Very shortly aeronautical satellite communications will be introduced on a world wide basis. By the end of the year, voice communications (both to the cabin and cockpit) and packet data communications will be available to both airlines and executive aircraft. During the decade following the introduction of the system, there will be many enhancements and developments which will increase the range of applications, expand the potential number of users, and reduce costs. A number of ways in which the system is expected to evolve over this period are presented. Among the issues which are covered are the impact of spot beam satellites, spectrum and power conservation techniques, and the expanding range of user services

Wood, Peter

English

NASA Technical Reports Server

N92-24167
Future Developments in Aeronautical
Satellite Communications
Peter Wood
Inmarsat
40 Melton Street
London, NW1 2EQ
United Kingdom
Phone: +44 71 387 9089
FAX: +44 71 383 0849
ABSTRACT
Very shortly aeronautical satellite
communications will be introduced on a world-
wide basis. By the end of the year voice
cormnunications (both to the cabin and the
cockpit) and packet data conununications will
be available to both aidhaes and executive
aircraft. During the decade following the
introduction of the system there will be many
enhancements and developments which will
increase the range of applications, expand the
potential number of users, and reduce costs.
This paper presents a number of ways in which
the system is expected to evolve over this
period. Among the issues which are covered
are the impact of spot beam satellites, spectrum
and power conservation techniques, and the
expanding range of user services.
INTRODUCTION
Aeronautical satellite communications have
been the subject of tests and studies for more
than 25 years. The earliest test was in 1964,
when communications were successfully
established with an PanAm aircraft in flight,
using the SYNCOM HI Satellite. The most
recent occurred earlier this year when an
aircraft operated by the U.S. National Science
Foundation was able to maintain
('ommunications to its home base to within 50
miles of the South Pole. Studies have included
the Oceanic Area System Improvement Study',
the RTCA SC-155 User Requirements for
Future Communications, Navigation, and
Surveillance Systems 2, and the report of the
ICAO Special Committee on Future Air
Navigation Systems 3.
Approximately five years ago Inmarsat began
to develop tile concepts for a global
aeronautical satellite service, and contracted for
the transponders in its Inmarsat-II satellites to
include 6 MhZ allocated to AMSS(R) use (3
MHz in each direction). Since that time the
system has evolved to the point where it will
enter into service in the very near future, and
will provide both voice and data
communications to aircraft operating world-
wide, with the exception of a lhnited area near
the poles.
The Inmarsat approach has been to use a
single-channel per carrier for voice
communications to and from the aircraft, a
combination of slotted Aloha and TDMA for
data communications from the aircraft, and
TDM for data communications to the aircraft.
During the course of development an
alternative to the Inmarsat approach based
entirely on TDMA techniques was proposed by
the AvSat Corporation, but although the
concepts are still retained in some
International Mobile Satellite Conference, Ottawa, t990
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specifications 4, development has been
terminated, and it is unlikely that a system
based entirely on TDMA will be introduced in
the forseeable future.
OUTLINE OF THE SYSTEM
The INMARSAT Aeronautical Satellite
System is based upon two types of antenna, a
nominally omnidirectional low gain antenna,
and a steerable high gain antenna (nominally
12 dB), together with a range of channel
transmission characteristics _Table 1) designed
to meet a wide range of applications.
Table 1. Channel Transmission Characteristics
Channel Channel
Rate Spacing
(bit/s) (kHz)
Modulation
21000 17.5 A-QPSK"
10500 10.0/7.5 A-QPSK
6000 5.0 A-QPSK
5250 5.0 A-QPSK
4800 5.0 A-QPSK
2400 5.0 A-BPSK b
1200 5.0/2.5 c A-BPSK
600 5.0/2.5 A-BPSK
a
b
c
A-QPSK is Aviation QPSK, a particular
fonn of O-QPSK _'6
A-BPSK (Aviation Binary Phase Shift
Keying) is a from of differentially
encoded BPSK in which alternate
modulation signals are transmitted in
notional In-Phase and Quadrature
chamlels 7
5.0 applies to P-channel, 2.5 to R- and
T-channels
Four types of channels are supported:
P-Channel: Packet mode time division
multiplex (TDM) channel, used in the
forward direction (ground-to-air) to carry
signalling and user data
R-Channel: Random access (slotted Aloha)
channel, used in the return direction
(aircraft-to-ground) to carry some signalling
and user data (short messages only)
T-Channel: Reservation Time Division
Multiple Access (TDMA) channel, used in
the return direction only.
C-Channel: Circuit Mode single channel per
carrier (SCPC) used in both the forward
and return directions. The use of the
channel is controlled by assignment and
release signalling at the start and end of
each call The C-channel includes sub-band
data fields for signalling purposes.
In the initial system, installations using the
omnidirectional low gain antenna will only
support low data rate communications over the
P-, R- and T-channels. High gain antennas
will support both low and high rate data, and
voice communications over the C-channel.
The majority of channel types use Forward
Error Correction (FEC) coding, consisting of a
convolutional encoder of constraint length k=7
and an 8-1evel soft decision Viterbi decoder.
The FEC coding rate is 1/2. Because of the
multi-path fading characteristics of the
aeronautical transmission path interleaving is
applied to all channels using FEC coding in
order to preserve the FEC coding gain.
Signalling and user data messages on the P,
T, and sub-band C-channels are formatted into
standard length signal units of 96 bits.
Signalling and user data messages on the R-
channel are formatted into extended length
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signal units of 152 bits. These allow for the
most common transactions to be carried out
within only one signal unit with a minimum of
spare unused capacity. More complex
messages (including use data) can be carried
by sequence of several signal units, up to a
maximum of 64.
Voice signals are digitally encoded using an
APC algorithm at a bit rate of 9.6 kbits/s. The
encoded voice data is combined with the sub-
band signalling channel for transmission over
a C-channel. The system is designed to allow
operation through the Public Switched
Telephone Network, although for operational
reasons initially it will only by possible to
initiate calls from the aircraft. This restriction
does not apply to communications to and from
the cockpit or with executive aircraft.
The system design is based upon the use of
a linear (class-A) High Power Amplifier in the
aircraft. This will support operation with
several channels simultaneously. The number of
charmels that the amplifier can support is
dependent upon the satellite through which an
aircraft is operating, and the location of the
aircraft with respect to the satellite. It is also
possible to use a class-C amplifier for single
channel installations.
SATELLITES
During the first few months of service
aeronautical communications will use the first
generation of Inmarsat satellites (Marecs and
Intelsat-MCS), and communications will be in
the maritime band. The main operational
limitation with these satellites is that they will
only support two channels simultaneously
(reduced to one channel at satellite elevation
angles less than 20°).
The first major operational change will
result from the relocation of one of the Marecs
satellites to 55 ° West, resulting in the creation
of four ocean regions (Atlantic-E, Atlantic-W,
Pacific and Indian). The effect of this
relocation will be to close the gap that
currently exists over part of North America and
the eastem Pacific Ocean.
This will be shortly followed by the launch
of the first of four Inmarsat-II satellites. Each
of these satellites will provide 3 MHz capacity
in the aeronautical band and, more importantly,
also have greater sensitivity, which will result
in doubling the number of channels that can be
supported by single HPA.
The third stage in the evolution of satellite
support for aeronautical communications will
be the launch of spot-beam satellites.
Currently AMSC in the United States, M-Sat in
Canada, and Imnarsat are plaiming to launch
spot beam satellites. The first two will provide
regional coverage; Imnarsat plans to provide
global coverage through its Inmarsat-llI
spacecraft. The first Inmarsat-III spacecraft is
expected to enter service late 1994 or early
1995.
Two important operational enhancements
will result from the introduction of spot beam
satellites. The first is an increase in the
number of channels that can be supported by
a single aircraft installation that complies with
industry standards. The second is the
possibility of supporting voice services through
a low-gain omnidirectional antenna. Although
Inmarsat does not see this as being the primary
method of operation (if for no other reason
than the increased interference to global beam
satellites that would result), nevertheless this
would be a method of maintaining a minimum
level of voice service for emergency use in the
event that the high gain steerable antenna
failed for some reason.
International Mobile Satellite Conference, Ottawa, 1990
589
SYSTEM DEVELOPMENTS
System Management
Initially each Ground Earth Station (GES)
that supports the hunarsat system will operate
on a stand-alone basis, using a dedicated set of
frequencies. Each GES will be required to
support a high-power P-channel to
communicate with aircraft that are only fitted
with low-gain antennas.
As usage of the system develops, Inmarsat
will be introducing Network Coordination
Stations (NCSs) which will allocate frequencies
on a dynamic basis according to demand. It
would also be possible to use space segment
resources more efficiently by providing a
common high-power P-channel at NCSs only.
As other satellite service providers begin to
offer service, the NCSs will also be responsible
for inter-system coordination.
Voice Coding
At the time the concepts for Inmarsat's
aeronautical system were maturing, the lowest
bit rate at which it appeared an acceptable
quality for voice communications could be
achieved was in the order of 9.6 kbits/s.
Extensive tests were carried out on behalf of
the Airlines Electronic Engineering Committee
to determine which of the algorithms available
had the highest performance in an aeronautical
environment, and an APC based algorithm
developed by British Telecom Research
Laboratories (BTRL) was selected as the
industry standard)
Subsequently developments in voice codec
technology have led to the possibility that
acceptable quality could be achieved at bit
rates as low as 4.8 kbit/s. Formal comparative
tests between 9.6 kbit/sec and 4.8 kbit/s have
still to be performed. If the performance of
these lower data rate codecs lives up to their
promise then it is probable that they will be
eventually selected as a future standard. No
major modifications to the basic system design
would be required, and the 9.6 kbit/sec codec
would continue to be supported while it was in
general use. The lower bit rates would, of
course, lead to a more efficient use of the
available spectrum; a key issue as the uses of
aeronautical satellite communications expands
to fill the frequencies allotted by the ITU for
that use.
Circuit Mode Operation
Circuit-mode data service is an optional
enhancement to Imnarsat's aeronautical system.
This service provides a basis circuit mode data
channel with a call set-up procedure shnilar to
that used for voice calls. The nature of the
end-to-end service between the aircraft and the
end user depends upon the type of
interconnection with the terrestrial network.
The use of voice-band analog modems provides
an analog-interconnect data service. Digital
interconnection is possible where a digital path
is available through the terrestrial network
(circuit switched digital data network, dedicated
digital network, or ISDN).
Each circuit-mode data call utilizes a pair of
C-channels for the duration of the call. The
digital bit stream of the C-channels, without
the use of voice band modems over the
satellite channel, is directly employed to
transport the data traffic. The voice codecs
normally associated with the C-channels are not
used during the data transfer state.
Rate adaptation must be performed to match
the user data bit rate, taking into account clock
rate variations, to the available satellite channel
capacity. The bit rate stability of the received
data from a PSTN modem is, in general, worse
than the required clock rate stability of the
satellite channel. It is therefore necessary to
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International Mobile Satellite Conference, Ottawa, 1990
implement a plesiochronous interconnection
arrangement in the forward direction. A
plesiochronous buffer is not required in the
return direction provided the PSTN modem is
synchronized to the incoming data stream from
the C-channel.
NEW SERVICES
When the concepts fi,r the system were
originally developed it was considered that
there were two basic types of service required;
packet mode data for air traffic service and
airline use, and voice for public correspondence
and, to a more limited extent, for non-routine
air traffic service and airline messages. As
more users became aware of the potential of
aeronautical satellite communications a wider
range of services appeared to justify support.
Examples of the services which will be
offered in the future are:
Facsimile
One of the earliest to be introduced will be
facshnile. An interface unit to permit
operation of standard Group-HI facsunile
equipment is now under development. The
ability to successfully transmit facsimile over a
satellite link has already been demonstrated.
Broadcast Messages
There is also a demand to broadcast voice
and data messages to a group of aircraft.
Voice messages can be accommodated quite
easily since only reception is required; this
allows the service to be introduced without
absorbing any of the linaited power available
from the aircraft HPA. Data messages (similar
to the Oracle and Seefax messages currently
being transmitted over TV chamlels in the
U.K.) could also be transmitted for subsequent
display over the aircraft's video equipment.
Computer-to-Computer Communications
With the increased use of portable and lap-
top computers a demand has arisen to support
their use by passengers while they are on the
aircraft. A simple interface unit would permit
this; the main linfitation is the feasibility of
certifying computer equipment for use by the
public on aircraft.
Mobile Telephone Service
There would be considerable benefits to
allowing the use of portable telephones on an
aircraft. Again, the main problem is related to
licensing of the equipment rather than technical
problems in its use. If this could be overcome
such a service would most likely be linfited to
the air-to-ground direction.
CONCLUSIONS.
Over the next few years, enhancements to
the Inmarsat aeronautical satellite system will
be incremental in nature. More exotic
developments, such as direct television
broadcasts from satellite to aircraft, will
eventually be hltroduced, but will required a
switch to a new portion of the spectrum. All
projections show that the L-band spectrum
currently allocated to aeronautical mobile
services will only be adequate for essential
demands by the end of the decade. As the
amount of traffic required to support safety and
regularity of flight increases there will be
pressure to move public correspondence to
these other frequencies. This could well be the
catalyst for introducing these new services.
International Mobile Satellite Conference, Ottawa. 1990
591
REFERENCES
1. Couluris, G.J. and B. Conrad. 1981
Oceanic Area System Improvement Study
(OASIS)--Volume 1 Final Report FAA-EM-
81-17 (Menlo Park, California, SRI
International)
2. ----1986. Report of RTCA Special Committee
165--User Requirements for Future CNS
Systems, Including Space Technology
Applications. DO-193 (Washington D.C., Radio
Technical Conunission for Aeronautics)
3. ----1988. Special Committee on Future Air
Navigation Systems--Fourth Meeting Report
Doc 9524, FANS/4 (Montreal, Quebec,
Intemational Civil Aviation Organization)
4. ---1989. Aviation Satellite Communication
System: Part 2, Equipment Design and
Equipment Functional Description. ARINC
Characteristic 741 (Annapolis, Maryland,
Aeronautical Radio Inc.)
5. Gronemeyer, S.A. 1976. MSK and
O-QPSK Communications IEEE Transactions
on Communications Vol. COM-24, pages
809-830
. Fang, R.J.F. 1981. Quartemary
Transmission Channels over Satellite
Channels with Cascaded Nonlinear Elements
and Adjacent Channel Interference IEEE
Transactions on Communications Vol.
COM-29, pages 567-581
7. Winters, J.H. 1984. Differential Detection
with Intersystem Interference and Frequency
Uncertainty. IEEE Transactions on
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33
8. Crowe, D.P. 1988. Selection of Voice
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(Ipswich, British Telecom Research Labs.)
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