INMARSAT decided to expand the spectrum covered by its new generation of satellites, INMARSAT-2, to include 1 MHz (subsequently increased to 3 MHz) of the spectrum designed for aeronautical use. It began a design study that led to the specifications for the system that is now being implemented. Subsequently, INMARSAT awarded contracts for the design of avionics and high gain antennas to a number of manufactures, while several of the signatories that provide ground equipment for communicating with the INMARSAT satellites are modifying their earth stations to work with the avionic equipment. As a resullt of these activities, a world-wide aeronautical satellite system supporting both voice and data will become operational in 1989

Smith, Keith

Wood, Peter

English

NASA Technical Reports Server

N88-25688
WORLD-WIDE AERONAUTICAL SATELLITE COMMUNICATIONS
Peter Wood, Manager-Aeronautical Department, INMARSAT
Keith Smith, Project Leader-Aeronautical Department, INMARSAT
INMARSAT
40 Melton Street
London, NWl 2EQ
EngLand.
ABSTRACT
Faced with the resurgence of interest in aeronautical satellite
communications, and with the encouragement of aviation organizations and its
Signatories, INMARSAT decided to expand the spectrum covered by its new
generation of satellites, INMARSAT-2, to include 1 MHz (subsequently
increased to 3 MHz) of the spectrum designated for aeronautical use. It began
a design study which led to the specifications for the system that is now being
implemented. Subsequently INMARSAT has awarded contracts for the design
of avionics and high gain antennas to a number of manufacturers, while several
of the Signatories that provide ground equipment for communicating with the
INMARSAT satellites are modilying their earth stations to work with the avionic
equipment. As a result of these activities, a world-wide aeronautical satellite
system supporting both voice and data will become operational in 1989.
SERVICE REQUIREMENTS.
There are four generic applications that will benefit from the use of satellite communications:
• air traffic services
communications for air traffic control purposes, including automatically relaying the position
of an aircraft as determined by the on-board navigation equipment to the air traffic control
center (automatic dependent surveillance)
• airline operational control
communications which involve the regularity and efficiency of flights, such as between an
aircraft and its operating aidine giving its estimated time of arrival.
• airline administrative communications
non-safety related communications which affect the administration of an airline. An example
would be passenger connecting flightarrangements.
• passenger correspondence
communications to and from a passenger in flight. These would normally relate to telephone
services, but could include telex and facsimile.
To meet the needs of these services, three basic forms of communication are required:
• a low data rate service (600 bit/sec). This has been defined by ICAO's FANS committee
as the "core" system.
• a medium data rate service (up to 12.6 kb/s), and
• a voice service, with a quality equivalent to that of a good telephone service ("toll"quality).
At a later date, other services (such as a lower quality voice channel) may be necessary.
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https://ntrs.nasa.gov/search.jsp?R=19880016304 2020-03-20T06:38:53+00:00Z
SYSTEM ELEMENTS
The major elements of the aeronautical satellite system comprise:
(a) the Space Segment, in particular the satellite communications transponders, and the associ-
ated frequency bands assigned for use by the aeronautical satellite system. Figure 1 shows
the provisions of INMARSAT-2 satellites compared with the outcome of Mobile-WARC-87;
(b) Aircraft Earth Stations (AES) which interface in the aircraft with ACARS/AIRCOM and other
data equipment, and with crew and passenger voice equipment, and with the space segment
at L-band;
(c) Aeronautical Ground Earth Stations (GES)* which interface with the space segment at C-
and L-bands, and with terrestrial voice and data networks;
(d) Network Coordination Stations (NCS), which interface with the GESs for the purpose of al-
locating satellite capacity.
INMARSAT's present space segment comprises nine satellites, three in each ocean region.
One satellite in each region is classified as the "operational" satellite, the other two as spares. The
three operational satellites provide effectively global coverage and INMARSAT reconfigures the
satellite disposition from time to time to match traffic demand. These first generation satellites will
be augmented from late 1989 by at least 3 2nd generation satellites (INMARSAT-2) which like the
first generation, each offer "global beam" coverage. Figure 2 shows the coverage at 0 and 5 degree
elevation angles. Plans are well under way for the introduction of spot-beam INMARSAT satellites
in the 1991/93 timeframe.
The major L-band parameters of the first and second generation INMARSAT satellites are (at
edge of coverage):
Satellite/ EIRP G/T
Package (dBW) (dB/K)
't
Marecs 34.5 -11.2 L
MCS 33.0 -13.0 S 1st Generation
Madsat 27.0 -17.0
INMARSAT-2 39.0 -12.5
SYSTEM DESIGN CONSIDERATIONS
The system design uses a hybdd FDMAKDMA/TDM approach. Data and signalling use a
TDM/TDMA packet-transmission technique, while digital voice circuits use single channel per carder
(SCPC) transmission in order to take advantage of the characteristics of speech to minimize satellite
resource demands. The factors that were considered included:
• The need to provide a global capability from the start of service.
• Spectral efficiency
• Power conservation
Many system designs were considered, but the reasons for choice of an SCPC approach to
voice transmission appear from the analysis summarised in Table 1.
Ground Earth stations are generally owned and operated by Signatories --organizations
nominated by their countries to invest in, and work with, INMARSAT. The Signatories are
the organizations actually responsible for the provision of aeronautical satellite
communication services.
58
Table 1 is based upon the SCPC design selected by INMARSAT, and a specific alternative
TDMA approach that has been considered by the aeronautical industry. Both of these have been
considered by the Aidines Electronic Engineering Committee (AEEC). It is apparent that, whether
operating with global or spot beam satellites, orwith high gain or low gain antennas,an SCPC system
offers more capacity.
Contributory factors are:
• the transmitted power from the satellite is proportional to the number of channels being
handled; for a TDMA system the power is constant, irrespective of the number of channels
in use at one time,
• the transmitted power can be adjusted on each channel to deal with variations in path
length, propagation conditions, and variations in antenna gain. This allows reduction of sys-
tem margins and consequent power savings. In a TDMA system, the power has to be at a
constant worst-case level.
• it is possible to use voice activation on voice circuits, using no power until someone is ac-
tually speaking.
• use of SCPC avoids the need to 'bundle' traffic into groups of circuits of preassigned size.
Such bundling not only restricts system flexibility, but incurs significant efficiency penalties
arising from the statistical nature of voice traffic.
Nevertheless, the present choice of SCPC does not preclude the future development of a
more suitable TDMA design following the widespread introduction of spot beam satellites.
SYSTEM CONFIGURATION
The INMARSAT aeronautical system isconfigured around two types of transmission channel,
data and voice, and two R.F. amplitler/antenna combinations on the aircraft, low-gain (0 dBi) and
high gain (12 dBi). Aircraft Earth Stations (AES) are classified according to their equipment
configurations and capabilities, as follows:
Class 1: Low gain antenna only, data services only;
Class 2: High gain antenna only, voiceservices only;
Class 3: High gain antenna only, voiceand data services;
In the initial configuration, transmissions over the data channel will be at either 600 or 1200
bits/second (selected by the ground earth station). As rate 1/2 forward error correction (FEC) coding
is used to reduce the bit error rate, the effective information data rates are 300 and 600 bits/second
respectively.
Voice signals are digitized at a 9.6 kbits/srate, and are combined with signalling data, resulting
in an information data rate of 10.5 kbits/s. Again, FEC encoding is used to reduce the bit error rate,
giving a data transmission rate of 21 kbits/s over the voice channel.
The specific service capabilities of an AES will vary according to its class and the number of
channel units installed. Examples of typical installations would be:
(a) one data channel only. This would supportdata services only, to a single GES;
(b) one data channel, plus two voice channels. This would support data and voice services sim-
ultaneously, with the voice and data able to use different GESs; and
(c) two data channels, plus six voice channels. The data services could be to two different GESs.
The voice services could use several GESs.
SYSTEM DESIGN SUMMARY
Modulation and Coding
Up to an RF transmission rate of 2400 bit/s, binary modulation is used with root 40% raised
cosine filtering, and above that a quadrature technique is used with root 60% raised cosine filtering.
59
Bothare "offset"techniqueswhichminimizeRFamplitudevariationsafterfiltering,andthereby
permitclose spacing of the RF channels. For the quadrature technique, initial channel spacings of
about 0.83 of the bitrate are achieved.
FEC coding is provided, with rates of 1/2, 3/4 and 1 (no coding). Constraint-length seven
convolutional coOingis used, and interleaving is applied to randomize the bursty errors arising from
the multipath L-band channel, which has a fade rate of about 30 Hz and above, with C/M of 10 dB
and above. A common intedeaver structure is applied at all bit rates, although the block size is vaded
to maintain a constant depth (randomizing power) to match the bit rate. When FEC-coded channels
are used for voice transmission, the maximum interleaving delay is kept to between 30 and 40 mS,
which slightly reduces the effectiveness of interleaving. It is also possible to operate without FEC.
Transmission Format (Data Link Layer)
Given the nature of the multipath fading channel, it is vital to design the transmission format
to ensure the maximum resistance to burst errors. This implies a need for consistent link-level error
detection performance, and the availability of error recovery procedures.
This has been handled at link layer by formatting all data, whether signalling, control, or
user-data, into standard packets of pre-defined length, each protected by a common CCI]-I- cyclic
redundancy (CRC) code to ensure consistent packet-error detection. The format and length of these
"Signal Units" (SUs) has been the subject of considerable study, representing a trade-off between
efficiency (link-layer overheads, compatibility with signals and other message lengths) and error
performance in the RF channel. The system user may request that packets are delivered to the
application without error checking (Direct Unk Service) or after ARQ (Reliable Unk Service). The
standard SU is 96 bits, of which t 6 are Cyclic Redundancy Check (CRC) for error detection. Where
included, aircraft addresses are 3 octets in length, and ground station addresses are one octet.
Data Applications (Network Layer)
The system provides full support for a network layer consistent with the ISO layered
architecture for connectionless, connection-oriented, and special (e.g., polling) services.
Transmission Channel Characteristics
To accommodate the range of services, four basic RF transmission channels have been
defined with different satellite access techniques. The four channel types are:
P-Channel:
Packet mode Time Division Multiplex (TDM) channel, used in the forward direction
(ground-to-air) to carry signalling and user data. The transmission is continuous from each earth
station in the satellite network.
R-Channel:
Random access (slotted Aloha) channel, used in the return direction (aircraft-to-ground) to
carry some signallingand user data (shod messages), specifically the initial signals of a transaction,
typically request signals.
T-Channel:
Reservation Time Division Multiple Access (TDMA) channel, used in the return direction
only. The receiving earth station reserves time slots for transmissions requested by aircraft earth
stations, according to message length. The sending aircraft earth station transmits the messages
in the reserved time slots according to priority.
C-Channel:
Bidirectional circuit-mode channel. The use of the channel is controlled by assignment
and release signalling at the start and end of each call. A C-channel has a BER after FEC decoding
of better than 10.3 (whereas the P, R and T channels have better than 10 -s) which is suitable for
voice communications. Each C-channel may carry one or more voice channels, but to support more
than two voice channels, more than one RF carder is needed. A C-channel also carries a multiplexed
low-rate data channel (about 200 bit/s) for signalling or user data.
60
TABLE 1
TDMAJFDMA COMPARISON: NUMBER OF AIR TO GROUND VOICE
CHANNELS PER AIRCRAFT AS A FUNCTION OF HPA POWER
AIRCRAFT ANTENNA GAIN (SAT. ELEVN.)
SPACECRAFT ANTENNA GAIN (EOC)
HPA POWER*
40 W {AEEC)
30W
20W
12W
6W
3W
• Linear HPA neeOed for" FDMA, examt nonlinear
(Class-C) may be used for s_ngle-carr_ cases
12deiC (>20*)
"G_ (17d_C)
FDMA
4
12(:181C (> 5')
sp_ (25_Bc)
"rDMA FDMA
0 14
0 10
0 7
0 4
0 2
0 1
0dmC (>2O*)
] Spo_(2sdam)
TDMA FDMA TDMA
6 2 0
6 1 0
6"* 1 0
0 0 0
0 0 0
0 0 0
*" Msq_w
TDMA/FDMA COMPARISON: NUMBER OF GROUND TO AIR VOICE
CHANNELS PER SATELLITE SPOT BEAM
Aircraft Antenna Gain 12 dB 0 dlB
Number of vo_(::echannels Number of voice channelsSatellite Beam
EIRP (dBW, eoc)
36
39""
42
• Requires 2 transponders
"* AvSat nominal (approx.)
System Designs:
FOMA rDMA
30
30+30"
FDMA'*" TDMA
4 0
g 0
20 0
"" Reduced voice acthmtion advantage
100
200
400
TOMA: as per ARINC Quick Check 39 (AvSat System Technical Dcmcrl;_n - 8.0 kblt/s voice)
FDMA: as per INMARSAT System Definition Manual - 9.6 kblt/s voice
eoc: edge of coverage = 5* elevation
61
FIGURE 1 - L-BAND FREQUENCY ALLOCATION
_T
GB',IBqAllON
I
1626.5
INMARSAT 2r,d I
SATE[LrrlF TO FARTH ALt r_J_T1t__,
I I It
1530 _1533 1544 1545
L I II
' I_//_ ',,
I I I I IS,6
TOSAT_UrrF ,_ rv_'nr__ I I,
I I
1116,.61631 It /634.$ 1636.5 1643.8 1646.3 ,
/I t_ t_.! ',t.,.I ]
1631.5 1636 1144.3 1646.$
16SS ISSt
I I
I I
I I
I I
,-,-_- ,l_//A
1666.6 161_,6
(_) 81wW ¢opmwy _1_ LM_
(_)81WId (Oene_ merle emile)
FIGURE 2 - INMARSAT GLOBAL COVERAGE
¢=z,c= Satellite locations:
AOR 26"W, fOR 63"E, POR 180"E
AERONAUTICAL GROUND EARTH STATIONS PLANNED •
MARITIME COAST EARTH STATIONS A
62


World-wide aeronautical satellite communications

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