Signal Processing Techniques for Aeronautical Communications via Satellite
Panos Fines
1
, Katerina Christofylaki
2
, Paul Febvre
3
1
Wireless Intelligent Systems Ltd, 64 Hatfield House, Baltic Street, London EC1Y 0SU, U.K.
2
University of Westminster, Dept. Electronic Systems, 115 New Cavendish Street, London W1W 6WS, U.K.
3
Inmarsat Ltd, Dept. of Advanced Systems, 99 City Road, London EC1Y 1AX, U.K.
ABSTRACT
Aeronautical communications are playing an important role in
the air traffic control as well as new passenger communication
services. Due to the global coverage required, satellite
transponders provide the required links which are traditionally
limited to narrowband throughputs (e.g. 64 kb/s) due to the
propagation channel characteristics. This paper analyses the
reasons that limit the throughput of the aeronautical satellite
links and presents advanced signal processing techniques that
allow satellite transceivers to break the narrowband barriers.
Keywords: Aeronautical Satellite Systems, Aeronautical Channel
Modelling, Iterative Equalization.
I. INTRODUCTION
There is a growing need for modern data communications
for aircraft passengers, crew and air-traffic control. The
demand for better aeronautical communications has led to
new system designs using powerful multi-beam satellites.
This paper addresses the transceiver signal processing
derived from detailed analysis of the propagation channel
aiming to overcome the limitations of the satellite based
systems. We show that the communication subsystem can
be aided from the positioning and meteorological data to
improve the link reliability and throughput.
The following sections, present the aeronautical
propagation channel, its effects in broadband transmissions
and an iterative receiver suitable for reliable broadband
communications.
II. PROPAGATION CHANNEL MODELLING
AND EFFECTS
Geostationary satellite networks are able to provide
communications over vast geographical areas and as such
are well suited to satisfying aeronautical communications
requirements. The effective satellite coverage area is
limited by the low elevation view of the satellite. Figure 1
shows an aircraft communicating via a geostationary
satellite (GEO) at 20 degrees elevation angle. In order to
derive an effective aeronautical transceiver system, the
aeronautical satellite channel should be carefully
considered. Aircraft safety considerations require high
communication integrity and reliability throughout three-
dimensional manoeuvres. At the same time, a small
antenna is required at the aircraft with very demanding
mounting requirements for reducing the air resistance. The
aeronautical satellite terminals operate over a wide
dynamic range in order to accommodate the long-term
impairments such as signal attenuation and the Doppler
effects. The short-term propagation impairments such as
multipath fading are equally important and they are the
main reason for the moderate data speeds currently
available in the aircrafts.
GEO Satellite
Alt=10Km, Lat=61.8d, El=20d
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
0 5000 10000 15000 20000 25000 30000 35000 40000 45000
Km
Km
Earth
GEO Satellite
Alt=10Km, Lat=61.8d, El=20d
LOS
Aero Horizon
Figure 1
Figure 2 shows the physical model for most signals
arriving at the airplane and demonstrates a typical
geometry. Reflections from the earth surface arrive at the
aeronautical antennas in addition to the line-of-sight (LOS)
signal. The reflections come from the visible earth area
under the aircraft (see Aero horizon in Figure 1). The
multipath component (M) is the sum of many rays
reflected by the earth. The ratio of the direct line of site
(LOS) power (C) to the multipath power (C/M) is a critical
parameter describing the severity of the fading and, hence,
the extra power required (link margin) for reliable
communication. The multipath component M consists of
the specular component and other diffused components
[1], and their relative powers depend on the roughness of
5070-7803-9314-7/05/$20.00©2005 IEEE
2005 IEEE International
Symposium on Signal Processing
and Information Technology