Trunk telecommunications

The International Telecommunication Satellite Organisation (IN­TELSAT) is controlled in matters of policy by the many govern­ments that are parties to the agreement that set it up and it is owned by the telecommunications undertakings that those governments have designated for the purpose. The INTELSAT system exhibits well the characteristics of a trunk telecommunications satellite system. The satellites, all geostationary, are located in three groups, for the Atlantic, Pacific and Indian Ocean regions. Some of the transponders are leased for domestic networks but the larger part of the system provides facilities for an international network linking earth stations owned by national telecommunications operating organisations. It is characteristic of this global network that the total required capacity is very large; that large numbers of national earth stations need access to it; that most of those earth stations need to be able to communicate directly with many of the other earth stations having access and that these country to country links vary greatly in scale, some requiring only one circuit but others requiring thousands of circuits (Hall and Moss, 1978; Alper and Pelton, 1984).

It would be ideal for INTELSAT users if a single satellite in each ocean region had enough fully interconnected capacity to carry all of the traffic that the region requires. Successive series of INTEL­SAT satellites have had larger capacities, satellite antenna beams have been kept wide to maximise connectivity and the required figure of merit (G/T) of earth station antennas has been kept high to maximise per-satellite capacity. However, demand per region has outpaced capacity per satellite consistently since 1967. There are currently 13 operational satellites, of the INTELSAT V, VAand VI series, seven of them serving the Atlantic Ocean alone. In each ocean region, one satellite is designated as the Primary Satellite and every country is allocated some capacity in it, so that earth stations with a relatively light total traffic flow can get access to all the routes of interest to them within their own ocean region without using more than one antenna. If the capacity required between two earth stations is large, it will usually be carried by one of the other satellites serving the region, called a Major Path Satellite.

All current INTELSAT satellite series have transponders opera­ting in both the 6GHz and 4GHz and the 14GHz and ll-12GHz bands. High gain spot beam dish antennas, steerable from the ground, are used for the transponders operating in the higher fre­quency band pair. For the lower band pair, some transponders use dual polar horn antennas covering the whole Earth visible from the satellite whilst other transponders use higher gain reconfigurable dual polar multi feed reflector antennas to cover smaller areas, the footprints being shaped to fit the geographical configuration of important service areas (see Figure 51.10). Both travelling wave tube and solid state power amplifiers are used with single carrier saturated output power ratings up to 20 watts, and transponder bandwidths ranging between 36MHz and 112MHz.

Earth station antennas having access to INTELSAT satellites for the global network are required to have high performance, to ensure that bandwidth and satellite power can be used efficiently. Until recently the standard receiving Figure of Merit (GAT) was 40.7dB/K at 4GHz and 39dB/K at UGHz, requiring antennas with primary reflectors about 32 metres and 18 metres in diameter respectively. Many antennas now in service meet these requirements, but the minimum G/T requirements for new all purpose antenna has been reduced to 37dB/K (lGHz) and 35dB/K (UGHz). Much lower performance is acceptable for some limited applications (INTEL­SAT, 1977; Thompson and Buchsbaum, 1985).

The high gain antennas must use active satellite tracking tech­niques to avoid loss of performance due to satellite orbital perturba­tions; typically earth station beam pointing errors are detected by observing the effect on the level of the telemetry beacon radiated by the satellite of small deliberate movements of the beam.

Both time division and frequency division multiple access methods are used in the INTELSAT global network for the trans­mission of narrow band channels which are designed primarily for telephony but which are, of course, also used for other narrow band telecommunications services. It is usual for the earth station to combine the telephone channels it transmits to many destinations onto one or a few multiplexed carriers; conversely, a typical earth station receives multiplex carriers from many other earth stations, extracting from the multiplexes only those channels which are its concern.

The INTELSAT TDMA systems operate at an information rate of 120.832Mb/s, using 4-phase PSK modulation and coherent de­modulation, with transponders of 72MHz bandwidth ((INTELSAT, 1972). Digital speech interpolation (DSI) is used to increase the capacity of the system for telephone traffic (Campanella, 1978). Satellite switched TDMA is available with INTELSAT VI satellites to enable up-beam or down-beam connectivity to be optimised.

However, most INTELSAT transponders are operated in the FDMA mode, with various modulation method, analogue and digi­tal. Multiplexed analogue systems use FM and FDM, with stand­ardised basebands of various capacities, the structure of the baseband being related to the CCIR and CCITT standards for analogue radio relay system basebands. The deviation and power level of the carriers are optimised to give channels which attain their performance objectives with economical use of transponder power and modulator allotments of bandwidth. Most digital systems use TDM and 4-phase PSK, with channels 8-bit encoded, although single channel PSK systems are also used.

Another important function of the INTELSAT global network is to provide temporary, brief wide band links for the transmission of news, in the form of television pictures, from the scene of origin to all parts of the world, for re-broadcasting. A transponder is reserved for this purpose in each Primary Satellite and frequency modulation by an analogue video signal, which may contain a digitalised sound signal, time division multiplexed amongst the synchronising pulses, is usual.

Wide band low noise first stage amplifiers, typically parametric amplifiers, are used to compliment the high receiving gain of the big earth station antennas, and these amplifiers are usually thermo-electrically cooled (Peltier effect) at 4GHz. High power is often required from the output stages of earth station transmitters, involving hun­dreds of watts output and sometimes several kilowatts, more espe­cially when the station will be transmitting television signals to a global coverage transponder or where large numbers of telephone channels are to be transmitted on several carriers. These high power amplifier stages may consist of wide band travelling wave tubes, or groups of klystrons operating in parallel through frequency selec­tive combining networks. In other respects the equipment of an earth station accessing the INTELSAT global network is similar, in principle, to that of a major radio relay station.

The INTELSAT system is by far the biggest provider of satellite facilities for trunk telecommunications but the EUTELSAT system provides digital international links with Europe, closely resembling the TDMA system operated by INTELSAT and using very similar satellite and earth station equipment (EUTELSAT, 1981). Similar configurations also arise in some of the North American domestic satellite networks. Recent satellite designs, like EUTELSAT II, tend to have transponders with higher output power, and a single carrier saturation rating of 50 wafts if often found now.

For systems which are used in this way, the gain of transponders and such earth station emission parameters as carrier power and (for FM) carrier deviation are typically determined so that the CCIR and CCITT channel noise and bit error ratio (BER) objectives for international telephone, data and other channels will be met (CCIR, 1980a; CCIR, 1980b; CCIR, 1980c). Foreseeable levels of inter­ference from other satellite networks are allowed for, and a margin is left for interference from terrestrial radio systems operating in the same frequency bands. The more important of these criteria are as follows:

1. For analogue telephone channels, the total noise, including interference reckoned as noise, psophometrically weighted, at a point of zero relative level in the telephone network, shall not
exceed l0000pW for more than 20% of the month. Nor shall this noise exceed 50000pW for more than 0.3% of any month.

2. Within(l), the margins allowedfor interference from terrestrial radio systems are l000pW for 20% of any month and 50000pW for 0.03% of any month (CCIR, 1980d).

3. For 64kbit/s digital channels which are to form part of an Integrated Services Digital Network (ISDN), the BER should not exceed one in 10⁷ for more than 10% of any month. For 2% and 0.03% of any month, the BER should not exceed one in 10⁶ and one in Mr respectively.

4. For a video signal the objective is for a weighted signal to noise ratio of not worse than 53dB, for 99% of any month, measured in a baseband 0.01MHz to 5.0MHz.

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