International Civil Aviation Organization
ACP/FSMP
WG-F/32
WP04
WORKING PAPER
2015-02-04
FREQUENCY SPECTRUM MANAGEMENT PANEL (FSMP)
(AERONAUTICAL COMMUNICATIONS PANEL (ACP))
THIRTY SECOND MEETING OF WORKING GROUP F (FREQUENCY)
Cairo, Egypt 16 – 24 February 2015
Agenda Item 6
Development of material for ITU-R meetings
Liaison Statement from ITU-R WP5B
“Characteristics of ADS-B Receivers on-board Satellite”
and a draft response, developed by the Technical Sub-group (TSG) of the
Aeronautical Surveillance Panel (ASP)
(Presented by the Secretary)
SUMMARY
Attachment 1 to this paper is a Liaison Statement from ITU-R WP5B, asking
specific questions in order to facilitate their on-going compatibility studies
with incumbent systems in the frequency band 1 088.7 – 1 091.3 MHz.
This Liaison Statement was addressed by the ASP TSG during their meeting
26 – 30 January. Attachment 2 to this paper contains their suggested response
to the first of two questions contained in the Liaison Statement.
ACTION
Using the attached material as basis, FSMP WG-F is invited to draft a
response to the Liaison Statement from ITU-R WP5B.
Attachment 1: Liaison Statement from ITU-R WP5B: “Characteristics of ADS-B Receivers on-board
Satellite”
Attachment 2: Preliminary draft response from ASP TSG
(10 pages)
Document1
ATTACHMENT 1
Radiocommunication Study Groups
Source:
Document 5B/TEMP/357
Annex 25 to
Document 5B/761-E
8 December 2014
English only
Annex 25 to Working Party 5B Chairman's Report
LIAISON STATEMENT TO ICAO
CHARACTERISTICS OF ADS-B RECEIVERS ON-BOARD SATELLITE
Working Party 5B (WP 5B) is developing a Report on Reception of automatic dependent surveillance
broadcast via satellite and compatibility studies with incumbent systems in the frequency band 1 088.7-1
091.3 MHz (Annex 15 of Doc. 5B/761). As part of this work it is examining the compatibility with nonICAO standardised systems that may be operated on frequencies overlapping the frequency used by ADSB. WP 5B is seeking any information that ICAO can provide to answer the following questions:
(1)
What is the minimum C/(N+I) required for ensuring the correct demodulation of ADS-B
messages at the input of the receiver on board the satellite?
(2)
Are there additional protection criteria such as a margin over a minimum C/(N+I), or other?
A response as soon as possible would be greatly appreciated, due to the urgency of the work in WP 5B.
Status: For action
Deadline:
May 2015 or early if possible
Contact:
Mr. Loftur Jonasson (ICAO)
(10 pages)
Document1
E-mail: LJonasson@icao.int
FSMP (ACP) WG-F/32 WP04
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ATTACHMENT 2
ASP WP18-20R1
26-30 January 2015
INTERNATIONAL CIVIL AVIATION ORGANIZATION (ICAO)
AERONAUTICAL SURVEILLANCE PANEL (ASP)
Technical Subgroup (TSG)
Fort Lauderdale, Florida
26 to 30 January 2015
Proposed Response to ITU Questions on ADS-B Satellite Reception
Revision 1
(Presented by Tom Pagano)
SUMMARY
This Working Paper (WP) is a proposed approach to respond to questions from the
International Telecommunication Union (ITU) regarding satellite ADS-B reception.
1.
Background
This WP is intended as TSG input to assist WG F of the Communications Panel in preparing their
response to a request received from Working Party 5B (WP 5B) of the Radiocommunication Study
Groups of the International Telecommunication Union (ITU). WP 5B is developing a report on
reception of Automatic Dependent Surveillance Broadcast (ADS-B) via satellite and compatibility
studies with other systems operating within a defined band of 1090 MHz. A request was received to
provide information to WP 5B regarding interference assumptions that will help them report on
interference expectations for satellite ADS-B reception.
2.
Discussion
The Radiocommunication Study Groups of the International Telecommunication Union (ITU),
WP 5B, is developing a report on reception of Automatic Dependent Surveillance Broadcast
(ADS-B) via satellite and compatibility studies with other systems operating within the 1090
MHz band and with systems that may penetrate into the 1090 MHz band. WP 5B specifically
requested information to answer the following questions:
(1)
What is the minimum C/(N+I) required for ensuring the correct demodulation of
ADS-B messages at the input of the receiver on board the satellite?
(2)
Are there additional protection criteria such as a margin over a minimum C/(N+I),
or other?
ADS-B reception standards do not specify performance in the manner that the above questions
from ITU assume. In order to provide WP 5B with helpful information, the TSG has considered
the noise and interference elements of ADS-B receiver performance separately. The carrier to
noise ratio (CNR) as well as the carrier to noise and interference ratio (CNIR) are means of
describing a communication system. Receiver performance requirements contained in ADS-B
standards do not utilize such performance metrics.
First, there is no direct treatment of noise in ADS-B receiver standards but it is considered in the
receiver requirements. Receiver performance is normally specified by a minimum receiver
sensitivity. The receiver sensitivity is defined as the signal level at which 90% decode
probability is achieved. The receiver sensitivity is specified in the absence of interference. This
implies that the receiver noise figure is sufficiently low enough to provide the margin required
for the decoder to overcome the noise amplitude.
Normally, baseband decoding requires 8 to 9 dB signal margin above the noise to enable
detection. The typical receiver design involves a chain of components including low noise
amplifiers, mixing with a local oscillator, an IF stage with filtering and demodulation. The most
sensitive ADS-B sensitivity defined in the current standards is -84 dBm at a standard antenna
which with the typical 3 dB assumed cable loss between the receiver and antenna, the sensitivity
at the receiver is -87 dBm. In the case of space-based ADS-B, however, a high performance
receiver and antenna system will be designed as a matched pair to meet the link budget for the
satellite orbit and intended use. This aspect is beyond the current purview of the ASP Technical
Sub-Group.
(10 pages)
Document1
FSMP (ACP) WG-F/32 WP04
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The noise figure of the receiver depends on the noise contribution of the hardware components
as well as the receiver bandwidth which is a function of the filters and amplifiers in the receiver
chain. Typical noise figures are in the order of 4 or more dB but with careful component
selection and other design considerations, reduction of the noise figure to 3 dB or less is possible.
The sensitivity of a receiver is related to the noise figure and CNR by:
Sensitivity = KBT + 10 Log(B) + NF + CNR
where B is the bandwidth and NF is the noise figure. As can be seen by the formula, there are
tradeoffs the implementer can balance to meet the minimum capability.
The questions from the ITU focus on CNIR since their goal is to understand and control the
interference from expected sources that could penetrate into the 1090 MHz band. Again, CNIR
isn’t a useful way to characterize interference for the purpose of characterizing interference
thresholds for ADS-B reception.
The dominate interference impacting ADS-B receiver performance is co-channel interference
from replies and broadcasts mainly from aircraft transponders. The interference is either
ATCRBS replies that are 21 microseconds in duration or Mode S signals which are either 64 or
120 microseconds. These are not the only interfering signals on 1090 MHz, as TACAN/DME
signals, as well as Mode 4/Mode 5 signals from military sources can occupy the frequency.
TACAN/DME signals are not considered significant as far as an interfering source since they
typically do not occur within the 1090 MHz band, and their short pulse duration is not enough
for them to be considered highly destructive for 1090 MHz ADS-B reception, because of error
correction.
To help ITU in characterizing satellite ADS-B interference effects, focusing on the co-channel
interference provides more meaningful information. Satellite ADS-B reception interference
analysis is vastly different than a typical airborne aircraft 1090 MHz or ground receiving station.
For example, an airborne ADS-B receiver which is attempting to receive and correctly decode
extended squitter messages from an aircraft at range, a message is potentially overlapped by the
ATCRBS and Mode S signals transmitted from other aircraft that may overlap the desired
extended squitter message.
The interfering signals arrive at various signal levels based on the range and transmit power of
the transponder emitting the signal. The interference rates and the signal level distribution
relative to the desired signal determine the decode probability of the desired message. The closer
the aircraft transmitting the signal is to the receiving signal increases the probability of
successful message decode since the probability of a signal to interference ratio that can achieve
successful decode is increased.
Satellite ADS-B reception significantly changes the interference that would be seen by the victim
satellite receiver. Since the receiver is located at greater ranges than receivers flying in the
normal airspace, the signal amplitude distribution of the interference is different as well. Also,
antenna characteristics for proposed satellite ADS-B receivers are not omni-directional, as is the
typical case for aircraft ADS-B receivers.
-3-
Challenges exist for satellite based receivers to narrow beams in the direction of interest over the
earth surface. There are potentially many signals the desired signal is competing against and
arriving at much lower signal levels. The distance involved and the weak signal levels
necessitate improved receiver sensitivity over specified MOPS compliant receivers. Narrower
antenna beam dwell as well as use of antenna gain are needed to overcome these challenges. The
design of the aggregate satellite receiving system must take into account the co-channel
interference rates. If satellite ADS-B is to provide a defined level of performance, it must
account for the impact of interference. On the other hand, the TSG has been advised that
ongoing work in other ICAO forums, in particular the ICAO NAT working groups, will define
performance requirements for surveillance applications in lower-density airspace to be served by
space-based ADS B. It is anticipated that this work will define longer position update intervals
for such applications, typically not greater than 15 seconds.
Satellite ADS-B surveillance is proposed to be provided by simultaneous multiple beams. Cochannel 1090 MHz message rates received in each beam depend upon the traffic density in the
beam footprint and the transmission rate of the aircraft contributing in the subtended area.
Reception of desired 1090ES messages from aircraft in low traffic density areas must compete
with this co-channel interference from aircraft in adjacent high density areas if both areas are in
the same beam footprint.
Discrimination between these two areas depends on the beamwidth resolution or spatial filtering
capability of the satellite array. As an example, this capability can be illustrated by assuming
three spot beams at angles covering the radial direction of the satellite field of view. Co-channel
interference rejection is determined by the subtended spot size relative to the distribution of
interference sources. Although this configuration represents a possible implementation that
could be used to compute resulting signal levels at the satellite receiver from multiple beams
pointing at different angles to the earth, no assumptions are made in this WP regarding satellite
antenna beams or gains. The approach is to attempt to provide ITU information that will allow
them to compute the signal level relationships based on the satellite antenna beam characteristics,
gain and receiver MTL.
There are complex relationships to determine competing interference signal levels relative to
desired signals. The geometry based on the location of the satellite receiver and the angles that
result from its location over the earth surface, the area subtended by the antenna beam forming
and the gain characteristics due to the elevation angles between the transmitted desired signal
and the interference sources, determine the signal to interference ratio at the satellite receiver.
Since there is no assumption made in this WP regarding satellite beam forming characteristics,
assumptions regarding interference estimates to ITU will allow them to estimate signal to
interference characteristics at the satellite receiver. Since this requires assumptions about aircraft
traffic densities, a baseline is provided so that future growth estimates can be accounted for and
simplifying assumptions can be provided. The highest density co-channel interference source
distribution is proposed to be represented by a rectangular surface area with traffic density and
transmission rates. The TSG used available information from the 2011 U.S. high density area
flight test measurements in order to respond to the request. However, this may not correspond to
the service volume intended by the application.
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FSMP (ACP) WG-F/32 WP04
It should be noted that the proposed baseline interference scenario is a simplification from a
realistic scenario but as pointed out above can be used to compute interference rates at a satellite
location and apply assumptions about the receiving antenna and receiver. It is assumed that the
instantaneous aircraft count (IAC) is a result of a uniform aircraft density across a defined
region. The following figure contains measurements that are the basis for aircraft density
assumptions:
1500
JFK
DCA
BWI
PHL
1400
1300
1200
1100
Traffic count
1000
900
800
700
600
500
400
300
200
100
0
0
25
50
75 100 125 150 175 200 225 250 275 300 325 350 375 400
Range from receiver, NM
Traffic Snapshot from Radar Data During April 2011 WJHTC Test Flight
Based on the aircraft count measurements over the New York Northeast corridor region in 2011,
the following represents the aircraft density, DAC, for the measured number of aircraft (NAC),
DAC = NAC / (π * R2),
where NAC is 1200 within range R = 300 NM.
DAC = .0042 aircraft per sq NM
For the purposes of converting density to area represented in earth center (EC) degrees, assuming
60 NM is 1 EC degree,
-5-
DAC * 3600 = 15.3 aircraft per sq EC degrees
If the New York high density area is represented by a 300 NM east-west and 900 NM northsouth region with uniform density, the IAC is:
IAC = 300 * 900 * .0042 = 1134 aircraft
The message transmission rates per aircraft are determined based on the measured message rates
averaged over the number of aircraft. Actual measured aircraft count is 1218 aircraft within 300
NM. The average reply/broadcast rates per aircraft from the measurements are:
63.9 Mode A/C replies/second
12.2 Mode S replies/second
The Mode S reply rate for this baseline scenario includes 5% that are extended squitter
broadcasts. As extended squitter equipage increases, the Mode S reply rate would need to reflect
the increase in equipage.
Other assumptions that can be used for interference and desired signal computation:
Assume transponder transmit power of 54 dBm.
Desired signal power is based on equipage class of extended squitter aircraft.
Minimum power is either 51 dBm or 54 dBm.
Antenna diversity operation needs to be reflected in signal power at the satellite receiver. For
interference calculations, it can be assumed that a percentage of transmissions are on the bottom
antenna. The antenna gain pattern assumptions can use the parameters that have been assumed
in 1090 MHz spectrum study activities. The elevation angle with respect to the satellite receiver
influences the signal level for both the top and bottom antenna. Since the antenna patterns are
relative to a horizontal plane, and top and bottom patterns are in opposite directions, bottom
antenna signal levels are appreciably attenuated. This hurts reception performance from a
desired signal perspective since extended squitters alternate top and bottom for each squitter
type.
However, the interfering signals from the aircraft competing with the desired signal that are
transmitted on the bottom antenna are attenuated as well. The majority of ATCRBS replies can
be assumed to be transmitted on the bottom antenna. A 60% assumption for Mode A/C and
Mode S replies from the bottom antenna is a conservative estimate that can be used.
All of the assumptions above can be used in the power computation including free space path
loss to determine signal levels at the satellite receiver. The distance from the high density areas
relative to the distance from the satellite receiver to the desired signal from the transmitting
aircraft provides the implementer with the signal environment that would need to be considered
to evaluate performance.
FSMP (ACP) WG-F/32 WP04
3.
-6-
Summary
Because of the unique challenges of satellite ADS-B reception, it is required to make
assumptions about the subtended area that satellite beam formation and characteristics of the
satellite receiving system that influence the interference to desired signal levels at the satellite
receiver.
The information in this WP is provided to enable computation of interference at the satellite
receiver which is dominated by the co-channel interference that would be experienced by a
satellite ADS-B receiver. The assumptions can be used by ITU for co-channel interference
assumptions from the primary source of these signals, aircraft transmissions on the frequency
consisting of ATCRBS and Mode S replies as well as broadcasts. These signals compete with
desired ADS-B signals from aircraft of interest.
The following are key inputs from the ICAO ASP TSG:

C/(N+I) is not the appropriate way to analyze interference of low duty cycle pulsed systems.

Each satellite of a space-based ADS-B system will employ multiple beams and receivers to cover
its “footprint”.

The effective footprint of a single beam is the region within which interfering signals from
ground or aircraft can impact the ADS-B reception.

Interfering co-channel signals will be received at amplitudes relatively similar (within a few dB)
to amplitudes of the desired ADS-B signals. (For both desired and undesired aircraft signals, it is
those from the top-mounted antennas that are most relevant to space-based ADS-B.)

Assessment of the impact of interference on position update interval of a space-based ADS-B
system should involve:
 Characterization of the interference environment in terms of emission sources and signal
types;
 Filtering for interfering signals based on the effective beam footprint of a particular spacebased ADS-B implementation;
 Assessment of the message collisions between desired and interfering signals, taking into
account error correction capabilities.

It is expected that surveillance applications in lower-density airspace to be served by space-based
ADS-B will be able to tolerate longer position update intervals, typically not greater than
15 seconds.