allocation of the necessary numbers of geostationary orbital slots can be formally approved at an international level. The accepted procedure for space exploration has always been the “first come, first served” principle, with the proviso that the exploration and use must be carried out for the benefits and in the interests of all countries, irrespective of their degree of economic or scientific development (1). It would seem that, using the interpretation that it is internationally beneficial in a general sense, the SPS programme would fulfill this condition. Formal confirmation of this will still have to be obtained, however. It is likely to be argued that the keeping of a GEOSAT in orbit for 30 years — the expected lifetime of an SPS — constitutes national appropriation, contrary to the 1967 Outer Space Treaty. This point will certainly need clarifying. Another potential problem area arises from the fact that there is no formal definition as to where sovereign airspace ends and outer space begins. This led to the 1976 Bogota Declaration, issued by 8 equatorial countries, claiming sovereignty over the segments of the geostationary orbit lying above their territories. This declaration has been rejected by the space resource countries and others, but again a formal ruling is required. The basic motivation behind the declaration was the belief that the space resource countries had monopolised the geostationary orbit. This is certainly a valid point in the light of talk about possible orbital overcrowding, before many developing countries have any satellites of their own. Because of limitations on the availability of the radio spectrum, an international organisation (the ITU) was set up to make allocations of radio frequencies to maximise the spectrum's efficient utilisation. It is still unclear whether the ITU has the power to deal with microwave frequency allocations — such power would automatically give it the right to allocate geostationary orbit positions for the SPS (1). Before any such procedures can be put into effect, however, thorough investigations are needed into the effects of the microwave beam on radio astronomy, shipbome radar, communications systems, and other services. The conflict of interests between present and potential users of the geostationary orbit, whether concerned with physical space, RF interference, shadowing, or some other effect such as collision risks, can only be resolved by international agreement. It remains to be seen whether the UN, the ITU, or a new international entity will be given the principle responsibility for protecting national and international wants and needs for the efficient, economic, and equitable use of the orbit. ORBITAL AVAILABILITY FACTORS The minimum separating distance required between geostationary satellites varies tremendously depending on the criteria used for arriving at an estimate. These criteria include the satellite size (and consequent shadowing and collision characteristics), the degree of tolerated electromagnetic interference, the stability of the orbit, the state of the technology, and many other factors. One estimate (2), has suggested that SPSs and other space objects may be obliged to maintain a separation of 2° from each other while in orbit above the equator — thus restricting the number of available slots to 180. Actually, since the geostationary orbit has considerable depth and width, and many present satellites are only a few metres in diameter, the orbit could physically
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