ACP-WGW24/WP-04
International Civil Aviation Organization
15 February 2011
WORKING PAPER
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
24th MEETING OF WORKING GROUP F
Paris,France 21 March - 25 March 2011
Agenda Item 6: Any Other Business
Use of 4 200-4 400 MHz Radio Altimeter Band
(Presented by Claude Pichavant)
SUMMARY
The band 4200-4400MHz has been identified as candidate band for
accommodation fixed and mobile broadband wireless services by the USA.
This contribution provides radio altimeter characteristics from different
suppliers used currently on Airbus programs.
ACTION
To note the paper and take the provided information on the technical
characteristics of radio altimeters into consideration as appropriate.
To initiate and foster activities among stakeholders of the civil aviation
administration and industry ensuring that the band 4200 - 4400MHz which
primary service is not usable by non-aeronautical users.
1
INTRODUCTION
During last 23rd meeting in Cairo, (Egypt), the information paper ACP WGF23/IP10 was presented by
USA to WG-F stated, that the National Telecommunication and Information Administration (NTIA) will
collaborate with the Federal Communications Commission (FCC) to “make available a total of 500 MHz
of Federal and nonfederal spectrum over the next 10 years, suitable for both mobile and fixed wireless
broadband use.
In order to satisfy this directive, a number of frequency bands are being studied. These studies will
determine current use of those bands in the U.S. and whether portions (or all) of them can be re-allocated
to support wireless broadband applications.
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ACP-WGF24/WP-04
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Of particular interest to aviation is consideration of whether portions of the 4200-4400 MHz band can be
used. NTIA has initially identified the 4 200-4 220 MHz and 4 380-4 400 MHz bands, the upper and
lower 20 MHz segments of the current 4 200-4 400 MHz aeronautical radionavigation service (ARNS)
band reserved on a worldwide basis1“exclusively for radio altimeters based on board aircraft and for the
associated transponders on the ground”. However, these ranges could change based on the further
determination of actual radio altimeter characteristics
The band 4200-4400 MHz is allocated to the aeronautical radionavigation service and is, according to
footnote 5.438 [1] of the ITU-R Radio Regulations, exclusively reserved for radio altimeters installed on
board aircraft and associated transponders on the ground. This band is predominantly used for low range
radio altimeter applications on board civil and military aircraft and it is obvious that radio altimeters need
to be protected from electromagnetic interference which could compromise their safe and reliable
operation.
The purpose of this paper is to provide an input about the main technical characteristics of Radio
Altimeters currently used on Airbus Air Transport aircraft. These characteristics were extracted from
Component Maintenance Manuel (CMM) issued by the three main Radio Altimeter suppliers.
Any additional information shall be asked to the Radio Altimeter suppliers if needed.
2
PRINCIPLE OF RADIO ALTIMETER OPERATION
On Airbus Aircraft two generations of Radio Altimeters are used;
the analogue one which is the most used and installed on Airbus fleet ;
and the digital one, the most recent, which offer some advantages and benefits like better integrity. and
reproducibility, new functions capability and cost reduction.
The use of analogue or digital RA units has no impact in terms of aircraft installation or manufacturing
activities.
2.1 Principle of RA operation
The purpose of Radio Altimeter (RA) equipment is to provide the aircraft with accurate measurement of
the minimal distance to the ground during the approach and landing phases of flight.
A Radio Altimeter uses the fact that the electromagnetic waves propagate through the air at a speed,
which is the constant speed of light.
[1] Per International Telecommunications Union (ITU) Radio Regulation Number 5.438: Use of the band
4 200-4 400 MHz by the aeronautical radionavigation service is reserved exclusively for radio altimeters
installed on board aircraft and for the associated transponders on the ground. However, passive sensing in
the Earth exploration-satellite and space research services may be authorized in this band on a secondary
basis (no protection is provided by the radio altimeters).]
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ACP-WGF24/WP-04
The Radio Altimeter then has to measure the shortest delay DT between the transmitted wave and the
received wave, linked to the minimum distance to the ground Ho by the formula:
T 
2 Ho
c
The RA is a wide-band frequency modulated continuous wave (FMCW) system measuring the distance
between the aircraft and the terrain below in the range of typically
-20 to +5500 feet.
It transmits an RF signal frequency modulated by a triangular waveform and receives this signal after it is
reflected from the ground. The RA system operates between 4.2 GHz and 4.4 GHz.
As illustrated by figure 1, an altitude is calculated by determining the difference between the frequency
F1 of the reflected signal and the frequency F2 of the signal being transmitted at the instant T2 the
reflected signal is received. This difference frequency DF is directly proportional to the time DT required
for the reflected signal to traverse the distance from the aircraft to the ground and back to the aircraft.
Figure 1 : Radio Altimeter Transmitted and Received Signals
At every instant, a beat signal is obtained by mixing the transmitted wave (with frequency F2) and the
received wave (with frequency F1). The frequency DF of this signal is equal to:
DF = F2 – F1.
As the modulation is linear, DF is linked to DT and then to Ho (minimum distance to the ground) by the
formulae:
F
T 2.H o


FH  FL TST c.TST
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2.2 Utilized bandwidth
The frequency bandwidth allocated to Radio Altimeter system is 4.2 – 4.4 GHz.
In fact, the RA sensor do not completely use this allocated bandwidth, the utilized bandwidth depends on
the sensor technology and design used, dedicated to each supplier and even each sensor family for a same
supplier.
So, the maximum utilized bandwidth is given here below by Supplier/RA family:

Supplier 1 :
a) Analog RA : 4,232 GHz – 4,368 GHz



Nominal bandwidth on initial tuning : 100 MHz
Tuning tolerance at lower frequency extreme = +/- 1 MHz
Tuning tolerance at upper frequency extreme = +/- 5 MHz
 Maximum occupied bandwidth = 106 MHz

Frequency stability specification = +/- 15 MHz
 Maximum utilized bandwidth = 136 MHz
b) Digital RA : 4,225 GHz – 4,375 GHz

Maximum occupied bandwidth = 150 MHz

Frequency stability specification = +/- 220 kHz so negligible
 Maximum utilized bandwidth = 150 MHz

Supplier 2 :
a) Analog RA : 4,218 GHz – 4,382 GHz

Maximum occupied bandwidth = 132.8 MHz

Frequency stability specification = +/- 15 MHz
 Maximum utilized bandwidth = 162.8 MHz
b) Digital RA : 4,211 GHz – 4,389 GHz

Maximum occupied bandwidth = 176.8 MHz, wide used bandwidth due to the
principle of frequency shift in order to avoid inter system coupling in case of
installation of maximum 3 systems.

Frequency stability specification = +/- 129 kHz so negligible
 Maximum utilized bandwidth = 176.8 MHz
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
ACP-WGF24/WP-04
Supplier 3 :
a) Analog RA 1st and 2nd generation : 4,233 GHz – 4,367 GHz

Maximum occupied bandwidth = 123 MHz +/- 0.5 MHz

Frequency stability specification = +/- 5 MHz
 Maximum utilized bandwidth = 133.5 MHz
b) Digital RA : Not applicable
In conclusion, in terms of utilized bandwidth, the worst case of RA sensor uses the following
bandwidth: 4,211 GHz – 4,389 GHz.
2.3 Receiver sensitivity
Another characteristic of Radio Altimeters is the receiver sensitivity which is chosen to answer to
specified receiver range (typically 5500 ft) and also depends on the sensor technology and design used.
So, the sensitivity is given here below by Supplier/RA family:

Supplier 1 :
a) Analog RA : No available information
b) Digital RA : < -114 dBm

Supplier 2 :
a) Analog RA : < -113 dBm
b) Digital RA : No available information

Supplier 3 :
a) Analog RA 1st and 2nd generation : between -114 and -120 dBm
b) Digital RA : Not applicable
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2.4 Receiver selectivity
Another characteristic of Radio Altimeters is the receiver selectivity that should be considered to prevent
interferences on Radio Altimeter system. It also depends on the sensor technology and design used.
Only information data concerning Digital RA from Supplier 2 is available so far:

Supplier 2 :


At Fc +/- 114 MHz (at 4.186 GHz or 4.414 GHz) = -25 dB
At Fc +/- 165 MHz (at 4.135 GHz or 4.465 GHz) = -45 dB
3 CONCLUSION
Today, all the types of Analog RA are installed on Airbus aircraft; furthermore, the Airbus strategy is to
propose in the next years on its new aircraft (current and future programs ) the digital technology
proposed by the different Suppliers.
So a large number of equipment that is or will be on board the aircraft uses an overall frequency band of
178 MHz (4211-4389 MH)].
It is useful to add that the remaining part of the frequency band, i.e. between 4200 – 4211 MHz and 43894400 MHz, is reserved for the « transition zone » which ensures the out of band non-interference,
accordingly with the ITU recommendations (Radio regulation edition 2004 RR5 table of frequency
allocation & appendix 3 rev WRC-03).
As a conclusion, it will be difficult to find available spectrum for other systems to share with the Radio
Altimeter equipment.
Moreover, it shall be noted that the Radio Altimeter are certified to perform safety critical tasks during
approach (Cat I, II & III) and so the risk of interference must be clearly managed.
4 ACTION BY THE MEETING
The ACP WGF is invited to note the information provided and, where appropriate, use the information in
preparing contribution.