (VICTIM) AND VOR (INTERFERER) ............. 84
A.9
RE. PARAGRAPH 4.2 – SEPARATION DISTANCES FOR VOR (VICTIM) AND GBAS (INTERFERER) ............. 84
APPENDIX B
B.1
B.2
B.3
PROPAGATION MODELS MONITOR INTERFERENCE .................................................. 85
FREE SPACE ................................................................................................................................................ 85
TWO RAY OR FLAT EARTH MODEL: .............................................................................................................. 85
EGLI MODEL. ................................................................................................................................................ 86
APPENDIX C
ILS MONITOR ANTENNA AT KUNGSANGEN AIRPORT. .............................................. 88
APPENDIX D ............................................................................................................................................................. 89
D.1
D.2
DETAILS ON VDL MODE 4 MISTUNING MITIGATING FUNCTIONS ................................................................. 89
ESTIMATION OF RESIDUAL INTERFERENCE IN NORMAL OPERATIONS ......................................................... 90
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List of tables
Table 1 – Measurement results ACR for VOR (victim) receivers interfered with VDL Mode 4 signal (22%)
.................................................................................................................................................................... 16
Table 2 – Separation distances between VOR (victim) receiver and VDL Mode 4 transmitter (22%) ....... 18
Table 3 – ACR and separation distances for different VDL Mode 4 channel loading at 115 MHz
(Honeywell receiver) ................................................................................................................................... 20
Table 4 – Results of measuring interference levels into VOR Monitor with 20% VDL Mode 4 channel
loading ......................................................................................................................................................... 26
Table 5 – Results of measuring interference levels into VOR Monitor with 2% VDL Mode 4 channel
loading ......................................................................................................................................................... 28
Table 6 – Parameters for antenna-to-antenna measurement..................................................................... 29
Table 7 – Observed behaviour of the monitor............................................................................................. 29
Table 8 – ACR for VDL Mode 4 (victim) and VOR (interferer) .................................................................... 32
Table 9 – Examples of minimum separation distances between VDL Mode 4 receivers and VOR
transmitters.................................................................................................................................................. 33
Table 10 – Measurement results ACR for ILS (localizer) victim receivers interfered with VDL Mode 4
signal (2.7% and 22% channel loading); VDL Mode 4 operating on 112 MHz and higher ......................... 37
Table 11 – Separation distances (km) between ILS Localizer (victim) airborne receiver and VDL Mode 4
.................................................................................................................................................................... 38
Table 12 – Measurement results ILS monitor alarms ................................................................................. 43
Table 13 – Adjacent channel rejection measurement results for VDL Mode 4 victim and ILS localizer
interferer ...................................................................................................................................................... 48
Table 14 – Separation distances between ILS localizer transmitter and VDL Mode 4 receivers (Re. Table
12) ............................................................................................................................................................... 49
Table 15 – ACR for VDB GBAS; GBAS (victim) and VDL Mode 4 (interferer) ........................................... 52
Table 16 – Separation distances between VDB GBAS (victim) and VDL Mode 4 (interferer) ................... 53
Table 17 – ACR for VDL Mode 4 (victim) and GBAS (interferer)................................................................ 56
Table 18 – Separation distances between VDL Mode 4 receiver (victim) and GBAS transmitter (interferer)
.................................................................................................................................................................... 57
Table 19 ACR; VDB GBAS victim and VOR interferer ............................................................................... 59
Table 20 – Separation distances; VOR interferer; VDB GBAS victim ........................................................ 60
Table 21 – ACR; GBAS interferer; VOR victim ........................................................................................... 62
Table 22 – Separation distances; GBAS interferer; VOR victim ................................................................. 63
Table 23 – Effect and mitigations of erroneous tuning in normal operations .............................................. 67
List of figures
Figure 1 – Critical area for ILS - Localizer .................................................................................................. 13
Figure 2 – Test setup for measuring interference from VDL Mode 4 into airborne VOR receivers ............ 15
Figure 3 – Measurement results as per Table 1 (Honeywell RNA34BF ILS/VOR receiver) ...................... 17
Figure 4 – Measurement results as per table 1 (KING KNR630 ILS/VOR receiver) .................................. 17
Figure 5 – Separation distance between VOR Honeywell receiver and VDL Mode 4 transmitter (22%) ... 18
Figure 6 – Separation distance between VOR KING receiver and VDL Mode 4 transmitter (22%) ........... 19
Figure 7 – ACR for different channel loading VDL Mode 4 (Honeywell receiver)....................................... 20
Figure 8 – Separation distances for different channel loading VDL Mode 4 (Honeywell receiver) ............ 21
Figure 9 – VOR Trosa ................................................................................................................................. 22
Figure 10 – Test setup for measuring the spectrum of the VOR transmitter .............................................. 23
Figure 11 – Spectral image of the VOR signal............................................................................................ 24
Figure 12 – Test setup for measurement of VOR monitor interference ...................................................... 24
Figure 13 – Power levels of VDL Mode 4 into VOR monitor (20% channel loading) ................................. 25
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Figure 14 – Power levels of VDL Mode 4 into VOR monitor (2% channel loading).................................... 27
Figure 15 – Test setup measuring interference from VOR into VDL Mode 4 ............................................. 32
Figure 16 – ACR as per Table 8 ................................................................................................................. 33
Figure 17 – Separation distances (VDL Mode 4 victim; VOR interferer) – 112 MHz
(first
adjacent channel 25 kHz not shown)
34
Figure 18 – Separation distances (VDL Mode 4 victim; VOR interferer) – 115 MHz
(first
adjacent channel 25 kHz not shown)
34
Figure 19 – Separation distances (VDL Mode 4 victim; VOR interferer) – 117.950 MHz
(first
adjacent channel 25 kHz not shown)
34
Figure 20 – Test setup for measuring interference from VDL Mode 4 into airborne ILS receivers ............ 36
Figure 21 – Measurement results as per Table 10; Honeywell RNA34BF ILS/VOR receiver) ................... 37
Figure 22 – Measurement results as per Table 10; KING KNR630 ILS/VOR receiver .............................. 37
Figure 23 – Separation distance as per Table 11; Honeywell RNA34BF ILS/VOR receiver ...................... 38
Figure 24 – Separation distance as per Table 11; KING KNR630 ILS/VOR receiver ................................ 38
Figure 25 – ILS localizer at Kungsangen airport ......................................................................................... 40
Figure 26 – Initial measurement test set-up................................................................................................ 41
Figure 27 – Test setup for measuring the spectrum of the ILS Localizer transmitter ................................. 41
Figure 28 – ILS localizer radiated spectrum ............................................................................................... 41
Figure 29 – Test set-up for measuring of ILS monitor interference ............................................................ 42
Figure 30 – VDL Mode 4 interference level necessary to switch to the stand-by localizer transmitter. ..... 44
Figure 31 – Localizer internal alarm settings .............................................................................................. 44
Figure 32 – Calculation of separation distance ........................................................................................... 45
Figure 33 – Test setup for measurement of ILS monitor interference ........................................................ 48
Figure 34 – ACR; VDL Mode 4 victim, localizer interferer .......................................................................... 48
Figure 35 – Separation distances between ILS localizer transmitter and VDL Mode 4 receivers (Re. Table
12) ............................................................................................................................................................... 50
Figure 36 – Test setup for measuring interference from VDL Mode 4 into VDB GBAS ............................. 51
Figure 37 – ACR as per Table 12 (GBAS victim and VDL Mode 4 interferer) ............................................ 52
Figure 38 – Separation distances as per Table 15 ..................................................................................... 53
Figure 39 – Test setup for measuring interference from VDB GBAS into VDL Mode 4 ............................. 55
Figure 40 – ACR as per Table 17 – ACR for VDL Mode 4 (victim) and GBAS (interferer) ........................ 56
Figure 41 – Separation distances as per Table 18 ..................................................................................... 57
Figure 42 – VOR interferer; VDB GBAS victim ........................................................................................... 60
Figure 43 – Separation distances; VOR interferer; VDB GBAS victim ....................................................... 61
Figure 44 – ACR; GBAS interferer; VOR victim .......................................................................................... 62
Figure 45 – Separation distances; GBAS interferer; VOR victim................................................................ 63
Figure 46 ..................................................................................................................................................... 64
Figure 47 – Abstraction of transmission chain showing failure modes ....................................................... 68
Figure 48 – Event that could lead to tuning to an incorrect frequency and their mitigation ........................ 69
Figure 49
Test setup for measuring antenna isolation .......................................................................... 73
Figure 50 – Location of antennas on an Beech King Air 200 ..................................................................... 74
Figure 51 – Isolation between NAV antenna #9 and the antennas COM1, COM2 and VDL 4 .................. 75
Figure 52 – isolation between NAV antenna #3 and the antennas VHF1, VHF2, VDL4 and COM5 ......... 76
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1 Introduction
This document contains the results of studies on the compatibility of the VHF air/ground
communication data link VDL Mode 4 with navigation systems operating in the VHF band 112 – 117.975
MHz. This frequency band is used by VOR and GBAS. Adjacent band compatibility with the ILS Localizer
which operates below 112 MHz was also assessed. For the purpose of international coordination of
frequency assignments, the studies have been coordinated (and agreed) in the ICAO Navigation Systems
Panel. The approval of internationally agreed frequency assignment planning criteria for VDL Mode 4 in
ICAO (NSP) on the basis of the results of the studies presented in this document has been completed.
Equipment testing which was performed during 2008 – 2011 was undertaken in
accordance with test plans developed within ICAO by the Aeronautical Communications Panel (ACP)
(2003 – 2007), the Navigation Systems Panel (NSP) and its predecessor, the Global Navigation Satellite
System Panel (GNSSP) (2002/2003) as well as from the NSP/Spectrum Subgroup (SSG) (2008/2009)
and by Eurocontrol (2004). Testing took place with production equipment.
Compatibility of VDL Mode 4 with VHF air/ground voice and data link communications
has already been addressed and approved in ICAO and the necessary compatibility criteria have been
incorporated in the ICAO Handbook on radio frequency spectrum requirements for civil aviation, Volume
II (Doc. 9718, in preparation)
1.1 Studies in the ICAO Navigation Systems Panel (NSP)
This document contains a comprehensive summary of the studies that took place in the NSP (Spectrum
Subgroup) in the period from 2008 – 2011.
The relevant working papers are available on the website for the NSP. :
A comprehensive paper (WP8) was presented and reviewed by the NSP (SSG) in November 2009, taking
into consideration a variety of comments from the NSP/SSG. WP13 to this meeting provided
measurement results by the Russian Federation on measurements of achievable on-board antenna
isolation on some aircraft.
Additional comments from the NSP/SSG were addressed in May 2011 in working papers addressing the
potential of interference into ILS and VOR monitor systems (WP5), the VDL Mode 4 mitigations against
erroneous tuning (WP6) and additional measurements of antenna isolation on a small aircraft (WP6).
After having reviewed all issues with regard to the use of the band 112 – 117.975 MHz by VDL Mode 4
the development of frequency assignment planning criteria for VDL Mode 4 in this frequency band were
completed.
A further review of these frequency assignment planning criteria by the NSP is not anticipated.
1.2 Summary of the compatibility criteria between VDL Mode 4
and ILS Localizer, VOR and GBAS
1. VOR and VDL Mode 4
Co-frequency and first adjacent (25 kHz) frequency: The separation distance between a
VDL Mode 4 transmitting station and a VOR receiving station shall be greater than the
distance to the radio horizon between the two aircraft operating at maximum range and
height of their respective designated operational coverage (DOC) area.
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Second adjacent 25 kHz frequency and higher: No frequency assignment planning
constraints.
2. ILS and VDL Mode 4
No frequency assignment planning criteria are necessary for the protection of airborne ILS
Localizer receivers from harmful interference from VDL Mode 4 transmissions
3. Interactions between GBAS and VDL Mode 4
Co-frequency use of GBAS and VDL Mode 4: The D/U ratio should be 20 dB. VDL Mode 4
equipped aircraft and VDB GBAS equipped aircraft should be separated to a distance
beyond the radio horizon of the two aircraft.
1st adjacent (25 kHz) channel) VDL Mode 4 equipped aircraft should not operate within each
DOC plus a buffer of 10 NM.
2nd adjacent channel and higher: No frequency assignment planning constraints between
VDL Mode 4 and GBAS frequency assignments
1.3 Frequency assignment planning criteria for VDL Mode 4
On the basis of the studies presented in this document, the following frequency assignment planning
criteria for VDL Mode 4 have been established:
 VDL Mode 4 and ILS
o No frequency assignment planning constraints
 VDL Mode 4 and VOR
o Co-frequency and first adjacent (25 kHz) frequency : VDL Mode 4 should be used beyond
the sum of radio horizon of the aircraft using VOR and the radio horizon of the aircraft
using VDL Mode 4
o VDL Mode 4 separated with 50 kHz or more from the VOR frequency: no frequency
assignment planning constraints
 VDL Mode 4 and GBAS
o Co-frequency: VDL Mode 4 should be used beyond the sum of radio horizon of the aircraft
using VOR and the radio horizon of the aircraft using VDL Mode 4
o 1st adjacent (25 kHz) frequency: VDL Mode 4 should be used beyond the designated
operational coverage of the GBAS plus a buffer of 10 NM
o 2nd adjacent (25 kHz) frequency and higher: no frequency assignment planning constraints
C
C
C

oNo frequency assigmnet planning criteria are required
VDL Mode 4 and ILS
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2 Organization of the studies
2.1 General
2.1.1 VDL Mode 4 channel loading
For the testing of interference that can be generated by VDL Mode 4, as presented in this
document, a channel loading of 22% (i.e. transmissions from one VDL Mode 4 aircraft radio) was used.
This is an unrealistic high level that can in practice not be achieved, but was used to simulate a
measurable level of potential interference into the radio navigation aids (ILS, VOR, GBAS) operating in
the band 108 – 117.975. Under worst case operating conditions transmissions from one VDL Mode 4
aircraft radio will result in a maximum channel loading of 2.7% in practice. Additional testing with 2.7%
channel loading was performed.
For the testing of interference from ILS, VOR and GBAS into VDL Mode 4, the channel
loading of the VDL Mode 4 was set at 100%. The level of the interfering signals was increased until a
message error rate of 2% was measured. The message error rate was measured over a period of 1
minute for each test.
2.1.2 Polarization discrimination
The polarization of ILS and VOR is horizontal and for VDL Mode 4 vertical. For
VDB/GBAS the polarization is also horizontal. Elliptical polarization for GBAS is permitted but not used in
Europe. The difference in polarization between ILS, VOR and GBAS versus VDL Mode 4 would allow in
practice for an antenna isolation of about 10-20 dB. The calculation of minimum separation distances
which are given in this paper that do not take into account any polarization discrimination. In some cases,
where expressively stated, a polarization discrimination of 10 dB has been considered.
2.1.3 ILS/VOR signal generator noise
Due to the relative high noise level of the ILS/VOR signal generator, the results of the
measurements showed worse performance for VDL Mode 4 than would be expected. This introduced a
measurement uncertainty in the order of 5 dB (the measured ACR should be 5 dB better) when assessing
interference into VDL Mode 4 systems.
2.1.4 Use of VDL Mode 4 at airports
Separation of VDL Mode 4 equipped aircraft and aircraft using GBAS at airports can be
as low as 100 m (e.g. one aircraft in the final stages of a landing and another aircraft taxiing on the
surface of the airport). Use of VDL Mode 4 on the airport needs to secure that in all cases the necessary
minimum separation between these aircraft is met. This may result in limitations to the use of certain
frequencies for VDL Mode 4. Such limitations are addressed in the frequency assignment planning
criteria. In any case, the GBAS protection requirements need to be observed throughout the Designated
Operational Coverage of the GBAS system, as promulgated in ICAO Annex 10 (Volume 1) and as
implemented by States. (Reference Annex 10, Volume I, paragraph 3.7.3.5.3.)
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2.1.5 On-board compatibility of VDL Mode 4 airborne transmissions
Considerations on the compatibility of VDL Mode 4 airborne transmissions with
navigation receivers on the same aircraft are presented for each interference mechanism (VDL Mode 4
vs. ILS/Localizer (see 4.1.4), VOR (see 3.1.7) and GBAS (see 5.1.3)) for information purposes only. It
should be recognized that the minimum required on-board antenna isolation may not be sufficient in all
cases to achieve compatibility. In such cases other mechanisms may be available to achieve compatibility
and, in case such measures are not feasible, certain aircraft may not be authorized to use frequencies
from the band 112 – 117.975 MHz by VDL Mode 4. Measurement results of on-board antenna isolation
between COM and NAV antennas are presented in Chapter 8.
On-board compatibility is not directly related to the development of frequency assignment
planning criteria by ICAO and should not be part of the frequency assignment planning criteria which are
[only] necessary to form a basis for international frequency assignment coordination. Achieving
compatibility between VDL Mode 4 and all other systems that may be installed on an aircraft is rather a
task for aircraft manufacturers and airworthiness authorities.
2.1.6 Frequency range
At the World Radiocommunication Conference 2007 (WRC-07) of the International
Telecommunication Union (ITU) the use of the frequency band 108-117.975 MHz for air-ground
communication systems (such as VDL-Mode 4) was restricted to the sub-band 112-117.975 MHz. As a
result, compatibility of VDL Mode 4 with the ILS-Localizer only has been assessed with the ILS-Localizer
operating at 111.950 MHz and the VDL Mode 4 operating at 112 MHz and higher frequencies.
Compatibility of VDL Mode 4 with VOR and GBAS has been assessed for co-frequency and adjacent
frequency use in the band 112 – 117.975 MHz.
2.2 Interference in ILS and VOR monitor receivers
2.2.1 General aspects on monitor interference
Electro-magnetic interference can be expected into the ILS-Localizer and VOR near-field
monitors from transmissions on frequencies inside (and outside) the frequency band 108 – 117.975 MHz.
Monitor systems are being used to verify the correct structure of the radio frequency signals-in-space for
the ILS-Localizer and the VOR. ILS-Localizer and VOR systems operate within the frequency band 108 –
117.975 MHz. The monitor receivers do not serve the purpose of detecting radio-frequency interference
as this is normally locally experienced in the vicinity of the airborne ILS-Localizer and VOR receivers in
the presence of interfering signals.
Sources of interference into the ILS or VOR monitor systems include transmissions from
VDL Mode 4 (ground and airborne transmissions, 112 – 137 MHz), GBAS (ground transmission, 108 –
117.975 MHz) and transmissions from VHF voice communication systems (ground and airborne, 117.975
– 137 MHz). Interference into ILS-Localizer and VOR monitors caused by FM broadcasting transmission
has also been reported. This document addresses interference into these monitor systems that can be
caused by VDL Mode 4 transmissions in the band 112 – 117.975 MHz into ILS and VOR monitor
systems. Details of measurement results on VOR monitor systems are contained in section 3.2 and on
ILS monitor systems are contained in section 4.2. Other sources that can cause interference to ILS
Localizer or VOR monitor systems (such as interference caused by GBAS and FM-broadcasting stations)
are NOT addressed in this document.
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ILS-Localizer and VOR monitor systems typically use broadband receivers capable of
receiving signals from (well) below the frequency 108 MHz to (well) above the frequency 117.975 MHz.
Very little effort has been made to improve the robustness of these monitor receivers to interference
outside the nominal operating frequency of the ILS-Localizer and VOR. This can be explained by the fact
that historically in many cases these systems used to operate in a fairly “clean” radio-frequency
environment, allowing for a low-cost design of the monitor receiver system.
With the introduction of VDL Mode 4 (112 – 117.975 MHz) and GBAS (108 – 117.975
MHz) the radio frequency environment is becoming more intensively used. The introduction of these
systems needs to secure continued safe operation of the ILS and VOR systems, including the relevant
monitor systems. A general measure that would mitigate potential interference into ILS and VOR monitor
systems could include implementing ILS-Localizer and VOR monitor receivers in the future that show
improved immunity from interference on frequencies outside the necessary bandwidth of the relevant ILSLocalizer and VOR system, as required.
As the ILS Localizer and VOR monitor systems are generally typically broadband
systems, harmful interference cannot be mitigated or removed through frequency assignment planning
techniques. Frequency assignment planning is primarily a tool to prevent harmful interference into
airborne ILS and VOR receivers and do not address the effect of interference into ILS-Localizer and VOR
monitor systems from any source.
2.2.2 ILS and VOR Monitor Interference scenarios
In general, monitoring equipment design is based on the principle of continuously
monitoring the radiated signals-in-space at specific points within the coverage volume to ensure their
compliance with the provisions of Annex 10, Volume I, Chapter 3, paragraph 3.1.3.11 and 3.1.5.7.
Monitor systems can detect anomalies in the ILS Localizer or VOR radio frequency signal
that can be caused by reflections (of the ILS-Localizer or VOR signal) of RF-signals from aircraft or
vehicles on the surface of the airport because of their near vicinity to the ILS or VOR systems. These
anomalies do not trigger a monitor alarm (switching of the ILS-Localizer or VOR) when these are transient
in nature if they do not result in the ILS-Localizer or VOR systems operating outside the limits set by
Annex 10. In general, Guidance Material in Annex10, Volume I points out that monitor systems should
not react to local conditions which do not affect the navigational information as seen by airborne systems.
Areas around the ILS Localizer that have been designated critical in this context are prohibited for aircraft
and vehicles to operate in while the ILS is in operational use. No such critical areas are specified in
Annex 10 for VOR.
To protect the ILS-Localizer systems from harmful interference that can be caused by reflection of
radio-frequency signals from (large) objects in the vicinity of the ILS-Localizer antenna, critical and
sensitive areas have been defined by ICAO. The ILS-Localizer critical area is an area of defined
dimensions around the localizer antenna within which vehicles, including aircraft, are excluded to operate
during all ILS operations (see Figure 1). Aircraft or vehicles, including those operating VDL Mode, 4 do
not enter this area while the ILS-Localizer or VOR is operational.
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120 m
75 m
300 m or the near end of the runway
Figure 1 – Critical area for ILS - Localizer
To protect the critical area it is necessary that the relevant authorities prohibit all entry of vehicles and the
taxiing and parking of aircraft within this area during all ILS operations.
Interference into an ILS-Localizer or VOR monitor system can be caused by static
systems operating from a fixed location and involve only ground based transmissions. These systems
include interference from other (nearby) ILS-Localizer and/or VOR systems, GBAS transmissions and FM
broadcasting transmissions. These interference scenarios are not addressed in this document. In
addition, interference can be caused by systems involving mobile transmissions from aircraft or vehicles
which are operating on the airport. Such systems include VDL Mode 4 and VHF air/ground
communication systems.
With regard to interference that can be caused by transmissions from VDL Mode 4 (and
air/ground communication systems) into the monitor receivers it should be noted that such interference
does NOT affect the signal structure of the ILS and VOR and does NOT cause interference into the
aircraft ILS-Localizer or VOR receivers when the frequency assignment planning parameters as
developed in this document are met. Such interference may cause the monitor to trigger an alarm that is
unrelated to the essential functions of the monitor system and that can eventually lead to either switching
to the standby ILS-Localizer or VOR transmitter or ceasing radiation of the ILS-Localizer or VOR. The
frequency assignment planning criteria developed in this document do not permit the co-frequency
operation of VDL Mode 4 within the designated operational coverage of the ILS or the VOR (including the
first adjacent 25 kHz channels). Essentially the interference into the monitor outside the necessary
bandwidth of ILS and VOR signals is caused by poor selectivity in the monitor receiver design.
Note: Interference that can be caused by inadvertently tuning of a VDL Mode 4 to an operational VOR
frequency is addressed in Chapter 7.
A monitor system for an ILS or VOR can trigger the following alarms by an interfering signal with the
following delays:
a. ILS-Localizer:
10 seconds for ILS-Localizer CAT I
5 [2] seconds for ILS-Localizer CAT II
2 [1] seconds for ILS-Localizer CAT III
b.
VOR:
30 seconds
Guidance material in Annex 10, Volume I further recommends that in order to reduce
failure of ILS equipment that may be operating near its monitor tolerance limits, it is useful for the monitor
system to include provisions to generate a pre-alarm warning signal to a designated control centre when
the monitored parameters reach a limit equal to a value in the order of 75 per cent of the monitor alarm
limit.
13 of 90
In addition to these alarm settings, the ILS system that was considered during the
measurements provides for the following internal warnings or alarms that, when triggered, do not affect
the operations of the ILS:
a.
a local warning at the ILS site that indicates that the radiated ILS radio frequency
signal has deviated from its original behaviour. The warning is only presented at the monitor
receiver and no indication is forwarded to the control centre.
b.
a local alarm that indicates that certain parameters of the ILS radio frequency
signal are (temporary) outside the acceptable levels. This local alarm is only presented at the
monitor receiver and no indication is forwarded to the control centre. One cause of such a local
alarm is because of reflections from aircraft landing which is typically transient in nature.
With regard to the potential interference that can be caused by transmissions of VDL
Mode 4 and air/ground communication systems into the ILS-Localizer monitor systems the following
observations are relevant:
a.
aircraft and vehicles do not operate within the critical area for ILS-Localizer while
the ILS is in operation.
b.
aircraft and vehicles do not operate VDL Mode 4 on the nominal ILS-Localizer or
VOR frequency or on the first adjacent 25 kHz frequency within the coverage of ILS or VOR.
c.
mitigation through improved filtering of the interfering transmissions from VDL
Mode 4 or air/ground communication systems is feasible should, in isolated cases, VOR or ILS
monitors be subject to harmful interference.
2.3
Equipment used in the testing
Testing was performed with production equipment.
The following equipment was used in the testing:
ILS-Localizer / VOR receiver; Honeywell RNA34BF s/n 01152
ILS / VOR receiver King Radiocorp. KNR 630
Rohde & Schwartz signal generator SMT 02 s/n 845376/017
VDL Mode 4 C.N.S. Systems AB VDL 4000-10-10 s/n 8.40-4000-100001
Telerad VDB transmitter EM 9009 s/n 491 ((for measurements with Honeywell ILS/VOR
receiver)
Telerad VDB/GBAS EM9009A s/n: 487 (for measurements with King ILS/VOR receiver)
Telerad VDB receiver RE 9009 s/n 74
HP Signal generator, VDL Mode 4 modulator E4431B s/n US38220123
Rohde & Schwartz spectrum analyzer FSL s/n 100341
VDL Mode 4 generator E4431B
VOR Monitor: Standard Elektronic Lorenz AG; D-VOR-S VOR Trosa, Sweden
ILS Monitor: NORMARC (Park Air), dual frequency system
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3 Compatibility of VDL Mode 4 with VOR
Compatibility criteria that would allow for using the band 112 – 117.975 MHz by VDL Mode 4 while
protecting VOR systems from harmful interference were established on the basis of interference
measurements with VOR systems, including airborne VOR receivers and VOR monitor receivers.
Interference from VOR into VDL Mode 4 systems was assessed using a VOR signal generator. The test
set-up used was agreed by the ICAO Navigation Systems Panel and the Aeronautical Communications
Panel.
3.1 Interference into VOR airborne receivers
3.1.1 Test set-up
The set-up for testing interference from VDL Mode 4 into the VOR receivers was as shown in Figure 2.
VOR
test equipment
VOR
receiver
-79 dBm
RF-combiner
RF-attenuator
RF-attenuator
VOR signal
generator
VDL Mode 4
transmitter /
signal generator
Figure 2 – Test setup for measuring interference from VDL Mode 4 into airborne VOR receivers
Test parameters:
a. VDL Mode 4
i.
ii.
iii.
Channel loading 22%
Additional measurements on 115 MHz with 2.7% and 50% channel loading
Frequencies 112 MHz, 115 MHz and 117.975 MHz
E.I.R.P.: 10 W ( 39 dBm, including antenna gain and feeder losses)
b. VOR
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i.
ii.
iii.
Frequencies: 112 MHz, 115 MHz and 117.950 MHz
VOR minimum signal level: -79dBm at the VOR receiver input
VOR interference threshold: 0.3 degree on VOR bearing
Note: measurements were taken at a range of 40 channels (25 kHz) above and below the tested
frequencies.
3.1.2 Measurement results
The measurement results obtained with the test parameters in paragraph 3.1.1 are presented in Table 1
and Figure 3 and Figure 4 below.
Channel
#
112 MHz
-40
ACR (dB)
66.3
ACR (dB)
64.3
-20
-10
67.3
69.3
-5
-4
115 MHz
117.950 MHz
Honeywell receiver
112 MHz
115 MHz
King receiver
117.950 MHz
ACR (dB)
65.3
ACR (dB)
73.2
ACR (dB)
67.2
ACR (dB)
77.2
65.3
68.3
66.3
68.3
74.2
74.2
75.2
75.2
77.2
76.2
71.3
69.3
72.3
68.3
70.3
67.3
73.2
71.2
75.2
72.2
76.2
73.2
-3
-2
67.3
65.3
68.3
65.3
66.3
64.3
72.2
70.2
72.2
69.2
72.2
70.2
-1
0
12.3
-14.7
10.3
-17.7
9.3
-19.7
16.2
-4.8
18.2
-6.8
18.2
-4.8
1
2
14.3
65.3
12.3
65.3
12.3
66.3
26.2
69.2
27.2
70.2
26.2
68.2
3
4
67.3
69.3
67.3
69.3
67.3
68.3
72.2
71.2
72.2
72.2
72.2
73.2
5
10
72.3
69.3
72.3
69.3
70.3
70.3
73.2
73.2
74.2
75.2
76.2
77.2
20
40
69.3
71.3
71.3
70.3
71.3
72.3
73.2
74.2
75.2
76.2
78.2
79.2
Table 1 – Measurement results ACR for VOR (victim) receivers interfered with VDL Mode 4 signal
(22%)
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Figure 3 – Measurement results as per Table 1 (Honeywell RNA34BF ILS/VOR receiver)
Figure 4 – Measurement results as per table 1 (KING KNR630 ILS/VOR receiver)
Minimum separation distances between a VDL Mode 4 transmitter and a VOR receiver were calculated
as described in Appendix A (paragraphs A.1 and A.5) and resulted in the following separation distances
that should be minimmaly maintained between a VDL Mode 4 transmitter and an (aircraft) VOR receiver.
The calculation results are presented in Table 2 and Figure 4 and Figure 6 below.
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112 MHz
115 MHz 117.950 MHz
Honeywell receiver
km
km
0.101
0.090
0.090
0.080
0.064
0.064
0.040
0.051
0.064
0.072
0.064
0.080
0.090
0.101
50.699
56.885
112 MHz
115 MHz
King receiver
km
0.072
0.028
0.028
0.028
0.040
0.040
0.057
20.417
117.950 MHz
Channel #
km
km
km
0.036
0.022
-40
0.080
0.032
0.022
-20
0.072
0.032
0.025
-10
0.057
0.036
0.025
-5
0.045
0.045
0.036
-4
0.057
0.040
0.040
-3
0.072
0.051
0.051
-2
0.090
25.703
20.417
-1
40.272
0
8.128
7.244
8.128
1
31.989
40.272
40.272
0.057
0.051
0.064
2
0.090
0.090
0.080
0.040
0.040
0.040
3
0.072
0.072
0.072
0.045
0.040
0.036
4
0.057
0.057
0.064
0.036
0.032
0.025
5
0.040
0.040
0.051
0.036
0.028
0.022
10
0.057
0.057
0.051
0.036
0.028
0.020
20
0.057
0.045
0.045
0.032
0.028
0.018
40
0.045
0.051
0.040
Table 2 – Separation distances between VOR (victim) receiver and VDL Mode 4 transmitter (22%)
Note: these separation distances do not take into account the effect of polarization discrimination
between VOR and VDL Mode 4 signals.
Figure 5 – Separation distance between VOR Honeywell receiver and VDL Mode 4 transmitter
(22%)
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Figure 6 – Separation distance between VOR KING receiver and VDL Mode 4 transmitter (22%)
These separation distances were calculated with the minimum VOR signal level (-79 dBm) at the input of
the VOR receiver with a channel loading of 22%. Under these conditions it was concluded that
transmissions from a VDL Mode 4 equipped aircraft will not interfere with the reception of VOR on
another aircraft when operating on the second adjacent (25 kHz) channel and more. The large separation
distance between VDL Mode 4 and a VOR receiver necessary to protect VOR from harmful interference
when the VDL Mode 4 is operating on the first adjacent channel, as well as the variations between the
ACR of different VOR receivers, would suggest that in this case the same criteria as for co-channel usage
should be recommended.
Note: The minimum (vertical) separation distance between aircraft in flight is 300 m.
3.1.3 Measurements with increased VOR signal level
The VOR signal was increased in 10 steps of 5 dB from the minimum signal level of -79 dBm.
Variations in the co-channel D/U were noted in the order of plus/minus 1 dB. For this measurement the
VOR frequency was 115.000 MHz. These variations have negligible effect on the calculated separation
distances
3.1.4 Measurements with increased channel loading
Although all tests measuring interference into VOR took place with a channel loading of 22%, additional
tests were performed with a 50% and a 2.7 % channel load. The tests with 50% channel loading were
performed to simulate the effect of a beat tone of 37.5 Hz that could occur when transmitting VOR
signals. In particular a decrease of the ACR on the second adjacent channel of 5 dB should be noted with
a channel load of 50 %. This effect should be carefully considered when implementing VDL Mode 4 on
board an aircraft, as the on-board compatibility may become marginal in this case.
Note: under normal operating conditions, the channel loading of an on-board VDL Mode 4 system will not
be more than 2.7%.
The results of these additional tests are in Table 3, Figure 7 and Figure 8 below.
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Channel #
115 MHz
22%
2.70%
50%
ACR
Km
ACR
km
ACR
km
-40
69.3
0.057
64.3
0.101
64.3
0.101
-20
70.3
0.051
65.3
0.090
66.3
0.080
-10
71.3
0.045
68.3
0.064
68.3
0.064
-5
72.3
0.040
72.3
0.040
71.3
0.045
-4
71.3
0.045
68.3
0.064
66.3
0.080
-3
71.3
0.045
68.3
0.064
65.3
0.090
-2
67.3
0.072
65.3
0.090
60.3
0.160
-1
12.3
40.272
10.3
50.699
11.3
45.186
0
-9.7
-17.7
-17.7
1
17.3
22.646
12.3
40.272
14.3
31.989
2
70.3
0.051
65.3
0.090
64.3
0.101
3
72.3
0.040
67.3
0.072
67.3
0.072
4
72.3
0.040
69.3
0.057
69.3
0.057
5
72.3
0.040
72.3
0.040
72.3
0.040
10
72.3
0.040
69.3
0.057
67.3
0.072
20
72.3
0.040
71.3
0.045
69.3
0.057
40
72.3
0.040
70.3
0.051
70.3
0.051
Table 3 – ACR and separation distances for different VDL Mode 4 channel loading at 115 MHz
(Honeywell receiver)
Note: these separation distances do not take into account the effect of polarization discrimination
between VOR and VDL Mode 4 signals.
Figure 7 – ACR for different channel loading VDL Mode 4 (Honeywell receiver)
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Figure 8 – Separation distances for different channel loading VDL Mode 4 (Honeywell receiver)
Although the measured ACR in some cases was different compared to the measurement
results with a VDL Mode 4 channel loading of 22%, the minimum separation distances to avoid harmful
interference to VOR remained well below 200m and would not affect the conclusions in paragraph 3.1.8.
3.1.5 Appearance of flag
With the maximum VDL Mode 4 signal fed into the VOR receiver, the flag was raised on the same
channel with a D/U ratio of -1 dB. This is about 20 dB more than necessary to meet the 0.3 degree
interference criterion. For this measurement the VDL Mode 4 channel loading was set at 22%. With a
channel loading of 2.7 %, the flag could not be raised with the maximum VDL Mode 4 signal fed into the
VOR receiver. For this test the frequency was 115.000 MHz.
3.1.6 Audible interference
During the tests, the effect of interference from VDL Mode 4 on the audible (Morse code) identification
signal was observed during each measurement; the interference was that low that the proper
identification of the Morse code by the flight crew was not impeded.
3.1.7 On-board compatibility of VDL Mode 4 transmissions and VOR
receivers
The VDL Mode 4 transmits with an EIRP of 39 dBm. Assuming an isolation between the VOR and the
VDL Mode 4 antenna of about 45 dB and 10 dB polarization losses (both antennas at the same side of
the fuselage) (see also Chapter 8), the energy from the VDL Mode 4 transmissions at the VOR antenna
would be in the order of -14 dBm. To protect the minimum VOR signal of -79 dBm, a D/U of -65 dB would
be required. This value is met at the second 25 kHz channel and higher. It should be noted that one
receiver tested showed significant better (about 7 – 9 dB) ACR on the third adjacent (25 kHz) channel
and higher which would facilitate implementation of VDL Mode 4 on board of smaller aircraft.
Note: The analysis of potential on-board compatibility between VDL Mode 4 and VOR in this paragraph is
only presented for general information. Actual implementation of VDL Mode 4 on-board an aircraft needs
to take into consideration a variety of elements specific to the aircraft. Approval of such implementation
rests with the national airworthiness authorities and is outside the scope of developing frequency
assignment planning criteria. (See also paragraph 2.1.5 above).
21 of 90
3.1.8 Conclusions
From the measurement results it can be concluded that the co-frequency D/U ratio between VOR and
VDL Mode 4 can be set at 20 dB. This would also apply when VDL Mode 4 is operating on the first
adjacent (25 kHz) channels to the VOR. This implies that VDL Mode 4 can only operate on the same
frequency and the first adjacent channels as the VOR frequency when the VDL transmitter is outside the
designated coverage of the VOR AND beyond the radio horizon of an aircraft at maximum range and
height of the designated operational coverage of the [desired] VOR.
To protect VOR from harmful interference that can be caused by VDL Mode 4 transmissions, the following
frequency assignment planning criteria apply:
Co-frequency and first adjacent (25 kHz) frequency: The separation distance between a VDL Mode 4
transmitting station and a VOR receiving station shall be greater than the distance to the radio horizon
between the two aircraft operating at maximum range and height of their respective designated
operational coverage (DOC) area.
Second adjacent 25 kHz frequency and higher: no frequency assignment planning constraints.
3.2 Interference into VOR Monitor receivers
Measurements were taken at the Trosa VOR in Vagnhärad in Sweden in 2011. This VOR
was a D-VOR operating on the frequency 114.300 MHz. (See Figure 9)
VOR site
Monitor antenna
Figure 9 – VOR Trosa
22 of 90
The VOR monitor antenna was a horizontally polarized three element Yagi antenna,
located approximately 200 m from the VOR transmitting antenna. Both antennas were at the same height
above the ground level, approximately 5m.
3.2.1
Test setup
The effect of VDL Mode 4 transmissions on the monitor receiver was initially measured
with the VDL Mode 4 system transmitting at maximum power (+40 dBm). The VDL Mode 4 transmissions
took place from an antenna that was installed on the monitor ground plane at the same height of the
monitor antenna and located in the direction of the maximum gain of the monitor antenna. This
measurement did not show any noticeable effect on the monitor. In this case, the VDL Mode 4 was
transmitting with a maximum channel loading of 2.7 %.
Measurements were taken with the following test setup:
a.
VOR received power.
Initial measurements of the VOR received power were taken with the test setup as in Figure 10. With the
spectrum analyser connected to the monitor antenna cable, the received VOR power was -29 dBm. A plot
of spectrum of the received VOR signal is in Figure 11.
This measurement did not show any noticeable effect on the VOR monitor; the signal level of the VDL
Mode 4 signal was -55 dBm,
VOR monitor antenna
VOR transmitter
Spectrum analyzer
Figure 10 – Test setup for measuring the spectrum of the VOR transmitter
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Figure 11 – Spectral image of the VOR signal
b.
Measurements on the effect of interference from VDL Mode 4 in the VOR monitor
The test setup for measuring the effect of interference from VDL Mode 4 into the VOR Monitor receiver
was as in Figure 12.
VOR
monitor antenna
50 Ω
VOR transmitter
VOR receiver
20 dB
VDL Mode 4
+
Spectrum Analyzer
Figure 12 – Test setup for measurement of VOR monitor interference
24 of 90
3.2.2
Results of measurements
3.2.2.1 VOR alarm indications
The VOR provides for two different alarm indications:
1.
2.
Alarm, indicating that the VOR operates outside the specifications (set by the
manufacturer and the operator). The alarm limits are roughly about at 75% of
the alarm specifications set by ICAO.
A VOR switch (or shutdown) is the alarm persist for a period of more than 30
seconds.
Both alarms are relayed to a remote control station.
Measurements were taken with VDL Mode 4 transmitting with a 2% channel loading and
with 20% channel loading.
3.2.2.2 Measurement results with a VDL Mode 4 channel loading of 20%.
Table 4 and Figure 13 show the results of the measured interference levels into the VOR monitor with a
VDL Mode 4 channel loading of 20%. Note that the horizontal scale is not linear over the full range.
The alarm levels shown are intermittent and do not trigger a VOR switch.
Figure 13 – Power levels of VDL Mode 4 into VOR monitor (20% channel loading)
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VOR course
alignment
Channel
offset
Frequency
[MHz]
VDL Mode 4
Power [dBm]
D/U [dB]
Alarm
Switch
Alarm
Switch
107,700
-240
108,300
-20
-15
-9
-14
-200
109,300
-21
-16
-8
-13
-160
110,300
-22
-17
-7
-12
-120
111,300
-24
-18
-5
-11
-80
112,300
-27
-21
-2
-8
-40
113,300
-28
-22
-1
-7
-20
113,800
-28
-22
-1
-7
-10
114,050
-27
-21
-2
-8
-5
114,175
-26
-20
-3
-9
-4
114,200
-25
-20
-4
-9
-3
114,225
-24
-19
-5
-10
-2
114,250
-24
-19
-5
-10
-1
114,275
-24
-19
-5
-10
0
114,300
-26
-23
-3
-6
1
114,325
-24
-18
-5
-11
2
114,350
-25
-19
-4
-10
3
114,375
-25
-19
-4
-10
4
114,400
-25
-19
-4
-10
5
114,425
-24
-19
-5
-10
10
114,550
-24
-19
-5
-10
20
114,800
-24
-19
-5
-10
40
115,300
-24
-19
-5
-10
80
116,300
-24
-19
-5
-10
120
117,300
-23
-18
-6
-11
160
118,300
-21
-16
-8
-13
200
119,300
-19
-14
-10
-15
240
120,300
-17
-12
-12
-17
280
121,300
-14
-9
-15
-20
320
122,300
-11
-6
-18
-23
360
123,300
-9
-5
-20
-24
Table 4 – Results of measuring interference levels into VOR Monitor with 20% VDL Mode 4
channel loading
26 of 90
With a channel loading of 20% the monitor provided a warning on the course alignment parameter. This
warning was stable for a period of about 15 seconds and then released. The course alignment indicator of
the monitor returned to zero but was vibrating heavily. Although this behaviour would not trigger a switch
of the VOR transmitter, this situation was considered as a valid alarm limit.
From the measured data it was concluded that the most stringent protection to avoid any alarm warning is
required at 113.300 MHz, about 1 MHz below the VOR operating frequency. For the VOR tested, this
value is -28 dBm (VDL Mode 4 channel loading 20%) and -22 dBm with the VDL Mode 4 channel loading
of 2%. The alarm levels shown are intermittent and do not trigger a VOR switch.
3.2.2.3 Measurement results with a VDL Mode 4 channel loading of 2%.
Table 5 and Figure 14 show the results of the measured interference levels into the VOR monitor with a
VDL Mode 4 channel loading of 2%. Note that the horizontal scale is not linear over the full range..
Figure 14 – Power levels of VDL Mode 4 into VOR monitor (2% channel loading)
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VOR
course
alignment
VDL Mode 4 Power [dBm]
Frequency
[MHz]
Alarm
Switch
D/U [dB]
Alarm
Switch
107,700
-11
-5
-18
-24
108,300
-11
-5
-18
-24
109,300
-13
-5
-16
-24
110,300
-15
-7
-14
-22
111,300
-17
-9
-12
-20
112,300
-20
-12
-9
-17
113,300
-22
-16
-7
-13
113,800
-22
-17
-7
-12
114,050
-21
-16
-8
-13
114,175
-20
-14
-9
-15
114,200
-18
-12
-11
-17
114,225
-19
-12
-10
-17
114,250
-18
-11
-11
-18
114,275
-17
-10
-12
-19
114,300
-18
-9
-11
-20
114,325
-18
-9
-11
-20
114,350
-19
-11
-10
-18
114,375
-19
-11
-10
-18
114,400
-20
-11
-9
-18
114,425
-19
-13
-10
-16
114,550
-20
-14
-9
-15
114,800
-20
-13
-9
-16
115,300
-19
-12
-10
-17
116,300
-17
-10
-12
-19
117,300
-15
-7
-14
-22
118,300
-13
-5
-16
-24
119,300
-11
-5
-18
-24
Table 5 – Results of measuring interference levels into VOR Monitor with 2% VDL Mode 4 channel
loading
With a channel loading of 2%, the warning, once activated, stayed activated and could
(eventually) trigger a VOR transmitter switch. In this case, because of the lower channel loading, a much
higher VDL Mode 4 signal was necessary to trigger an alarm compared to the case where the channel
loading was 20%.
28 of 90
From the measured data it may be concluded that the most stringent protection to avoid any alarm
warning is required at 113.300 MHz, about 1 MHz below the VOR operating frequency. For the VOR
tested, this value is -22 dBm (VDL Mode 4 channel loading 2 %).
VDL Mode 4 signal level at monitor receiver input
Further measurements were taken to establish the level of a VDL Mode 4 signal, when
transmitted from the antenna on the counterpoise, in the direction of the maximum gain of the three
element Yagi monitor antenna. These measurements are contained in Table 6.
Parameter
Value
Unit
Comments
-55
dBm
--
114.800
MHz
Received power
Frequency
Used for reference
measurement
Distance to monitor
According to on-site
antenna
200
m
information, not measured
Table 6 – Parameters for antenna-to-antenna measurement
The transmitted VDL Mode 4 signal was +40dBm (10 W), meaning that the attenuation between
transmitter and receiver was 95dB. The VDL Mode 4 antenna used was a standard VHF antenna, one
quarter wavelength dipole.
The VOR monitor antenna was a horizontally polarized three element Yagi antenna, mounted
approximately 200 m from the transmitting antenna. Both antennas were at the same height above the
ground level, approximately 5m.
The effect of the VDL Mode 4 transmission was too small to be measured and therefore the behaviour of
the monitor indicators was observed. The result of the measurement on five different frequencies is
presented in Table 7.
F [MHz]
Observed behaviour
114,275 No effect
114,300 Minor deflections on instrument
114,325 No effect
114,350 No effect
114,800 No effect
Table 7 – Observed behaviour of the monitor
3.2.3 Separation distances
The maximum undesired level of the VDL Mode 4 signal at the VOR monitor receiver
input as is used in the calculations below is -28 dBm. Appendix B includes several methods that can be
used to predict the received power from an interferer into an ILS monitor receiver. These models include:
(i)
the free space model
(ii)
the two-ray or flat Earth model
(iii)
the Egli model
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The models under (ii) and (iii) provide a more accurate prediction of propagation losses, taking into
account the received direct and indirect wave. These separation distances have been calculated with the
following criteria:
EIRP (VDLM4) = 10 W (40 dBm) (f = 112 MHz)
GR = 6.5 dB (monitor receiver antenna gain)
LR = 2 dB (monitor receiver cable losses)
LPOL = 10 dB (polarization discrimination)
Height of VDL Mode transmitting antenna = 2 m
Height of receiving (monitor) = 5 m.
PU = -22 dBm (maximum interference level in the VOR monitor as measured with
2% channel loading)
Re. (i)
When using the free space propagation model, the minimum separation to be maintained
between a VDL Mode 4 transmitter (antenna) would be 143 m. Beyond this distance no effect of the VDL
Mode 4 signal on the VOR monitor would be observed.
Re. (ii)
When using the two-ray or flat Earth propagation model, the minimum separation to be
maintained between a VDL Mode 4 transmitter (antenna) would be 81 m. Beyond this distance no effect
of the VDL Mode 4 signal on the VOR monitor would be observed.
Re. (iii)
When using the Egli propagation model, the minimum separation to be maintained
between a VDL Mode 4 transmitter (antenna) would be 50 m. Beyond this distance no effect of the VDL
Mode 4 signal on the VOR monitor would be observed.
With regard to the application of these three propagation models, it should be considered
that the measurements showed an attenuation of the VDL Mode 4 signal from the VDL Mode 4 antenna
(on the counter poise of the VOR) to the monitor receiver input of 95 dB. This is about 20 – 30 dB more
than can be calculated with any of the three propagation models used in this section. (The same applies
to the ILS measurements). Applying these propagation models leads to very conservative separation
distances.
3.2.4
General observations
Monitor receiver behaviour
The pulsed VDLM4 signal does not introduce an error in the monitoring of the FM
modulated element of the VOR signal.
There are errors introduced in the monitoring of the AM modulated element of the VOR
signal but not large enough to trigger the alarm.
The monitoring of the VOR course alignment is affected by the pulsed signal and the
monitor behaviour varies with the channel loading of the VDL Mode 4 signal. To analyse this behaviour,
detailed knowledge about the design of the monitor receiver is needed.
The measurements show that the monitor receiver has very poor filtering of received RF
signals. The filtering seems to be only 2dB per MHz from 116 MHz and up. This implies that the monitor
should be sensitive to VHF communications in the band from 118MHz and up. Also, the filtering is equally
poor at the lower end of the band and therefore the monitor may be sensitive to broadcast FM signals
transmitted in the band between 88MHz to 107.9MHz.
The monitor shows very strange frequency response. The minimum immunity to the
interference is approximately 1MHz below the nominal frequency. This response is independent of the
VDLM4 channel loading.
30 of 90
Identification signal
The interference level on the identification (Morse) signal, received by the monitor
system, was judged by listening to the Morse code signal in a speaker. With 2 % channel load the
interference does not introduce a problem in reading the identification signal nor can it be regarded as
disturbing even at high power levels at any frequency.
With 20% channel load the interference is disturbing when the VDL Mode 4 signal is
tuned to the same frequency as the VOR transmitter itself and at power levels of -23 dBm or more (i.e.
above the level where a transmitter switch has been triggered). On the first assignable channel from the
VOR nominal frequency, the power level needed to disturb the reading of the Morse code is -12dBm
which is 12dB above the power level needed to trigger a switch.
3.2.5
Summary of the VOR monitor measurement results
The VOR monitor receiver may trigger a switch caused by interference of a VDL Mode 4
signal at very high interference power levels. At a channel load of 2%, the interfering signal level is in the
range of -17dBm at the worst measured frequency. This means that the interfering signal has to be
increased by 38dB compared to the measurement done where the VDL signal was transmitted from the
antenna of a regular VDL Mode 4 system.
At the 20% channel loading the power levels have to be -22dBm, i.e. an increase of 33dB
compared to the measurement done where the VDL signal was transmitted from the antenna of a regular
VDL Mode 4 system.
The monitor receiver is susceptible to interferences from transmissions within the NAVband (e.g. GBAS and VDL Mode 4) as well as interferences from transmissions within the COM-band
(119.975 – 137 MHz) and from broadcast FM stations (operating below 108 MHz). However, the required
power levels, for pulsed signals, are very high before a switch is triggered. The monitor receiver immunity
towards continuous signals has not been investigated in this measurement.
3.2.6
Conclusions
Interference from VDL Mode 4 transmissions into VOR monitor receivers can occur at very short
separation distances (less than 100 m)
Interference from other systems (GBAS, FM broadcasting) needs to be considered
Industry should consider improving RF selectivity of ILS and VOR monitor receivers
3.3 Interference into VDL Mode 4 from VOR transmissions
3.3.1
Test setup
The set-up for testing interference from VOR transmissions into the VDL Mode 4 receiver was as shown
in Figure 15. The test set-up was agreed by the ICAO Aeronautical Communications Panel.
31 of 90
VDL Mode 4
test equipment
VDL Mode 4
receiver
-82 dBm
RF-combiner
RF-attenuator
RF-attenuator
VOR signal
generator
VDL Mode 4
transmitter /
signal generator
Figure 15 – Test setup measuring interference from VOR into VDL Mode 4
Measurements were taken with VDL Mode 4 operating at 112 MHz, 115 MHz and
117.950 MHz. The channel loading of the VDL Mode 4 received signal was set at 100%. Measurements
were taken with the VOR signal generator being tuned to the adjacent 25 kHz channels and the results
are presented in Table 8 and Figure 16. The measurements were made over one minute. The maximum
bit error rate for VDL Mode 4 is 1 in 103, which corresponds to a message error rate of 2%.
3.3.2 Measurement results
Adjacent Channel Rejection (dB)
Channel #
112 MHz
115 MHz
117.950 MHz
-40
66
69
70
-20
66
68
67
-10
63
64
63
-5
60
60
59
-4
58
58
58
-3
57
56
56
-2
55
55
54
-1
38
38
38
0
-8
-9
-8
1
34
34
34
2
55
56
55
3
57
57
57
4
59
59
58
5
60
60
59
10
65
64
63
20
68
67
66
40
68
69
66
Table 8 – ACR for VDL Mode 4 (victim) and VOR (interferer)
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Figure 16 – ACR as per Table 8
3.3.3
Separation distances
Table 9 and Figure 17, Figure 18 and Figure 19 provide examples of the minimum
separation distance between VOR transmitter and VDL mode 4 receivers for respectively 50 W, 100 W
and 1000 W EIRP values for the VOR transmitter. The ACR values used are as in Table 8.These
separation distances, which are calculated without applying the effect of polarization discrimination, can
be further reduced if a polarization discrimination of 10 dB would be applied. Calculation of separation
distances is further explained in Appendix A (Paragraph A1 and A4).
112 MHz
Adjacent
Channel
#
-40
ACR
115 MHz
VOR EIRP (W)
100
50
1000
ACR
117.950 MHz
VOR EIRP (W)
100
50
1000
ACR
VOR EIRP (W)
100
50
1000
dB
66
km
0.4
km
0.3
km
1.3
dB
69
km
0.3
km
0.2
km
0.9
dB
70
km
0.3
km
0.2
km
0.8
-20
-10
66
63
0.4
0.6
0.3
0.4
1.3
1.9
68
64
0.3
0.5
0.2
0.4
1.0
1.7
67
63
0.4
0.6
0.3
0.4
1.2
1.9
-5
-4
60
58
0.8
1.0
0.6
0.7
2.6
3.3
60
58
0.8
1.0
0.6
0.7
2.6
3.3
59
58
0.9
1.0
0.7
0.7
3.0
3.3
-3
-2
57
55
1.2
1.5
0.8
1.0
3.7
4.7
56
55
1.3
1.5
0.9
1.0
4.2
4.7
56
54
1.3
1.7
0.9
1.2
4.2
5.2
-1
0
38
88
10.5
-
7.4
-
33.1
-
38
89
10.5
-
7.4
-
33.1
-
38
88
10.5
-
7.4
-
33.1
-
1
2
34
55
16.6
1.5
11.7
1.0
52.5
4.7
34
56
16.6
1.3
11.7
0.9
52.5
4.2
34
55
16.6
1.5
11.7
1.0
52.5
4.7
3
4
57
59
1.2
0.9
0.8
0.7
3.7
3.0
57
59
1.2
0.9
0.8
0.7
3.7
3.0
57
58
1.2
1.0
0.8
0.7
3.7
3.3
5
10
60
65
0.8
0.5
0.6
0.3
2.6
1.5
60
64
0.8
0.5
0.6
0.4
2.6
1.7
59
63
0.9
0.6
0.7
0.4
3.0
1.9
20
40
68
68
0.3
0.3
0.2
0.2
1.0
1.0
67
69
0.4
0.3
0.3
0.2
1.2
0.9
66
66
0.4
0.4
0.3
0.3
1.3
1.3
Table 9 – Examples of minimum separation distances between VDL Mode 4 receivers and VOR
transmitters
33 of 90
Figure 17 – Separation distances (VDL Mode 4 victim; VOR interferer) – 112 MHz
(first adjacent channel 25 kHz not shown)
Figure 18 – Separation distances (VDL Mode 4 victim; VOR interferer) – 115 MHz
(first adjacent channel 25 kHz not shown)
Figure 19 – Separation distances (VDL Mode 4 victim; VOR interferer) – 117.950 MHz
(first adjacent channel 25 kHz not shown)
34 of 90
3.3.4
Conclusions
It was concluded that with the separation distances required to protect VDL Mode 4 from
interference that can be caused by VOR transmissions, no co-frequency use of VDL Mode 4 and VOR is
recommended when the VDL Mode 4 equipped aircraft is within radio line of sight of the VOR ground
station.
When VDL Mode 4 is operating on the first adjacent (25 kHz) channel of VOR frequency
it is possible to operate VDL Mode 4 outside a range of 10 to 40 km from the VOR station, depending on
the EIRP of the VOR station. (See also paragraph 3.1.8 which recommends, in order to protecting VOR
against interference from VDL Mode 4 to not using the first adjacent (25 kHz) channel within line of sight
of the VOR).
When VDL Mode 4 is operating on the second adjacent (50 kHz) channel of the VOR
frequency, no frequency planning constraints are necessary to protect VDL Mode 4 from interference
from VOR transmissions. However, when implementing VDL Mode 4, attention should be given to the
possibility of interference into the VDL Mode 4 receiver within a radius of 1-5 km around a VOR
transmitting station, depending on the EIRP of the VOR (see Table 9).
For VDL Mode 4, operating on the second adjacent (25 kHz) channel from a VOR station, no
frequency assignment planning constraints are required.
Note: attention should be given to the possibility of interference into VDL Mode 4 when operating
within a radius of 1 – 5 km from a VOR station. Such interference is typically transient in nature.
35 of 90
4 Compatibility of VDL Mode 4 with ILS
4.1 Interference into ILS airborne receivers
4.1.1
Test set-up
The set-up for testing interference from VDL Mode 4 into the ILS receivers was as shown in Figure 20.
The test set-up was agreed by the ICAO Navigation Systems Panel and the Aeronautical
Communications Panel.
ILS-Localizer
test equipment
ILS Localizer
receiver
-86 dBm
RF-combiner
RF-attenuator
RF-attenuator
ILS signal
generator
VDL Mode 4
transmitter /
signal generator
Figure 20 – Test setup for measuring interference from VDL Mode 4 into airborne ILS receivers
As the ILS Localizer can only operate in the frequency band below 112 MHz and VDL
Mode 4 only above 112 MHz, the ILS Localizer frequency used during the tests was 111.950 MHz (which
is the highest assignable ILS frequency) and the VDL Mode 4 was operating at 112.000 MHz (which is
the lowest VDL Mode 4 assignable frequency) and higher.
The input level of the ILS-Localizer signal was set at -86 dB; measurements were taken
with a maximum error of 4.5 µA in the ILS Localizer receiver. Measurements were taken with a channel
loading of the VDL Mode 4 transmitter at 22% and 2.7 %.
The measurement results are contained in Table 10. (f(0)=111.950 MHz; second adjacent
(25 kHz) channel is 112.000 MHz) and in Figure 21 and Figure 22.
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Adj. 25 kHz channel #
2
3
4
5
10
20
40
Honeywell RNA 34 BF
ACR with loading 22%
54.3 63.3 70.3 79.3
79.3 79.3 79.3
ACR with loading 2.7%
74.3 78.3 79.3 79.3
79.3 79.3 79.3
King Radio Corp. KNR 630
ACR with channel loading 22%
77.2 81.2 81.2 81.2
81.2 81.2 81.2
ACR with channel loading 2.7%
81.2 81.2 81.2 81.2
81.2 81.2 81.2
Table 10 – Measurement results ACR for ILS (localizer) victim receivers interfered with VDL Mode
4 signal (2.7% and 22% channel loading); VDL Mode 4 operating on 112 MHz and higher
Note: At an ACR of 79.3 the VDL Mode 4 signal level of 0 dBm was input to the ILS receiver.
Figure 21 – Measurement results as per Table 10; Honeywell RNA34BF ILS/VOR receiver)
Figure 22 – Measurement results as per Table 10; KING KNR630 ILS/VOR receiver
Note: In Table 10 and Figure 21 and 22 the results of the first adjacent channel (25 kHz) are not
presented as VDL Mode 4 cannot operate on 111.975 MHz.
4.1.2
Separation distances
37 of 90
Separation distances (km) between ILS receivers and VDL Mode 4 transmitters were
calculated as explained in Appendix A (paragraphs A1 and A3) with the results as below in Table 11 and
Figure 23 and Figure 24:
Adjacent channel #
2
3
4
5
10
20
40
Honeywell RNA34BF ILS/VOR receiver
VDL M 4 loading 22%
0.716 0.254 0.114 0.04
0.04
0.04
0.04
VDL M 4 loading 2.7%
0.072 0.045 0.04
0.04
0.04
0.04
0.04
KING KNR630 ILS/VOR receiver
VDL M 4 loading 22%
0.051 0.032 0.032 0.032 0.032 0.032 0.032
0.032 0.032 0.032 0.032 0.032 0.032 0.032
VDL M 4 loading 2.7%
Table 11 – Separation distances (km) between ILS Localizer (victim) airborne receiver and VDL
Mode 4
Figure 23 – Separation distance as per Table 11; Honeywell RNA34BF ILS/VOR receiver
Figure 24 – Separation distance as per Table 11; KING KNR630 ILS/VOR receiver
4.1.3
Conclusions
The separation distances in paragraph 4.1.2 were calculated with the minimum ILS signal
level (-86 dBm) at the input of the Localizer receiver. Under worst case operating conditions a channel
loading of more than 2.7 % is not expected. It had been concluded that transmissions of VDL from an
38 of 90
aircraft will not cause harmful interference to another aircraft equipped with ILS. Separation distances
between an aircraft at final approach or landing and a VDL Mode 4 equipped aircraft on the surface of the
airport in the order of 30 – 50 m are not expected. The lowest assignable VDL Mode 4 frequency is
112.000 MHz
No frequency assignment planning constraints are necessary for the protection of ILS (airborne) ILS
localizer receivers from harmful interference for VDL Mode 4 transmissions.
4.1.4
On-board compatibility of VDL Mode 4 transmissions
and ILS receivers
The VDL Mode 4 transmits with an EIRP of 39 dBm. Assuming and isolation between the
VOR and the VDL Mode 4 antenna of about 45 dB and 10 dB polarization losses (both antennas at the
same side of the fuselage), the energy from the VDL Mode 4 transmissions at the ILS antenna would be
in the order of -14 dBm. To protect the minimum ILS signal of -86 dB, a D/U of - 72 dB would be required.
This criterion is met with VDL Mode 4 operating at the 2 nd (25 kHz) adjacent channel (112 MHz) from the
highest assignable ILS frequency with a positive margin of 9.8 dB with a maximum VDL Mode 4 channel
loading of 2.7%.
Higher antenna isolations (in the order of 60-70 dB) can be expected when the ILS and VDL Mode 4
antennas in use are located on different sides of the fuselage.
Note: The analysis of potential on-board compatibility between VDL Mode 4 and ILS in this paragraph is
only presented for general information. Actual implementation of VDL Mode 4 on-board an aircraft needs
to take into consideration a variety of elements specific to the aircraft. Approval of such implementation
rests with the national airworthiness authorities and is outside the scope of developing frequency
assignment planning criteria in ICAO. (See also paragraph 2.1.5 above)
4.1.5
Appearance of flag
With the maximum VDL Mode 4 signal fed into the Localizer receiver, the flag was raised
on the Honeywell receiver on the second adjacent channel with a D/U ratio of -79 dB; at this point the
error in the ILS indicator was 20 µA. For this measurement the VDL Mode 4 channel loading was set at
22%; the frequency of the localizer was 111.950 MHz and the frequency of the VDL Mode 4 was 112
MHz. No flag was raised when the interference was less than 4.5 µA with a (worst case) channel loading
of 2.7%. No flag was raised with the KING receiver with VDL Mode 4 operating on 111.950 MHz or higher
with a channel loading of 2.7%. When the channel loading was increased to 22%, the flag was raised only
in case the VDL Mode 4 would operate on the first adjacent 25 kHz channel (111.975 MHz).
4.1.6
Increased level of wanted ILS-Localizer signal
The ILS-Localizer signal was increased in 6 steps of 5 dB from the minimum signal level of -86
dBm and, at the same time, the level of the (unwanted) VDL Mode 4 signal was also raised with the same
value. No changes in the adjacent channel D/U ratios were observed. For this measurement the ILSLocalizer frequency was 111.950 MHz and the frequency for the VDL Mode 4 was 112 MHz.
4.1.7
Audible interference
During the tests, the effect of interference from VDL Mode 4 on the (Morse code)
identification signal was observed during each measurement; the interference was that low that the
proper identification of the Morse code by the flight crew was not impeded.
39 of 90
4.2
Interference into ILS-Localizer monitor receivers
Note: during all measurements, the VDL Mode 4 system was transmitting data with a duty cycle of 22%.
This is an unrealistic high duty cycle factor which was used to create a measure of interference. VDL
Mode 4 operates with a maximum channel loading of 2.7%.
Measurements were taken at Kungsangen airport in Norrkoping, Sweden in September
2010. (See Figure 25).
Figure 25 – ILS localizer at Kungsangen airport
The ILS Localizer at Kungsangen airport is operating on the frequency 109.500 MHz. The ILS Localizer
monitor is located about 100m from the ILS antenna.
The effect of VDL Mode 4 transmissions on the monitor receiver was initially measured
with the VDL Mode 4 system transmitting at maximum power (+40 dBm). The monitor antenna was a two
element Yagi antenna. This antenna had a gain of 6.5 dBi towards the localizer antenna and a front/back
ratio of 9 dB. Details of the antenna are in Appendix C. The VDL Mode 4 transmitter was installed in a car
and connected to an antenna on the roof of the car at a distance of 130 m from the monitor antenna (See
Figure 26). This measurement did not show any noticeable effect on the monitor. In this case, the VDL
Mode 4 was transmitting with a maximum channel loading of 2.7 %
40 of 90
Figure 26 – Initial measurement test set-up
4.2.1
Test setup
Measurements were taken with the following setup:
a.
Test setup of the received spectrum of the ILS-localizer transmitter was as in
Figure 27. The measurement results are in
Figure 28.
ILS Localizer
monitor antenna
ILS Localizer transmitter
Spectrum analyzer
Figure 27 – Test setup for measuring the spectrum of the ILS Localizer transmitter
41 of 90
Figure 28 – ILS localizer radiated spectrum
The measurement results are in
Figure 28 and did not show any anomaly in the spectrum radiated by the ILS Localizer that could affect
the interference measurements.
b.
The test set-up for the measurements of the effect of interference from VDL Mode 4 into
the ILS monitor system is in Figure 29.
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Figure 29 – Test set-up for measuring of ILS monitor interference
4.2.2
Results of measurements
Measured was the level of the VDL Mode 4 signal at the input of the ILS-Localizer
monitor receiver on different frequencies necessary to trigger the three levels of alarm:
1. Warning
2. Alarm
3. Switch of the ILS system to the standby transmitter.
Note: The warning and the alarm were internal alarms at the ILS system that were not
reported to the control centre.
The measurements are presented in detail in Table 12 and Figure 30 and Figure 31.
During these measurements, the VDL Mode 4 was operating with a channel loading of
20%. The channel loading of 20% created a deterministic response with regard to the each of the alarm
notifications. One out of each 5 time slots were used with a maximum burst length of 13 milliseconds.
This channel loading is unrealistic high as VDL Mode 4 normally operates with a maximum channel
loading of 2.7%. VDL Mode 4 stations that would operate with a channel loading of 20% would seriously
disrupt the VDL Mode 4 network. The measurement results show a fairly constant behaviour of the
various alarm triggers over a wide band with a somewhat better immunity performance at higher
frequencies (higher than 116 MHz).
No alarm could be triggered into the ILS Localizer monitor with a VDL Mode 4 signal with
a channel loading of 2.7% during any of the measurements and it was decided to do the measurements
with 20% in order to get deterministic responses from the monitor.
Frequency
Alarm [dBm]
Warning [dBm]
Tx Switch [dBm]
108.000
-21.2
-22.2
-20.2
108.500
-21.2
-22.2
-20.2
109.000
-22.2
-23.2
-20.2
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109.250
-21.2
-23.2
-19.2
109.375
-20.2
-21.2
-19.2
109.400
-20.2
-21.2
-19.2
109.425
-18.2
-20.2
-17.2
109.450
-17.2
-18.2
-17.2
109.475
-18.2
-19.2
-16.2
109.500
-27.2
-27.2
-18.2
109.525
-18.2
-19.2
-16.2
109.550
-18.2
-19.2
-16.2
109.575
-19.2
-20.2
-16.2
109.600
-19.2
-20.2
-16.2
109.625
-19.2
-20.2
-17.2
109.750
-21.2
-22.2
-17.2
110.000
-20.2
-22.2
-19.2
110.500
-21.2
-23.2
-20.2
111.000
-21.2
-23.2
-18.2
112.000
-22.2
-23.2
-19.2
113.000
-22.2
-23.2
-15.2
114.000
-19.2
-21.2
-17.2
115.000
-18.2
-21.2
-16.2
116.000
-18.2
-19.2
-13.2
117.000
-15.2
-17.2
-11.2
117.975
-13.2
-16.2
-11.2
118..000
-11.2
-11.2
-11.2
119..000
-11.2
-11.2
-11.2
120..000
-11.2
-11.2
-11.2
Table 12 – Measurement results ILS monitor alarms
44 of 90
Figure 30 – VDL Mode 4 interference level necessary to switch to the stand-by localizer
transmitter.
The internal alarms (internal warning and internal alarm) were triggered with VDL Mode 4
signals exceeding the values as shown in Figure 31 below (see paragraph 2.2.2 for a description of these
alarms).
Figure 31 – Localizer internal alarm settings
The three alarms (warning, alarm and switch) are set and used as follows:
i.
ii.
iii
Warning. The warning is set at 75 % of the alarm levels.
When this alarm is triggered, an amber light is displayed at the remote control unit.
Alarm. When one of the monitored functions exceeds 75% of the maximum values determined in
Annex 10 a red light is displayed at the remote control unit
Switch. When the alarm levels are exceeded for a period about 50% of the time specified by
ICAO, the ILS-Localizer transmitter will switch to the standby transmitter (or be shutdown).
Alarm levels i and ii above have no operational consequences.
The protection levels for the warning and alarm are more stringent at the nominal ILS
Localizer frequency 109.500 MHz (-27.2 dBm was measured for the VDL Mode 4 signal) compared to the
switch-over (or shutdown) alarm. These (internal) alarms do show a large difference with the actual switch
of the ILS (to the standby transmitter or to switch off the ILS) on the operating frequency. For this
frequency, it needs to be observed that the frequency assignment planning criteria do not allow for VDL
Mode 4 and ILS (or VOR) frequency to operate on the same frequency (and the first adjacent 25 kHz
frequencies) within designated operational coverage for the ILS and the VOR. Therefore, this (lower) “onfrequency” interference is not further considered. To avoid warning/alarm to be triggered, the maximum
interference level for VDL Mode 4 should be lower than minus 23.2 dBm at the ILS Localizer monitor
receiver input. (This was the worst case level of interference that would trigger a warning outside the
Localizer bandwidth). The difference between the various alarm levels is mainly due to the time the alarm
level is exceeded. Due to the stochastic behaviour of the monitor receiver, it was not possible to
determine which particular parameter measured by the monitor triggered the alarm.
45 of 90
4.2.3
Separation distances.
Minimum separation distances between VDL Mode 4 transmitters and ILS localizer monitor antennas
were calculated as explained in Appendix B.
G=6.5 dB
LRx=2 dB
Lpol=10 dB
EIRP=40W
ILS Monitor
receiver
PU-25 dBm
VDL Mode 4
Figure 32 – Calculation of separation distance
The undesired level of the VDL Mode 4 signal at the ILS monitor receiver input as is used
in the calculations below is -24 dBm. Appendix B includes several methods that can be used to predict
the received power from an interferer into an ILS monitor receiver. These models include:
(i)
the free space model
(ii)
the two-ray or flat Earth model
(iii)
the Egli model
The models under (ii) and (iii) provide a more accurate prediction of propagation losses,
taking into account the received direct and indirect wave. These separation distances have been
calculated with the following parameters (see also Figure 32):
EIRP(VDLM4) = 10 W (40 dBm)
f = 115 MHz
GR = 6.5 dB (monitor receiver antenna gain)
LR = 2 dB (monitor receiver cable losses)
LPOL = 10 dB (polarization discrimination)
Height of VDL Mode transmitting antenna = 2 m
Height of receiving (monitor) = 2 m.
PU = -24 dBm (maximum interference level in the ILS monitor as measured was
- 23.2 dBm)
(i)
When using the free space model, the minimum separation to be maintained
from a VDL Mode 4 transmitter (antenna) would be 175 m. Beyond this distance no effect of the VDL
Mode 4 signal on the ILS monitor would be observed.
(ii)
When using the two-ray or flat Earth model, the minimum separation to be maintained
from a VDL Mode 4 transmitter (antenna) would be 58 m. Beyond this distance no effect of the VDL Mode
4 signal on the ILS monitor would be observed.
(iii)
When using the Egli model, the minimum separation to be maintained from a VDL Mode
4 transmitter (antenna) would be 43 m. Beyond this distance no effect of the VDL Mode 4 signal on the
ILS monitor would be observed.
46 of 90
The above calculations provide a conservative estimation of the minimum necessary
distance between a VDL Mode 4 transmitter and the ILS Localizer antenna. The following comments are
offered with regard to the above calculations:
a.
the effect of the antenna diagram has not been taken into consideration when
calculating the minimum separation distance between the monitor antenna and the VDL
Mode 4 antenna.
b.
6.5 dB antenna gain (omnidirectional) was used in the calculation (A simple
dipole would show about 3 dB antenna gain)
c.
free space attenuation was assumed; using a more realistic model (e.g. the tworay model of the Egli model) would predict significant larger path losses.
As a result, the calculated minimum separation distances can be considered as overly
conservative.
Note: in this respect is should also be considered that the measurements and calculations are based on
an unrealistic high VLD Mode 4 channel loading of 20%.
4.2.4
General observations
The monitor response to VDL Mode 4 interfering signals became very stochastic when
high signal levels of the VDL Mode 4 signals were used (e.g. necessary to trigger an alarm or a switch of
the ILS system. In particular the monitored modulation depth varied from very high to very low levels
randomly. It was concluded that this behaviour, as well as the fairly consistent response of the monitor
system to interference over a wide bandwidth was caused by saturation of the monitor receiver. This
behaviour is consistent with the observations from measurements that were taken by Eurocontrol in 2009
and presented to ICAO (NSP)..
4.2.5
Conclusions on the ILS monitor measurement results
With regard to interference that can be caused by VDL Mode 4 system into the ILS
localizer monitor receiver, only at relative small separation distances between the ILS Localizer monitor
and the VDL Mode 4 transmitter interference can be caused that may trigger a switch-over of the ILS
systems. These distances would require the source of interference to be located within the ILS Localizer
critical area (during actual operation of the ILS).
It should be recognized that different monitor systems may have different behaviour and
show better or less immunity. Furthermore, the above analysis is (for the ILS at Kungsangen airport)
worst case as it assumes an omnidirectional antenna gain of the monitor antenna with 6.5 dB antenna
gain. Actually, the front-back ratio of the monitor antenna is 9dB, which would add a margin of 9 dB for
interference sources in front of the ILS antenna system. Additional, where a single dipole antenna is used
in the monitor system, the separation distances, as calculated above, can be reduced with the equivalent
of 3.5 dB (a monitor antenna gain of 6.5 dB was used in the calculations of separation distances).
Interference that can be caused by aircraft flying over the monitor antenna at a short
distance to the monitor antenna is transient and will not cause a monitor alarm.
To secure in all cases safe operation of the ILS system at airports where VDL Mode 4
frequencies in the band 112 – 117.975 MHz are in operation it is recommended to add to the frequency
assignment planning criteria the potential of harmful interference into ILS Localizer monitor systems and
47 of 90
to advise States to secure (through measurements and, if necessary, installing additional filtering into the
monitor receiving system) that no harmful interference can be caused to the ILS Localizer monitor by VDL
Mode 4 transmissions.
The measurement results further lead to the conclusion that there is no need for a
general measure throughout the designated operational coverage of the VDL Mode 4 system to improve
ILS Localizer immunity performance against interference that may be caused by VDL Mode 4 operations
to existing ILS monitor systems.
It should be recommended that ILS system manufacturers be advised to secure better
immunity from out-of-band interference in the future. This may assist in implementing other systems (such
as VDL Mode 4 and GBAS) in the frequency band 108 – 117.975 MHz. Interference caused by GBAS
operations have been reported as well as interference from FM broadcasting transmitters, operating
below 108 MHz.
The better immunity of the localizer monitor systems for frequencies higher than 116 MHz
secures most likely provides protection from interference from aircraft air/ground voice communication
transmissions operating in the band 117.975 – 137 MHz.
States are advised that in some cases interference from VDL Mode 4 transmissions into ILS monitors
may occur when operating VDL Mode 4 at distances less than about 200 m from the ILS monitor
antenna, depending on the local situation. This can be verified through measurements and mitigated
by additional filtering. In most cases, interference is not expected when VDL Mode 4 operates outside
the ILS-Localizer critical area. .
It is recommended that manufacturers should improve the RF immunity characteristics of ILS monitor
receivers outside the nominal ILS-Localizer frequency. This would assist in reducing the potential of
harmful interference from various sources.
4.3
Interference from the ILS Localizer into VDL Mode 4
4.3.1
Test set-up
The set-up for testing interference from localizer transmissions into VDL Mode 4 was as shown in Figure
33. This test set-up was agreed by the ICAO Aeronautical Communications Panel.
48 of 90
VDL Mode 4
test equipment
VDL Mode 4
receiver
-82 dBm
RF-combiner
RF-attenuator
RF-attenuator
ILS signal
generator
VDL Mode 4
transmitter /
signal generator
Figure 33 – Test setup for measurement of ILS monitor interference
Although VDL Mode 4 can only operate in the band 112-117.975 MHz, potential
interference that can be caused into VDL Mode 4 has been tested with the ILS-localizer (signal generator)
operating on 112.000 MHz as well as frequencies above and below, with VDL Mode 4 operating on 112
MHz. Measurements were taken with the ILS Localizer signal being tuned to the adjacent 25 kHz
channels as shown in Table 13 and Figure 34. The level of the (unwanted) ILS signal was increased until
a message error rate of 2% was measured. The measurements were made over one minute. The
maximum bit error rate for VDL Mode 4 is 1 in 103, which corresponds to a message error rate of 2%.
Measurement results are presented in Table 13 and Figure 34.
Channel
-40
-20
-10
-5
-4
-3
-2
-1
0
1
2
3
4
5
10
20
40
ACR
69
69
65
63
61
59
58
49
-8
49
57
59
61
63
67
69
70
Table 13 – Adjacent channel rejection measurement results for VDL Mode 4 victim and ILS
localizer interferer
Figure 34 – ACR; VDL Mode 4 victim, localizer interferer
49 of 90
4.3.2 Separation distances
Table 14 and Figure 35 provide examples of the minimum separation distance (in km)
between an ILS transmitter and a VDL Mode 4 receiver for given EIRP values for the ILS-Localizer (these
values are typical for the course sector of the ILS-Localizer). The ACR values used are as in Table 13.
Also separation distances are given if a polarization discrimination of 10 dB is applied. Calculation of
separation distances is further explained in Appendix A, paragraphs A1 and A2.
Channel
-40
-20
-10
-5
-4
-3
-2
-1
0
1
2
3
4
5
10
20
40
Separation distance between ILS transmitter (interferer) and VDL
Mode 4 receiver (victim); all distances in km
EIRP ILS
EIRP ILS
1kW
polarization
500W
polarization
correction
(60 dBm)
(10dB)
(57 dBm)
correction (10 dB)
0.933
0.295
0.661
0.209
0.933
0.295
0.661
0.209
1.479
1.862
2.344
2.951
3.311
9.333
0.468
0.589
0.741
0.933
1.047
2.951
1.047
1.318
1.660
2.089
2.344
6.607
0.331
0.417
0.525
0.661
0.741
2.089
9.333
3.715
2.951
2.344
1.862
1.175
0.933
0.832
2.951
1.175
0.933
0.741
0.589
0.372
0.295
0.263
6.607
2.630
2.089
1.660
1.318
0.832
0.661
0.589
2.089
0.832
0.661
0.525
0.417
0.263
0.209
0.186
Table 14 – Separation distances between ILS localizer transmitter and VDL Mode 4
receivers (Re. Table 12)
50 of 90
Figure 35 – Separation distances between ILS localizer transmitter and VDL Mode 4 receivers (Re.
Table 12)
*Include a polarization discrimination of 10 dB
These separation distances apply to the course sector of the ILS Localizer (+/- 10⁰
relative to the (extended) runway center line. Outside this area, the EIRP of the ILS transmitter is about
10 dB lower.
4.3.3
Conclusions
With these results it was concluded that VDL Mode 4 can be used without frequency
assignment planning constraints vis-à-vis the ILS if separated by at least 50 kHz (e.g. ILS-Localizer
operating on 111.950 MHz and VDL Mode 4 on 112.000 MHz). The minimum separation distance
necessary to protect VDL Mode 4 from harmful interference is about 1 km. This frequency separation is
already provided by ICAO provisions and the ITU Radio Regulations.
With regard to the separation distances it should be noted that these can possibly be
further reduced by correcting the measurements because of the high noise level of the ILS signal
generator (see Appendix 3). Also, since normally the desired field strength of the (wanted) VDL Mode 4
signal in the vicinity of airports is higher than the minimum assumed in the calculations, the actual
minimum separation distance can be less than indicated. This would be an aspect to be considered when
implementing the use of VDL Mode 4 on or around airports.
VDL Mode 4 can be used without frequency assignment planning constraints if separated by and ILS
operating frequency of at least 50 kHz.
Note: VDL Mode 4 may be subject to harmful interference if the separation distance to the ILS
transmitter in the front course sector is less than about 1 – 5 km.
51 of 90
5 Compatibility of VDL Mode 4 with VDB GBAS
5.1 Interference from VDL Mode 4 into VDB GBAS
5.1.1 Test set-up
The set-up for testing interference from VDL Mode 4 into VDB GBAS receivers was as shown in Figure
36. The test set-up was agreed by the ICAO Navigation Systems Panel and the Aeronautical
Communications Panel.
GBAS
test equipment
GBAS
receiver
-86 dBm
RF-combiner
RF-attenuator
RF-attenuator
GBAS
transmitter
VDL Mode 4
transmitter /
signal generator
Figure 36 – Test setup for measuring interference from VDL Mode 4 into VDB GBAS
The MFR (message failure rate) for VDB GBAS measured is less or equal to 1.10-3, over
500 GBAS messages. Tests were undertaken confirming that this method is equivalent to 0 errors in
1500 or more messages. The GBAS equipment was set to transmit and receive on all timeslots. The
message error rate was measured on all GBAS timeslots.
The frequencies on which the GBAS testing took place were 113, 115 and 117.950 MHz.
During the tests, the GBAS signal level (wanted signal at the GBAS receiver input) was set at -86 dBm.
The VDL Mode 4 (interferer) channel load was set at 100 %
The measurement results for the ACR (dB) are presented in Table 15 and Figure 37
52 of 90
Channel no ACR 113 ACR 115 ACR 117.950
-40
79
79
79
-20
79
79
79
-10
79
79
79
-5
70
70
70
-4
66
66
67
-3
60
60
60
-2
60
60
60
-1
35
37
36
0
19
17
17
1
36
37
37
2
61
61
60
3
60
60
60
4
66
67
67
5
70
70
70
10
79
78
79
20
79
79
79
40
79
79
79
Table 15 – ACR for VDB GBAS; GBAS (victim) and VDL Mode 4 (interferer)
Figure 37 – ACR as per Table 12 (GBAS victim and VDL Mode 4 interferer)
53 of 90
5.1.2
Separation distances
Separation distances have been calculated as shown in Table 16 and Figure 38 (distance in km) using
the method as described in Appendix A (paragraphs A1 and A7):
Channel no Distance 113 Distance 115 Distance 117950
-40
0.042
0.042
0.042
-20
0.042
0.042
0.042
-10
0.042
0.042
0.042
-5
0.132
0.132
0.132
-4
0.186
0.186
0.209
-3
0.295
0.263
0.295
-2
0.372
0.417
0.417
-1
6.607
6.607
6.607
0
1
5.248
5.248
4.677
2
0.372
0.372
0.372
3
0.263
0.263
0.295
4
0.166
0.186
0.234
5
0.117
0.117
0.132
10
0.042
0.047
0.042
20
0.042
0.042
0.042
40
0.042
0.042
0.042
Table 16 – Separation distances between VDB GBAS (victim) and VDL Mode 4 (interferer)
Figure 38 – Separation distances as per Table 15
Note: These measurement results are consistent with earlier similar measurements with the VDB/GRAS
in SCAT1 mode system.
54 of 90
5.1.3
On board compatibility of VDL Mode 4 transmissions
and GBAS receivers
It was noted that the minimum wanted signal level for the GBAS signals is -71.7 dBm (or
– 76.7 dBm) in accordance with the provisions of Annex 10, Volume 1, paragraph 3.7.3.5.4.4.1.2 (or
3.7.3.5.4.4.2.2 for the vertical component of the GBAS signal). The minimum receiver sensitivity is, as per
Annex 10, -87 dBm.
The VDL Mode 4 transmits with an EIRP of 39 dBm. Assuming the isolation between the
VDB GBAS and the VDL Mode 4 antennas of 45 dB (both antennas at the same side of the fuselage) the
energy of the VDL Mode 4 transmissions at the GBAS antenna would be in the -6 dBm. To protect the
GBAS signal of -72 (77) dBm, a D/U of -66 (71) dB would be required. With the antennas located on
different sides of the fuselage and an assumed antenna isolation of 60 dB, the energy from the VDL
Mode 4 transmission at the GBAS antenna would be -21 dBm. The required D/U would be 51 (56) dB.
Note: The analysis of potential on-board compatibility between VDL Mode 4 and VDB (GBAS) in this
paragraph is only presented for general information. Actual implementation of VDL Mode 4 on-board an
aircraft needs to take into consideration a variety of elements specific to the aircraft. Approval of such
implementation rests with the national airworthiness authorities and is outside the scope of developing
frequency assignment planning criteria in ICAO (See also paragraph 2.1.5)
5.1.4
Conclusions
From the measurement results it would be safe to conclude that a co-channel protection
(D/U) of 20 dB could be agreed. This would require a separation between the GBAS and the VDL Mode 4
equipped aircraft to beyond the radio horizon (i.e. the sum of the radio horizon of the GBAS equipped
aircraft and the radio horizon of the VDL Mode 4 equipped aircraft. Both aircraft are at the edge of their
respective designated operational coverage areas).
If the GBAS and the VDL Mode 4 channels are separated by 25 kHz (one channel) a buffer zone of about
10 km around the GBAS DOC would be sufficient to protect GBAS from interference from VDL Mode 4.
If the GBAS and VDL Mode 4 channel is separated by 50 kHz, 75 kHz or 100 kHz (resp.
2nd, 3rd and 4th adjacent channel, VDL Mode 4 is not recommended for use by aircraft on the surface of
the same airport where GBAS is operating in case the no adequate separation (re Table 16) between
aircraft while on final approach or landing and a taxiing VDL Mode 4 equipped aircraft can be maintained.
For aircraft in flight no separation criteria are required.
No frequency assignment planning constraints would be necessary in case GBAS and
VDL Mode 4 are separated by 125 kHz or more (i.e. 5th or higher adjacent channel). VDL Mode 4
equipped aircraft would not interfere with GBAS reception on another aircraft while at final approach or
landing if at the airport surface a separation of more than 130 m between the two aircraft can be secured.
Co-frequency use of GBAS and VDL Mode 4. The D/U ration should be 20 dB. The VDL Mode 4
equipped aircraft and the VDB GBAS equipped aircraft should be beyond the radio horizon.
1st adjacent channel: Within the Designated Operational Coverage of the VDB GBAS and a buffer
zone of 10 km around the GBAS, VDL Mode 4 should not operate on the 1st adjacent frequency
2nd adjacent channel and higher: no frequency assignment planning constraints between VDL Mode 4
and GBAS frequency assignments.
VDL Mode 4 should not operate on the 2nd, 3rd or 4th adjacent channel to a VDB GBAS frequency at
the surface of an airport in case the separation between a VDB GBAS and a VDL Mode 4 equipped
aircraft can be less than 130 m. Note: This is an operating restriction
55 of 90
5.2 Interference from VDB GBAS into VDL Mode 4
5.2.1 Test set-up
The set-up for testing interference from VDB GBAS into VDL Mode 4 was as shown in Figure 39. The test
setup was agreed by the ICAO Aeronautical Communications Panel.
VDL Mode 4
test equipment
VDL Mode 4
receiver
-82 dBm
RF-combiner
RF-attenuator
RF-attenuator
GBAS
transmitter
VDL Mode 4
transmitter /
signal generator
Figure 39 – Test setup for measuring interference from VDB GBAS into VDL Mode 4
Testing took place with VDB/GBAS and VDL Mode 4 operating on 113, 115 and 117
MHz. The output power of the GBAS transmitter was set to 49 dBm (80 W). The GBAS transmissions
were on all time slots with maximum pulse length. The VDL Mode 4 signal was set at -82 dBm at the VDL
Mode 4 receiver. The measurements were made over 1000 (GBAS) messages
The measurement results are presented in Table 17and Figure 40 below.
56 of 90
ACR level [dB]
Adj. channel no
113 MHz
115 MHz
117 MHz
-40
62
64
65
-20
61
60
60
-10
57
56
56
-5
53
52
52
-4
51
51
51
-3
47
49
49
-2
44
46
46
-1
40
40
40
0
-19
-19
-20
1
39
39
40
2
41
41
46
3
43
44
47
4
45
44
48
5
49
49
51
10
58
57
57
20
62
60
61
40
62
64
63
Table 17 – ACR for VDL Mode 4 (victim) and GBAS (interferer)
Figure 40 – ACR as per Table 17 – ACR for VDL Mode 4 (victim) and GBAS (interferer)
5.2.2
Separation distances
The separation distances as shown in Table 18 and Figure 41 have been calculated, as specified
in Appendix A (paragraph A1 and A6).
57 of 90
km
115 MHz
0.46
0.74
1.18
1.86
2.09
2.63
3.71
7.41
Adj. channel no 113 MHz
117 MHz
-40
0.59
0.41
-20
0.66
0.74
-10
1.05
1.18
-5
1.66
1.86
-4
2.09
2.09
-3
3.31
2.63
-2
4.67
3.71
-1
7.41
7.41
0
1
8.32
8.32
7.41
2
6.61
6.61
3.72
3
5.24
4.67
3.31
4
4.17
4.68
2.95
5
2.63
2.63
2.08
10
0.93
1.04
1.04
20
0.58
0.74
0.66
40
0.59
0.47
0.53
Table 18 – Separation distances between VDL Mode 4 receiver (victim) and GBAS transmitter
(interferer)
Figure 41 – Separation distances as per Table 18
5.2.3
Conclusions
Co-channel frequency assignments for GBAS and VDL Mode 4 should secure that the
separation distance between a GBAS transmitter station and a VDL Mode 4 equipped aircraft is greater
58 of 90
than the distance to the radio horizon of the VDL mode 4 equipped aircraft. This would result in a
protection ratio (D/U) of 20 dB or more. (See also paragraph 5.1.4 as the protection of GBAS from
interference from VDL Mode 4 transmission are more stringent and should apply)
When GBAS and VDL Mode 4 are operating on the first adjacent channel, a geographical
separation of about 10 km between a GBAS (ground) station and the VDL Mode 4 receiver (either on the
ground or on-board an aircraft) would be sufficient to protect the VDL mode 4 signal. (See also paragraph
5.1.4 as the protection requirements for GBAS from interference from VDL Mode 4 are more stringent)
No frequency assignment planning criteria would need to be applied when GBAS and
VDL mode 4 are operating on the second adjacent (25 kHz) or more adjacent channel. In an area of
about 5 km around a GBAS station (on the second adjacent channel) a VDL Mode 4 equipped aircraft
may be interfered by GBAS transmissions; this range becomes less when the frequency separation
increases. Considering the transient effect of interference when the aircraft is in flight, this interference
range would be acceptable for VDL Mode 4. However, for aircraft operating on the surface of an airport, it
may be necessary, taking into account also the local conditions on an airport, to use the second to fifth
adjacent channel with caution. (See also paragraph 5.1.4 as the protection requirements for GBAS from
interference from VDL Mode 4 may be more stringent.)
Co-channel and first adjacent (25 kHz) frequency assignments of VDL Mode 4 and GBAS: see 5.1.4
as the separation distance to protect the reception of GBAS signals from interference from VDL Mode
4 transmissions is greater than the separation distance to protect the reception of VDL Mode 4 signals
from interference from GBAS transmission.
Second adjacent (25 kHz) frequency assignments of VDL Mode 4 and GBAS: No frequency
assignment planning constraints.
59 of 90
6 Interference between VOR and GBAS
6.1
Interference from VOR into GBAS
For these measurements the test setup was similar to the test setup used for assessing
interference between VOR and VDL Mode 4 and described in paragraph 3.3 above as GBAS is basically
an air/ground data link similar to VDL Mode 4 (which is also capable of broadcasting typical GBAS
signals). For measurements of interference from VOR into GBAS the GBAS signal at the GBAS receiver
input was set at -86 dBm. The GBAS was receiving on all time slots. The MFR (message failure rate)
measured for GBAS was less or equal to 1.10-3 over 500 GBAS messages.
6.1.1
Measurement results
The measured adjacent channel rejection of the GBAS receiver from VOR signals is
shown in Table 19 and Figure 42 below. The VOR frequency was 115 MHz.
Channel no ACR (dB)
-40
74
-20
68
-10
59
-5
49
-4
53
-3
58
-2
56
-1
41
0
-24
1
36
2
57
3
60
4
50
5
52
10
60
20
69
40
73
Table 19 ACR; VDB GBAS victim and VOR interferer
60 of 90
Figure 42 – VOR interferer; VDB GBAS victim
Note: To facilitate comparison with the measurements that were taken with the VOR as interferer and
VDL Mode 4 as victim, the measurement results (on 115 MHz) as presented in paragraph 4.3 are
inserted.
The separation distances for different eirp of VOR transmitters are in Table 20 and Figure
43 below.
Separation distance in meters
Channel #
50 W
100W
1kW
-40
209
295
933
-20
417
589
1862
-10
1175
1660
5248
-5
3715
5248
16596
-4
2344
3311
10471
-3
1318
1862
5888
-2
1660
2344
7413
-1
9333
13183
41687
0
1
16596
23442
74131
2
1479
2089
6607
3
1047
1479
4677
4
3311
4677
14791
5
2630
3715
11749
10
1047
1479
4677
20
372
525
1660
40
234
331
1047
Table 20 – Separation distances; VOR interferer; VDB GBAS victim
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Figure 43 – Separation distances; VOR interferer; VDB GBAS victim
Figure 43 presents the separation distances (in meters) to be maintained between a
GBAS equipped aircraft and a VOR (ground based) transmitter in order to avoid interference from VOR
transmissions into the reception of GBAS signals. The measurements were taken with VOR being tuned
at 115 MHz. The separation distance calculations are for a VOR of 50 W, 100 W and 1 kW. The
separation distances were calculated as shown in Appendix A (paragraph A1 and A8). The measured
ACR and calculated separation distances are consistent with those for interference from VOR into VDL
Mode 4 (see paragraph 3.3 above).
The measurement results for the ACR and the corresponding separation distances on the
4th and 5th adjacent channels were not optimal due to the VOR generator presenting unexpected signal
levels (peaks) on these channels. Further measurements on these channels would be necessary to
properly assess potential interference from VOR into GBAS, resulting in smaller separation distances
between VOR and GBAS.
6.2
Interference from GBAS into VOR
Interference from GBAS into VOR was measured using the same test setup as was used
to measure interference from VDL Mode 4 into VOR (as described in paragraph 3.1 above).
The frequencies on which the interference was measured were 112 MHz, 115 MHz and
117 MHz. The signal level at the VOR receiver was set at -79 dBm; the GBAS was transmitting on 2
channels (channel load of 25%).
Measurements were undertaken to test the effect of the GBAS channel loading. It was
observed that the difference in ACR when all 8 GBAS channels are being used compared to using 4
channels, 2 channels or one channel results in a difference in ACR of not more than 2 dB. As a result of
these measurements it was concluded that the GBAS channel loading would not affect the measured
ACR values for the (victim) VOR.
The ACR measured is contained in Table 21 and Figure 44 respectively.
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Channel #
ACR 112
ACR 115
ACR 117
-40
56
65
66
-20
57
63
65
-10
57
64
65
-5
57
62
65
-4
57
60
64
-3
56
59
63
-2
55
58
59
-1
10
10
8
0
-6
-7
-6
1
-5
-7
15
2
49
54
56
3
47
51
57
4
50
48
57
5
58
61
61
10
56
62
67
20
56
64
66
40
57
65
67
Table 21 – ACR; GBAS interferer; VOR victim
Figure 44 – ACR; GBAS interferer; VOR victim
The corresponding separation distances calculated as explained in Appendix A (paragraph A.1 and A.9)
are in Table 22and Figure 45 below.
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Separation distance in meters
Channel # Distance 112 MHz Distance 115 MHz Distance 117 MHz
-40
832
295
263
-20
741
372
295
-10
741
331
295
-5
741
417
295
-4
741
525
331
-3
832
589
372
-2
933
661
589
-1
165959
165959
208930
0
1
933254
1174898
93325
2
1862
1047
832
3
2344
1479
741
4
1660
2089
741
5
661
468
468
10
832
417
234
20
832
331
263
40
741
295
234
Table 22 – Separation distances; GBAS interferer; VOR victim
Figure 45 – Separation distances; GBAS interferer; VOR victim
Note: verical axis: separation idstance in meters)
4.5.4
Figure 46 presents a comparison of the measured results for interference from GBAS into
VOR (King), line AA, VDL Mode 4 into VOR (Honeywell, line BB) and VDL Mode 4 into King (line CC).
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Figure 46
The measurements demonstrate that frequency assignment planning criteria for GBAS when
operating nearby VOR frequencies can be modified to improve GBAS frequency assignment
planning. Further work, that would include measurements with GBAS equipment is recommended
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7 Erroneous tuning of VDL Mode 4
7.1 Introduction
This Chapter addresses the ability of an aircraft to cause the loss of integrity or continuity
for receiving the desired navigation signal in case the VDL Mode 4 transmitter is transmitter inadvertently
tuned to a frequency that is being used for a navigation function (e.g. VOR and GBAS).
This Chapter identifies any provisions already in place to prevent or correct mistuning of
the VDL Mode 4 transmitter. It presents a holistic view on potential failure modes that could lead to a VDL
Mode 4 radio erroneous tuning. This Chapter addresses the recovery modes for such events and
determines whether the current provisions in the relevant ICAO SARPs and industry Standards provide
adequate mitigation [1][2][3][4]
The failure modes and recovery modes apply to VDL Mode 4 operating throughout the
frequency range 112 – 137 MHz and provide protection to both NAV systems operating in the band 112 –
117.975 MHz and to VHF communication systems operating in the band 117.975 – 137 MHz.
Three mechanisms apply to tuning of the transmitting function of VDL Mode 4:
i.
ii.
iii.
Autonomous tuning using hardwired GSC channels and an on-board database
of published frequencies;
On-board selection of services by the operator e.g. flight crew (the context of
manually tuning is discussed below).
Directed tuning (auto tuning)by the ground stations;
The first two are automated mechanisms while the latter involves human intervention on
the basis of information received from the ground system. This document identifies and assesses the
erroneous tuning modes for these three tuning mechanisms, as described in paragraph 7.3.
7.2 VDL Mode 4 channels
VDL Mode 4 is governed by the ICAO provisions in Annex 10 to the Convention on
International Civil Aviation. These provisions allow VDL Mode 4 equipment to be capable of tuning
between 112 MHz and 137MHz. VDL Mode 4 nominally operates on two Global Signalling Channels 1.
The Global Signalling Channels (CSC) carry essential channel management functions
and enable effective frequency tuning by announcing the availability of any authorised additional
frequency (Local Signalling Channel, LSC) which is assigned either on a Regional or a sub-Regional
basis. LSCs provide additional data link capacity for specific data link services including broadcast
surveillance services and point to point air/ground data link communications. In Europe, 136.925 MHz is
assigned for this purpose. A second GSC channel is anticipated in the frequency band 112 – 117.975
MHz.
7.3 Modes of tuning in normal operations
7.3.1 The tuning function
Data link radios operate in fundamentally different ways from conventional analogue
(voice) radios, in particular with regard to tuning. Frequency tuning of (aircraft) data link radios is tightly
1
Annex 10, Volume III, Part I, paragraph 6.9.2.2.
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controlled by automated processes with varying levels of autonomy (as explained below) which give
access to the frequency the (aircraft) radio transmits on.
Through (autonomous) selection of services by the aircraft, the authorised frequencies for
use by the aircraft are announced by the VDL Mode 4 ground station in one of two ways:
1. Through updating or overriding of the on board data base of available frequencies;
2. Through direct ground control (directed tuning, using the system’s management functions
transmitted on the GSCs).
Selection of the operating frequency is handled by the link management entity (LME).
This function is responsible for:
 building and tearing down links on any assigned frequency;
 background monitoring of the health of the digital connection on the channel, irrespective of
whether it is a GSC or an LSC;
thereby governing the frequencies on which a VDL Mode 4 radio is allowed to transmit on 2 an LME
instantiation exists for each mobile link operating on each channel.
7.3.2 Autonomous tuning using on-board sources
Autonomous tuning is the ability of the mobile radio to autonomously select the
transmission frequency without any outside influence. This type of tuning information is either hard-wired3
in the radio tuning function4 or stored on on-board databases.
The GSCs are the primary channels which the transponder tunes to by default when
started up. These channels are hard wired in the VDL Mode 4 radio (Re. 7.1 i.). The on-board data base
is initialised with the GSC frequencies on start-up. The database is then complemented with other
frequencies announced by the ground system (see below).
7.3.3 On-board selection of services (by the flight crew)
This is a mode of tuning that relies on tuning information relayed from the ground (Re. 7.1
iii). This tuning information is relayed to the transmitter frequency control function by the Link
Management Entity (LME). The on-board selection of services involves human intervention. Although the
required service is selected by the flight crew (radio operator), the frequency is not manually selected.
Instead, the operator selects from a number of services announced on specific periodically broadcast
ground transmissions by the Directory of Services (DoS) 5. The radio then automatically tunes to the
frequency associated with the service.
The DoS presents the operator with any services available in the region, including the
frequencies which are available for tuning. On selection of the required service by the aircraft, the
transponder tunes automatically to the frequency announced by the ground station in the DoS 6
transmission. Channel selection time in the transponder is instantaneous 7,8.
The DoS message can be used to synchronise on-board database information in order to
accommodate regional changes in policy, frequency management, and level of service. Selection of
2
An LME instantiation exists for each mobile link operating on each channel.
Data link radios default to a frequency that is hard wired in their tuning function upon startup.
4 The tuning function in a modern data link radio could be a combination of hardware and software.
5 DoS are transmitted in periodically broadcast ground transmissions known as Ground Station
Information Frames (GSIFs).
6 Doc 9816 Part II 1.5.7.1.1.
7 Under 13ms following selection by the user.
8 Annex 10 Vol III 6.9.5.1.2.
3
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services may be made automatically by using the autotune function (as described below), or ‘manually’ by
the pilot/controller through the use of DoS.
7.3.4 Directed tuning by ground stations (Autotune)
The GSCs are the primary channels which are initialised by the transponder by default
when starting transmissions (Re. 7.1 ii). Once network acquisition has taken place and the radio is
registered on a given network (and frequency), the airborne transponder can then be directed to tune to
secondary frequencies for additional services. Erroneous frequency selection can occur through
 An erroneous or corrupt database
 An erroneous transmitting function
Autotune is a mode of tuning that allows remote tuning of the VDL Mode 4 transmitter for
broadcast applications, such as ADS-B and periodic DAP reporting characteristics. It is used by a ground
station to force the airborne transmitters to transmit on specific frequencies. Auto tuning is used to control
the broadcasts of a group of aircraft in a particular geographical region.
7.4 Categorisation of failure modes
The possible modes of erroneous frequency tuning are identified below. The table
identifies the modes that potentially introduce a risk for erroneous tuning to a NAV channel and the
mitigation.
Category
Autonomous
Mechanism
Incorrect database
Incorrect TX function
On board selection
Corrupt database
Incorrect TX function
Incorrect
interpretation
of
uplinked
DoS
(correct
frequencies incorrectly decoded)
Poor DoS configuration (wrong
frequencies correctly decoded)
Malfunction of mapping DoS
channel to service
Mis-selection of service
Autotune
Mitigation
Approved upgrades of airborne
database
Avionics
testing
and
certification
Approved upgrades of airborne
database
Avionics
testing
and
certification
System
level
mitigation
through repeat transmissions
by the ground station and error
detection algorithms in the
aircraft.
Ground station authentication
Certification
of
avionics
function
No interference since misselection will result in radio
tuning to another VDL Mode 4
channel.
Ground equipment certification
Wrong data due to ground
station error
Data corruption in ground DoS Approved upgrades of ground
database
database
Table 23 – Effect and mitigations of erroneous tuning in normal operations
An abstract system representation of the receiver (or transmitter) process chain reflecting
the identified failure modes is shown below. Notably, the only source of potential erroneous tuning that
involves a human actor (mis-selection of service) will not cause any harmful interference because it will
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simply tune the radio to another unintended (but authorized) VDL Mode 4 channel. The error modes are
therefore confined to automatic built-in functions that are subject to rigorous certification as with any
aeronautical radio.
Figure 47 – Abstraction of transmission chain showing failure modes
7.5 Mitigating factors addressing mistuning in normal operations
A number of built in system functions in VDL Mode 4 allow for the detection and recovery
from potential mistuning. The correct operation of these functions is tested as part of the certification
process. These tests are specified in the relevant equipment specifications [3][4].
However during the course of normal operations, it is possible for errors to be introduced
in the reception chain, for example from the presence of natural RF noise and unintentional interference
from other VDL Mode 4 stations operating under normal conditions. This could raise the potential for
erroneous tuning even for fully functional and certified equipment. The possible causes of erroneous
mistuning under normal operation are9:






Periodic repeat transmissions
Quarantining of ground tuning instructions
Error detection algorithm
Channel sensing
System timers
Autotune (detection of missed transmissions by the ground station on the expected frequency)
The functions built in in VDL Mode 4 to mitigate these possible causes of mistuning are addressed below.
The figure below illustrates the possible events in the reception chain that could result in
the potential of a mobile transmitter tuning to an incorrect frequency. The action and outcome of the
mitigating functions are discussed below.
9
Further details are in Appendix D
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Figure 48 – Event that could lead to tuning to an incorrect frequency and their mitigation
7.5.1 Periodic repeat transmissions
Critical channel management instructions dispatched by the ground station are repeated
(re-transmitted) on a periodic basis in order to allow new entrants to gain an overview of the available
services and the available frequencies within a reasonable time. These channel management functions,
including the Directory of Service (DoS), are broadcast on the global signalling channels (GSC) in
transmissions known as GSIFs10. The frequency of this periodic broadcast is set by the ground system
and depends on local conditions such as the nature of airspace and traffic density. The rate of this
periodic broadcast is at least once a minute. The airborne stations are required to continuously monitor
the GSC thereby providing:
 a means of detecting erroneously decoded tuning information at the aircraft by providing multiple
opportunities to decode DoS messages, in the rare event that the error is not detected by onboard error detection;
 a lifeline to the aircraft through the GSC allowing continuous reachability and instructions to tune
the aircraft to another frequency in case the ground system does not receive the aircraft’s
transmissions on the required frequency.
7.5.2 Quarantining of ground tuning instructions
To mitigate against unintentional corruption of the ground station instruction containing the frequency
information, the ground station is only allowed to transmit autotune instructions in especially designated
(or quarantined) time slots. Ground quarantining is a set of time slots set aside every second for the sole
use of the ground station. Transmitting autotune instructions in quarantined slots reduces the probability
10
Ground Station Information Frames.
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of intentional or unintentional interference and corruption of the directed frequency tuning instructions in
an autotune message, as well as for other essential channel management functions11.
7.5.3 Error detection
The data contained in each VDL Mode 4 transmission is protected by an error detection scheme called
Cyclic Redundancy Check. This is a standard scheme used widely in fixed and mobile digital
communications to allow detection of data corruption with the following outcomes:
 preventing the use of incorrect tuning information;
 prompting the request of a retransmission of the tuning data (e.g. from a directed autotune
request) from the ground station.
In general, an n-bit CRC will detect all consecutive bit errors that are less than n bits long. When an error
transmission is longer than n bits, an n-bit CRC scheme will detect at a rate of 1-2-n. The 16-bit CRC
scheme adopted in VDL Mode 4 therefore has a 99.9984% chance of detecting an error burst. In VDL
Mode 4 the CRC applies to the contents of the entire burst that encompasses all fields including the ones
containing the tuning frequency. Considering that this is the probability of detecting an error across the
entire transmission, the probability of an undetected error being in the frequency tuning data is finite but
very low.
In addition to error detection, the VDL Mode 4 technical provisions on frequency tuning require the tuning
function to ensure that the tuning range is restricted to the allowable limits [1][2]. In the event of an
undetected error falling in the frequency tuning part of the transmission, the validity check (post error
detection) carried out on the decoded frequency tuning data rejects all values that fall outside the
admissible range e.g. below 112 MHz.
7.5.4 Channel sensing
When in the process of autonomously acquiring a network on a new frequency, a VDL Mode 4
transponder automatically carries out an estimation of the noise floor on the frequency to which it is
tuned12. The purpose of the estimation is to verify that the frequency is not busy, and therefore avoid
interference to any on-going transmissions from stations already on the network. The estimation involves
sensing the channel for RF activity and for a valid VDL Mode 4 training sequence (burst preamble).This
process allows a mobile VDL Mode 4 transmitter to assess whether the frequency is available for
transmission at a particular point in time. In the event the VDL Mode 4 transmitter is tuned to an incorrect
frequency that is not a VDL Mode 4 frequency, the channel sensing mechanism prevents the transmitter
from transmitting at all if it detects RF background activity, such as the case when a VDL Mode 4
transmitter is erroneously tuned on a NAV channel. A detailed explanation of the power measurement
function is provided in Appendix D.
7.5.5 System timers
System timers provide a mechanism for monitoring of the tuning function. In the event a mobile
transponder is not tuned to the appropriate frequency, the system timers provide an additional layer of
protection against unintentional mistuning. The default timer parameter setting for a “leave” event
generation when a station is declared unreachable is 20 seconds.
A number of failure modes which are internal (e.g. synchronization) and external to the system (e.g.
frequency selection) are defined. In the event an aircraft fails to act on a frequency switching instruction
from a ground station, or if a ground station fails to instruct an aircraft two possible actions are taken in
the context of transmitting on a local channel (LSC) 13:
11
Doc 9816 Part I 2.4.10.5.1.
Annex 10 Vol III 6.9.5.3.
13 Doc 9816 Part I Appendix A to Chapter 4, section 4.
12
71 of 90


For inbound aircraft, the aircraft remains reachable on the GSC and reporting to/from the
aircraft is still possible albeit at a lower rate. This gives the ground station several
opportunities to autotune the aircraft to the appropriate frequency in that period.
For outbound aircraft, the aircraft will continue transmitting on the local signalling
channel. This event would time out after a maximum of 15 minutes.
The amount of time that an aircraft can operate on an erroneous non VDL Mode 4 frequency before the
event can be detected is governed by a set of timers designed to limit the exposure of a VDL Mode 4
transmitter. The most critical timer is one that controls the re-transmission of special control transmissions
for which an expected response has not been received14.
In practice, this timer is set after the transmission of a control parameter for which a response is
expected, for example a control parameter sent by an aircraft attempting to log onto a new channel. The
timer is then cleared upon receipt of a response to the control instruction from the ground station. If the
timer expires, such as the case when the aircraft’s transmitter is tuned to the wrong frequency, the aircraft
will attempt to hand off to another ground station (on another frequency) using the GSC. The default
setting for this timer is 6 seconds.
7.5.6 Autotune
In the case of the sending station being a ground station, failure to detect an aircraft responding on the
expected frequency can trigger a forced directed retune (though an autotune sent on the global channel)
forcing the errant peer station (e.g. erroneously tuned) to take one of the following actions:
 retune to the intended frequency;
 initiate connection on another channel, or
 stop transmitting altogether
7.6 Conclusions
The failure mode analysis and assessment presented has found that the current provisions for control
and monitoring of frequency selection in the relevant ICAO, Eurocae and ETSI Standards provide
adequate mitigation to prevent harmful interference to other systems operating in the same frequency
band.
14
In VDL Mode 4 this is called the TL2 timer.
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8 On-board isolation between VHF-COM (VDL Mode 4)
antenna and ILS/VOR/GBAS antenna
8.1 Introduction
The analyses presented above in Chapters 3, 4 and 5 indicate that with an antenna isolation of 45 dB
(plus 10 dB polarization discrimination) between the VDL Mode 4 antenna and the ILS, VOR or GBAS
antenna it would be feasible to use VDL Mode 4 on the same aircraft without causing harmful
interferencne to the reception of ILS, VOR or GBAS signals if the VDL Mode 4 system is transmitting on
the second adjacent 25 kHz channel or further away from the NAV frequency (See also paragraphs 2.1.5,
3.1.7, 4.1.4 and 5.1.3 above)
Measurement results from the Russian Federation were presented in 2009 to ICAO showing that not in all
cases the required minimum antenna isolation (45 dB) could be achieved on a small aircraft (Antonov An140 short haul turbo prop) but on a (larger) Antonov AN-148 regional jet, the minimum isolation of 45 dB
could be achieved with the proper selection of the VDL Mode 4 antenna with respect to the ILS/VOR
antenna. (See NSP Nov 9 WGW/WP13).
The (generic) analysis on the on-board compatibility between VDL Mode 4 and ILS/VOR/GBAS in this
document are only presented for general information. Actual implementation of VDL Mode 4 on board an
aircraft needs to take into account, by the aircraft manufacturer, a variety of elements specific to the
aircraft. Approval of such implementation rests with the relevant airworthiness authorities and falls outside
the scope of this document.
8.2 Measurements
Measurements were undertaken in September 2010 by LFV-Sweden. Measured was the antenna
isolation between a variety of COM antennas and the two NAV antennas installed on an Beech King Air
200 aircraft. This aircraft is used for flight inspection purposes. The communication and navigation
antennas located on this aircraft are shown in Figure 50.
Measurements were undertaken with a CW signal being fed into the following antennas
COM antennas:
VHF1(#30)
VHF2(#7)
VDL4(#12)
COM1(#6)
COM2(#25)
COM5(#10)
The received signal level in the NAV antennas for ILS and VOR was measured at antenna:
NAV (#9)
VOR/LLZ/COM (#3)
(Note: number in brackets refer to the antenna numbers in Figure 50)
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8.3 Test setup.
ILS/VOR antenna (hor)
COM antenna (vert)
Aircraft
Signal generator
112-118 MHz
Spectrum analyzer *
112-118 MHz
* or other measuring
equipment
ILS/VOR antenna (hor)
COM antenna (vert)
Figure 49 Test setup for measuring antenna isolation
For testing the antenna isolation between ILS/VOR and COM antennas on board an aircraft, the test
setup as in Figure 49 was used.
The signal generator was set to generate a signal in the band 112-117.975 MHz with a modulation tone
(AM) of 1000 Hz to mimic a typical VHF COM signal. The spectrum analyzer was set to measure, in a 50
kHz band width, the signal that can be received by the ILS/VOR antenna. The signal level at the end of
the antenna cable was measured. The difference between the transmitted signal level and the received
signal level provides a measure of the antenna isolation.
The measurements were undertaken with the antennas as identified in 8.2 below and included
configurations when both transmit and receive antenna were at the same side of the fuselage and when
the transmit and receive antennas were on different sides of the fuselage.
Measurement procedure:
1. Disconnect the NAV and COM units from the pair of antennas (NAV and COM antenna) with
which the measurements are taking place.
2. Connect the output of the RF signal generator to the COM antenna.
3. Connect the NAV antenna to the input of the spectrum analyzer or other measuring dev ice
4. Tune the signal generator to 112 MHz.
5. The output level of the signal gewnerator was set to 0 dBm. Adjust the output of the signal
generator, taking into account the expected antenna isolation (e.g. to +40 dBm in case an
antenna isolation of 40 dB is expected.)
6. Tune the Spectrum analyzer (or other measuring device) to 112 MHz and measure the RF level .
7. Repeat steps 4-6 at 113 MHz, 114 MHz, 115 MHz, 116 MHz, 117 MHz, 118 MHz, 119 MHz and
120 MHz.
8. The difference in signal level of the signal generator and the measured signal level at the input of
the spectrum analyzer (or other measuring device) is a measure of the antenna isolation. From
this difference, the cable losses for each cable needs to be subtracted to achieve the antenna to
antenna isolation.
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Figure 50 – Location of antennas on an Beech King Air 200
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8.4 Measurement results
8.4.1 Results with NAV antenna #9 (in the tail of the aircraft)
The isolation between the antennas COM1, COM2 and VDL 4 (#6, #25 and #12 respectively in figure
1) and the NAV antenna #9 (in the tail of the aircraft) was measured as in figure 51.
.
Figure 51 – Isolation between NAV antenna #9 and the antennas COM1, COM2 and VDL 4
The antenna COM1 is on the same side of the fuselage of the aircraft as the NAV antenna; the
antenna COM2 and VDL Mode 4 are on the opposite side of the aircraft as the NAV antenna.
8.4.2 Results with NAV antenna #3 ( on the top of the fuselage of
the aircraft)
The isolation between the antennas VHF1, VHF2, VDL4 and COM5 (#30, #7, #12 and #10
respectively in figure 1) and the NAV antenna #3 was measured as in figure 52.
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Figure 52 – isolation between NAV antenna #3 and the antennas VHF1, VHF2, VDL4 and COM5
The antenna VHF2 COM1 is on the same side of the fuselage of the aircraft as the NAV antenna; the
antennas VHF1, VDL4 and COM 5 are on the opposite side of the aircraft as the NAV antenna.
Both series of measurements showed that in all cases the achieved isolation is better than 45 dB and
antenna isolation of between 50 – 60 dB can be achieved, in particular when the VDL Mode 4
antennas are installed on the opposite side of the aircraft where the NAV antennas are installed.
Since the measurements were taken while the aircraft was on the ground, the mesurements of
inparticular the antenna isolation between antennas on the top of the fuselage and those on the
ground may be influenced by reflections. In most cases, the isolation between these antennas would
be better when the aircraft is in the air. When implementing VDL Mode 4 on board an aircraft, the
actual isolation between COM and NAV antennas while the aircraft is airborne should be verified.
8.5 Conclusions
Although the antenna isolation that can be achieved on board a (small) aircraft is an important
parameter, it may be concluded that installing VDL Mode 4 on a small aircraft is feasible. Larger (e.g.
air transport airplanes) are expected to achieve better antenna isolation.
As an antenna isolation of at least 45 dB would be required to install VDL Mode 4 on an aircraft. This
value is based on the measurement results that were presented to ICAO in 2008 and 2009. In the
light of the results of antenna measurements presented in this document, as well as in working paper
NSP Nov09 WGW/WP13, implementation of VDL Mode 4 on a small aircraft can be considered to be
feasible. It is expected that on larger aircraft, better isolation factors are achievable. The
measurement results presented in this paper are compatible with those presented in NSP Nov09
WGW/WP13. However, considering the results presented in NSP Nov09 WGW/WP13, installation of
VDL Mode 4 may not be possible in all cases on (some) aircraft.
With regard to the 45 dB antenna isolation that would be required to implement VDL Mode 4 on an
aircraft. It should be noted that this value is based on the measurement results presented in NSPSSG/WP16 in 2008 These measurement were taking place with a VLD Mode 4 channel loading of
20% in order to present a stable level of interference into the measured ILS and VOR receivers. In
practice, VDL Mode 4 cannot operate with a channel loading more than 2.7% which would require a
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higher level of interference from VDL Mode 4 into ILS or VOR before is can become harmful. This
should be taking into account when implementing VDL Mode 4 on board an aircraft.
Note: during the measurements, the aircraft was on the ground. This could influence the
measurement of the antenna isolation when the bottom mounted antennas are considered.
78 of 90
9 Summary of conclusions
9.1 VOR and VDL Mode 4
9.1.1 Interference from VDL Mode 4 into VOR receivers
To protect VOR from harmful interference that can be caused by VDL Mode 4 transmissions, the
following frequency assignment planning criteria apply:
Co-frequency and first adjacent (25 kHz) frequency: The separation distance between a VDL Mode 4
transmitting station and a VOR receiving station shall be greater than the distance to the radio horizon
between the two aircraft operating at maximum range and height of their respective designated
operational coverage (DOC) area.
Second adjacent 25 kHz frequency and higher: no frequency assignment planning constraints.
9.1.2 Interference from VDL Mode 4 into VOR monitor systems
Interference from VDL Mode 4 transmissions into VOR monitor receivers can occur at very short
separation distances (less than 100 m)
Interference from other systems (GBAS, FM broadcasting) needs to be considered
Industry should consider improving RF selectivity of ILS and VOR monitor receivers
9.1.3 Interference from VOR transmitters into VDL Mode 4
For VDL Mode 4, operating on the second adjacent (25 kHz) channel from a VOR station, no
frequency assignment planning constraints are required.
Note: attention should be given to the possibility of interference into VDL Mode 4 when operating
within a radius of 1 – 5 km from a VOR station. Such interference is typically transient in nature.
9.2 ILS and VDL Mode 4
9.2.1 Interference from VDL Mode 4 into ILS receivers
No frequency assignment planning constraints are necessary for the protection of ILS (airborne) ILS
localizer receivers from harmful interference for VDL Mode 4 transmissions.
9.2.2 Interference from VDL Mode 4 into ILS monitor systems
States are advised that in some cases interference from VDL Mode 4 transmissions into ILS monitors
may occur when operating VDL Mode 4 at distances less than about 200 m from the ILS monitor
antenna, depending on the local situation. This can be verified through measurements and mitigated
by additional filtering. In most cases, interference is not expected when VDL Mode 4 operates outside
the ILS-Localizer critical area. .
It is recommended that manufacturers should improve the RF immunity characteristics of ILS monitor
receivers outside the nominal ILS-Localizer frequency. This would assist in reducing the potential of
harmful interference from various sources.
9.2.3 Interference from ILS transmitters into VDL Mode 4
VDL Mode 4 can be used without frequency assignment planning constraints if separated by and ILS
operating frequency of at least 50 kHz.
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Note: VDL Mode 4 may be subject to harmful interference if the separation distance to the ILS
transmitter in the front course sector is less than about 1 – 5 km.
9.3 GBAS and VDL Mode 4
9.3.1 Interference from VDL Mode 4 into ILS receivers
Co-frequency use of GBAS and VDL Mode 4. The D/U ration should be 20 dB. The VDL Mode 4
equipped aircraft and the VDB GBAS equipped aircraft should be beyond the radio horizon.
1st adjacent channel: Within the Designated Operational Coverage of the VDB GBAS and a buffer
zone of 10 km around the GBAS, VDL Mode 4 should not operate on the 1st adjacent frequency
2nd adjacent channel and higher: no frequency assignment planning constraints between VDL Mode 4
and GBAS frequency assignments.
VDL Mode 4 should not operate on the 2nd, 3rd or 4th adjacent channel to a VDB GBAS frequency at
the surface of an airport in case the separation between a VDB GBAS and a VDL Mode 4 equipped
aircraft can be less than 130 m. Note: This is an operating restriction
9.3.2 Interference from GBAS transmitters into VDL Mode 4
Co-channel and first adjacent (25 kHz) frequency assignments of VDL Mode 4 and GBAS: see 5.1.4
as the separation distance to protect the reception of GBAS signals from interference from VDL Mode
4 transmissions is greater than the separation distance to protect the reception of VDL Mode 4 signals
from interference from GBAS transmission.
Second adjacent (25 kHz) frequency assignments of VDL Mode 4 and GBAS: No frequency
assignment planning constraints.
9.4 VOR and GBAS
The measurements demonstrate that frequency assignment planning criteria for GBAS when
operating nearby VOR frequencies can be modified to improve GBAS frequency assignment
planning. Further work, that would include measurements with GBAS equipment is recommended
9.5 Erroneous tuning of VDL Mode 4
The failure mode analysis and assessment presented has found that the current provisions for control
and monitoring of frequency selection in the relevant ICAO, Eurocae and ETSI Standards provide
adequate mitigation to prevent harmful interference to other systems operating in the same frequency
band.
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10 References
Reference to Main Documentation, Delete If Not Required
[1]
[2]
[3]
[4]
ICAO Annex 10, Vol III, Part I, 2nd ed. 2007
Manual on VHF Digital Link (VDL) Mode 4, Doc 9816, AN/448, 1st Ed. 2004
ETSI European Norms for VDL Mode 4 ground and airborne equipment (EN 301 and EN302)
Eurocae Minimum Operational Performance Specifications (ED-108A)
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11 Acronyms
11.1 Acronyms and Terminology
Term
Definition
ACP
Aeronautical Communications Panel
ACR
Adjacent Channel Rejection
CSC
Common Signalling Channel
GBAS
Ground Based Augmentation System
ICAO
International Civil Aviation Organization
ILS
Instrument Landing System
ITU
International Telecommunication Union
NSP
Navigation Systems Panel
SSG
Spectrum Sub Group
VDL
VHF Digital Link
VDLM4
VDL Mode 4
VHF
Very High Frequency
VOR
VHF Omnidirectional Range
WRC
World Radiocommunication Conference
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Appendix A
- Calculation of separation distances
Note: Separation distances between (airborne) ILS, VOR and GBAS receivers with VDL Mode 4
transmitters
A.1
Interference model
Undesired
Transmitter
Tu
Feeder
Loss
Antenna
Transmission
Gain
Loss
Lu
Gu
Lbf
Antenna
Gain
Co-channel or
adjacent channel with
the desired signal
Lbf
(
(dB)
1)
=
32.4
Pu
Desired
Signal
+20logf
+
20
logd;
Pd
Feeder
Loss
Desired
Receiver
Pd
where
f
=
MHz
Pd - Pu = D/U = Pd - {(Tu +Lu + Gu) -Lbf} where: Pd, Pu and Tu are expressed in dBm
Lu, Gu and Lbf are expressed in dB
and
d
=
km
(2)
From (1) and (2) the separation distance can be calculated by:
20log(d) = D/U - Pd + Tu + Lu + Gu - 20log(f) - 32.4
(3)
The EIRPu of the undesired transmitter equals Tu + Lu + Gu which converts formula
(3) into
20log(d) = D/U - Pd + EIRPu - 20log(f) - 32.4
(4)
The factor 20log(f) introduces a variation of 1.24 dB in the results of calculating 20 log(d). (20log(f)) is
for the frequency 118 MHz 41.43 and for 115 MHz 41.2. Therefore minimum separation distances are
at the lower band edge slightly larger than at the higher band edge (117.975 MHz).
For 115 MHz:
20log(d) = D/U - Pd + EIRPu – 73.6; D/U= - Adjacent Channel Rejection (ACR)
A.2
Re. paragraph 3.1 – Separation distances for
VDL Mode 4 (victim) and ILS Localizer (interferer)
For VDL Mode 4, Pd = -82 dBm; EIRP for ILS is 500 W (57 dBm) or 1000 W (60 dBm)
EIRPu = 500W:
20 log (d) = -ACR – (-82) +57 – 73.6 = -ACR + 65.4
EIRPu=1000W
20log (d) = -ACR – (-82) + 60 – 73.6 = -ACR + 68.4
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A.3
Re. paragraph 3.2 – Separation distances for
ILS Localizer (victim) and VDL Mode 4 (interferer)
For ILS, Pd = -86 dB; EIRP for VDL Mode 4 is 39 dBm
20log(d) = - ACR – (-86) + 39 – 73.6 = - ACR +51.4
A.4
Re. paragraph 4.1 – Separation distances for
VDL Mode 4 (victim) and VOR (interferer)
For VDL Mode 4, Pd = -82 dBm; EIRP for VOR is either 50 W (47 dBm), 100 W (50
dBm) or 1000 W (60 dBm)
A.5
EIRPu = 50W:
20 log (d) = -ACR – (-82) +47 – 73.6 = -ACR + 55.4
EIRPu=100W
20log (d) = -ACR – (-82) + 50 – 73.6 = -ACR + 58.4
EIRPu=1000W
20log (d) = -ACR – (-82) + 60 – 73.6 = -ACR + 68.4
Re. paragraph 3.1.1.1 – Separation distances for
VOR (victim) and VDL Mode 4 (interferer)
For VOR, Pd = -79 dB; EIRP for VDL Mode 4 is 39 dBm
20log(d) = - ACR – (-79) + 39 – 73.6 = - ACR +44.4
A.6
Re. paragraph 5.1 – Separation distances for
VDL Mode 4 (victim) and GBAS (interferer)
For VDL Mode 4 Pd = -82 dBm; EIRP GBAS = 80 W ( 49 dBm)
20log(d) = - ACR – (-82) + 49 – 73.6 = - ACR +57.4
A.7
Re. paragraph 5.1 – Separation distances for
GBAS (victim) and VDL Mode 4 (interferer)
For GBAS Pd = -86 dBm; EIRP VDL Mode 4 = 39 dBm
20log(d) = -ACR – (-86) + 39 – 73.6 = - ACR +51.4
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A.8
Re. paragraph 4.4 – Separation
distances for GBAS (victim) and VOR (interferer)
For GBAS, Pd = -86 dBm; EIRP for VOR is either 50 W (47 dBm), 100 W (50 dBm) or
1000 W (60 dBm)
A.9
5.1
EIRPu = 50W:
20 log (d) = -ACR – (-86) +47 – 73.6 = -ACR + 59.4
EIRPu=100W
20log (d) = -ACR – (-86) + 50 – 73.6 = -ACR + 62.4
EIRPu=1000W
20log (d) = -ACR – (-86) + 60 – 73.6 = -ACR + 72.4
Re. paragraph 4.2 – Separation distances for
VOR (victim) and GBAS (interferer)
For VOR, Pd = -79 dB; EIRP for GBAS is 49 dBm
20log(d) = - ACR – (-79) + 49 – 73.6 = - ACR +54.4
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Appendix B
Propagation models monitor interference
G=6.5 dB
LRx=2 dB
Lpol=10 dB
EIRP=40W
ILS Monitor
receiver
PU-25 dBm
VDL Mode 4
B.1
Free space
To calculate the received power in free space propagation conditions, the following generic formula
can be applied:
(1)
In this formula,
PR = received power (PU in Figure 1)
PT = transmitted power
D = distance (m)
GT = antenna gain of transmitter, including cable losses
GR = antenna gain of receiver, including cable losses
λ (wave length) and d (distance between the antennas) have the same unit
L are system losses (there are no further system losses considered)
This formula can be re-written into:
(2)
Where:
PU = Received power (dBm) at the receiver input
EIRPVDLM4 = eirp of the VDL Mode 4 transmitter, including antenna gain and cable losses
d = distance (km)
f = frequency (MHz)
GR = monitor receiver antenna gain (dB)
LR = monitor receiver cable losses (dB)
Lpol = polarization discrimination (dB) (assumed to be 10 dB)
When EIRPVDLM4 = 40 dBm, GR = 6.5 dB, LR = 2 dB and Lpol = 10 dB:
(3)
(PU in dBm, d in km and f in MHz)
B.2
Two ray or flat Earth model:
To calculate the received power taking into account the effect of the ground, the two ray ground
reflection model or flat Earth model can be applied and gives a more accurate prediction of the
received power compared to the free space model:
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(4)
In this formula:
PR = received power (PU in Figure 1)
PT = transmitted power
HT is transmitter antenna height (m)
HR is receiver antenna height (m)
d = distance between the antennas (m)
GT = antenna gain of transmitter, including cable losses
GR = antenna gain of receiver, including cable losses
L are system losses (there are no further system losses considered)
This formula can be re-written into
(5)
Where:
PU = Received power (dBm) at the receiver input
EIRPVDLM4 = eirp of the VDL Mode 4 transmitter, including antenna gain and cable losses
d = distance (m)
HT is transmitter antenna height (m)
HR is receiver antenna height (m)
GR = monitor receiver antenna gain (dB)
LR = monitor receiver cable losses (dB)
Lpol = polarization discrimination (dB) (assumed to be 10 dB)
When EIRPVDLM4 = 40 dBm, GR = 6.5 dB, LR = 2 dB and Lpol = 10 dB:
(6)
B.3
Egli model.
The Egli model can be used to predict propagation losses taking into account the effect of the terrain,
when both the transmitting and the receiving antenna are located relatively close to the ground. The
Egli model offers a more accurate prediction of path loss compared to the free space model. The Egli
model is based on measured path losses and converted into the following mathematical model and
provides an alternative generic method to predict propagation losses when the antennas are close to
the ground and includes an empirical frequency dependent correction for frequencies greater than 30
MHz:
(7)
This formula can be re-written into:
(8)
Where:
PU = Received power (dBm) at the receiver input
EIRPVDLM4 = eirp of the VDL Mode 4 transmitter, including antenna gain and cable losses
f = frequency (MHz)
d = distance (m)
HT is transmitter antenna height (m)
HR is receiver antenna height (m)
GR = monitor receiver antenna gain (dB)
LR = monitor receiver cable losses (dB)
Lpol = polarization discrimination (dB) (assumed to be 10 dB)
When EIRPVDLM4 = 40 dBm, GR = 6.5 dB, LR = 2 dB and Lpol = 10 dB:
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(9)
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Appendix C
ILS Monitor antenna at Kungsangen airport.
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Appendix D
D.1 Details on VDL Mode 4 mistuning mitigating functions
What is autotune?
This channel tuning function is performed by the Link Management Entity (LME). It allows a ground
VDL Mode 4 station to command an aircraft to change the operating characteristics of its
synchronization burst transmissions, including the frequency15. For example, a VDL Mode 4 ground
station may autotune a station operating on the GSCs to special LSCs and back again when entering
or leaving a specified airspace or geographical area 16.
Directed versus autonomous mode
Mobile VDL Mode 4 transmitters can transmit in directed or autonomous mode. Directed mode affords
the ground station overall control over slot allocation between participating mobiles. It can do this on a
one-to-one basis or in broadcast mode.
In autonomous mode, typically in broadcast scenarios, mobile stations are able to gain access to the
channel (time slots) without requiring control and oversight by a master station such as a ground
entity.
The role of channel sensing
When a VDL Mode 4 transmitter is exposed on a new network (channel), an automatic channel
estimation function is triggered. This estimation function allows a transmitter to announce itself and
log in to the network before it has gained a synchronised state. This is to avoid disruption to
transmissions from other stations already on the network.
When attempting to transmit on a given frequency, if the transponder’s power measurement function
detects at least 4 times the noise floor for a period of at least 0.5 ms, the transponder considers the
channel busy and therefore unavailable for transmission with a probability of 95%. This process
occurs within 1millisecond of the measurement.
In the event of a VDL4 transmitter erroneously tuned to a NAV channel in reception range of a VOR
signal17, the channel estimation function would interpret the activity from the NAV beacon within range
as a busy channel thereby preventing any transmission on the erroneous channel.
Rules governing access to channels
The channel access protocols defined for VDL Mode 4 allow tight control over channel resource. Such
control is essential to maintain the efficient running of a time division (slotted) system like VDL Mode
4. Practically speaking, this means that at each stage of the data transfer process, a definite portion of
channel time is reserved so that the participating stations have controlled access to the channel.
The channel access protocols allowing a VDL Mode 4 radio to gain access to a particular frequency
are governed by a suite of channel access protocols [2]. These protocols are tightly specified and are
key to the proper functioning of a time synchronised system such as VDL Mode 4 18.
15
Doc 9816 Part II 1.1.3.1.
Doc 9816 Part 1, 2.4.10.7.4.
17 VOR stations transmit two signals: a constant, omnidirectional reference signal and a directional,
phase-shifting signal.
18 A (time) slotted channel system is known as Time Division Multiple Access (TDMA).
16
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During nominal operating conditions, the VDL Mode 4 transfer protocols rely on a time (slot)
reservation system whereby reservations are announced on the channel for further transmissions by
same station or other peer stations. All participating stations tuned to the frequency listen to this
reservation information to update their reservation and PECT19 tables.
The reservations are announced beforehand as part of their transmissions on the channel so that all
other peer stations have an accurate picture of who’s who and any available time slots on the
channel. This process also allows the stations to detect in near real time any missed transmissions on
the peer recipient’s side, and thereafter to take appropriate action.
Missed transmissions happen for the following reasons in normal operations:
 RF interference events of finite duration impeding proper decoding of data bursts (units of
transmission) e.g. through local interference sources on the same mobile platform;
 Genuine overlapping of distant Mode 4 transmissions allowed through the cellular co-channel
scheme leading to garbled reception.
 A peer responding station being unreachable on a given channel due to mistuning 20.
This provides a mechanism for early detection of potential erroneous tuning events. Generally, when
a transmitting station does not detect a response from the peer station 21 in the allotted time slot on a
given channel, the sending station attempts to re-establish contact with the peer station by
retransmitting the original message. This process recurs until the originating station detects a reply on
the intended channel and typically occurs in less than a second. If no reply is detected on that
channel, the station is declared unreachable 22.
D.2 Estimation of residual interference in normal operations
During the period of time a transmitter is in an erroneous state of tuning, the residual interference that
can be expected while in this state can be estimated. The duration of interference introduced into a
victim channel is bound by the length of window of the VDL Mode 4 (time) slot reservation cycle, and
the setting of the re-transmission parameters prevailing at that time.
The default upper bound for the retransmission cycle when operating in point to point mode is 60
seconds. During this period a VDL Mode 4 transmitter may transmit a series of short bursts of nominal
duration between 13.33ms and 66.65ms23. Assuming a default maximum of 4 attempts in this period
using the higher of the two durations, an indicative interference duty cycle before detection and
recovery from a state of erroneous tuning is approximately 0.4%.
-------------------------------------------------
19
A Peer Entity Contact Table is a register of all other aircraft that are reachable on the VDL Mode 4
network.
20 Note all stations are always reachable on the GSC,
21 Every addressed transmission in VDL Mode 4 is followed by an automatic acknowledgment on the
same channel from the peer station.
22 If no reply is detected and the peer station is the ground station, the aircraft will be instructed to
tune to an alternate channel through the GSC.
23 Most VDL Mode 4 transmissions are limited to between 1 and 5 slot bursts.
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