suggested (3), up to 5 satellites per degree of orbital space, giving a total of 1800. Thus, the question of orbital slot availability seems to have little to do with purely physical limitations. This does not necessarily apply to the immense dimensions of the SPS, however, as discussed below. RADIO FREQUENCY INTERFERENCE It is presently proposed that a 100 MHz bandwidth, centred on 2.45 GHz, will have to be reserved for the SPS system in order to protect other users from interference. In addition, if harmonics prove troublesome, other exclusive bands may have to be designated. The 100 MHz band should protect ground based communication links in adjacent bands— although this may not be justified in the case of communications satellites, as they will be much closer to the SPSs. One estimate (4) has postulated that a separation of 26° is needed between a Comsat and an SPS to avoid gain compression effects at the ground station, and 9° is needed to avoid similar effects at the Comsat. These estimates are subject to vast areas of uncertainty, but it is obvious that the above conditions are totally impractical and that much work is required in this direction. SPS harmonics could cause problems in various ways, and it is estimated that they must be suppressed at least 90 dB below the fundamental to avoid difficulties with most users. More stringent limits than this may be necessary to protect earth stations which receive signals from Comsats. It has been suggested (4) that at least a 3° separation may be needed to enable a Comsat-to-Earth link to be able to operate successfully at harmonics of the SPS fundamental frequency. Once again, uncertainties abound. The overriding message is the importance of the optimisation of the use of the radio spectrum. To this end, a number of design variables need to be investigated. These include — The degree of common frequency usage — The degree to which two satellites may illuminate the same area of the Earth's surface — Earth station antenna size and design — Antenna polarisation — Reversal of frequency assignments — Modulation type and degree — Interference allowances. All of these variables must be considered in all their combinations and variations in examining the concept of discrete orbital slots and the danger of spectrum/orbit scarcity (3). COLLISIONS Calculations carried out for 5 m diameter satellites indicated that 100 inactive satellites of such a size in geostationary orbit would produce less than one collision every 500 years. Thus, for active and passive satellites of this size, the danger of collision is negligible (3). However, a totally different picture emerges if we consider large space structures. A 5 GW SPS, for instance, could have a total area of arond 50 km2.
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