Satellite communication for mobile stations

An important advantage of satellite communication over all other media for long distance communication is its ability to operate with mobile terminals. Indeed, satellites provide the only feasible way of providing reliable communication with ships and aircraft which are far out of sight of land. The microwave frequencies which are used for communication between satellites and fixed earth stations are, however, unsuitable for use with mobile stations. Lower frequencies provide a better trade off between the satellite carrier power re­quired and the cost of mobile earth station antennas. As shown in Table 51.2, frequency bands have been allocated for satellite mobile services around 1.6GHz and, in some parts of the world, at 800MHz. However, the 1.6GHz bands are narrow and the 800MHz bands are crowded by terrestrial services.

The first commercial satellite communication service for ships was provided in 1976 by the MARISAT satellites, owned by Comsat Corporation (Lipke et al., 1987). These satellites covered most significant sea areas, although with some gaps in arctic and antarctic waters. In 1982 the International Maritime Satellite Organisation, a global consortium similar to INTELSAT, took over and expanded this service. The INMARSAT system initially used the MARISAT satellites leased from COMSAT, some transponders which had been added for the purpose to three INTELSAT V satellites and two MARECS satellites on loan from the European Space Agency. All of these satellites operated their ship satellite links at 1.6GHz and their feeder links with earth stations ashore at 6GHz and 4GHz. Their transponders had bandwidths of a few Megahertz and the saturated output power of their transmitters was rated at a few tens of watts. All satellite antennas had footprints which covered the whole earth visible from the satellite.

The standard INMARSAT ship earth station antenna in the receive mode has a Figure of Merit (G/T) of-4dB/K, typically achieved by an antenna gain of 23dBi from a 1.2 metre dish and a receiver with a system noise temperature of 500K. Such an antenna has a -3dB beamwidth of about 10° and a simple beam stabilising and control system is sufficient to compensate for the movement of the ship and slow changes in the direction of the satellite as the ship changes its location. Single channels of narrow bandwidth analogue telephony signals frequency modulate voice switched carriers, and the trans­ponder power per channel requirement is further reduced by com­panding. A low speed TDMA system provides many channels of telex, for use with all equipped ships. A second TDMA system as used for signalling and order wire purposes to control both tele­phone and telex facilities.

A second generation of INMARSAT satellites is in the process of deployment, replacing the various satellites that brought the system into being (Berlin, 1986). These satellites operate in the same frequency bands as did their predecessors but the transponder band­width has been increased to 16MHz and the available down-link power is also considerably increased.

With the additional capacity that the second generation of satel­lites will bring to the INMARSAT system, it will be possible to provide for growth of maritime use but it also becomes possible to consider opening up new services. In 1987 the INMARSAT Con­vention was amended to permit the organisation to offer service to aircraft and a start has been made in offering connections to the public telephone network to passengers on some aircraft on trans-Atlantic flights. INMARSAT is also offering telex service to road vehicles.

Various other satellite systems are also being set up to provide communication services to mobile stations and in particular to road vehicles. Some of these will operate in the same frequency bands as INMARSAT. Others, and particularly some that are emerging in North America, will use the frequency allocations at 800MHz to 960 MHz.

51.6.4 Satellite broadcasting

Subject to various constraints to protect terrestrial radio services, and in particular terrestrial television broadcasting, which make extensive use of the same band and has superior allocation status, paragraph 693 of the ITU Radio Regulations permits satellite broad­casting to be done in the band 620GHz to 790GHz. The USSR has been using EKRAN satellites to broadcast FM TV in this band for a number of years, providing signals that can be received over a wide area. However, there seems to be little prospect that satellite broadcasting will expand in this band.

The main frequency bands foreseen for satellite broadcasting are at 12GHz. Frequency assignment plans for broadcasting down-links in these bands, and the corresponding feeder links in other bands at 18GHz and 15GHz, were agreed at various administrative con­ferences of the ITU between 1977 and 1988. These plans define the ways in which the band is to be used in some considerable detail, but in essence:

1. With few exceptions, down-link footprints may not exceed national boundaries;

2. With allowances made for the climate of the country con­ cerned, the power flux density at the ground at the edge of the coverage area should be -103dB(W/m) for ITU Regions 1 and
3; -107dB(W/m²) for Region 2.

3. Every country has been assigned a share of the channels that the plan can provide. In Region 1 the shares are equal, every country being assigned 5 channels, except where the country is so large that provision must be made for extra channels to enable each time zone to have its own programmes.

These plans have been ratified by national governments, provid­ing strong protection against cross frontier interference. Several countries had experimented with satellite broadcasting at 12 GHz before the plans were finalised (Siocos, 1978; Roscoe, 1980; Ishida et al., 1979). Others have launched satellites more recently to make use of the planned assignments, including TV-SAT, TDF-1 and Marco Polo for France, Germany and the United Kingdom, respec­tively. However, in general the take up of these planned assignments has been slow. The specified power flux density requires satellite transmitter output powers of several hundreds of watts for the larger countries, which causes capital costs to be high and it is arguable that recent advances in the design of low cost satellite broadcasting receivers has made such a high power flux density unnecessary. Furthermore, the limiting of coverage to national boundaries is no longer attractive in, for example, Western Europe. The general public is tending to get its satellite broadcasting signals from other sources.

Many satellites of the fixed satellite service, in particular domestic systems of the USA, are used to distribute substantial numbers of TV programme channels to terrestrial radio stations, cable network head ends, hotels and so on. However, members of the public also set up domestic antennas to receive these transmissions for their own enjoyment. This process has spread, for example, to Europe, and account has now been taken of it in the design of satellites. Thus, the ASTRA satellites of SES and the EUTELSAT II series, both transmitting in the 10.7GHz to 11.7GHz band, have relatively large beam footprints for international coverage and are used, for example, to distribute programmes to designated fixed earth station-s for onward distribution to the public. However, the transponders of these satellites, being equipped with 50 watt transmitters, provide a signal strength on the ground which is not so very much less than the objective set for high power satellites in the planned 12GHz bands; in consequence these satellites tend to be seen by the public as broadcasting satellites and large numbers of homes treat them as such.

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