which is simply a duplication of the present/planned satellite distribution. The second case assumes that the additional satellites are spread evenly over the entire longitudinal range, and the third assumes an additional distribution which is the inverse of the present/planned satellite distribution — a sort of “filling in the gaps” scenario (6). The total 1991 predicted satellite numbers were then proportionally reduced to allow for satellites reaching the end of their life, and to lead us to the figures of 239 active geostationary satellites mentioned in the Introduction. The predicted number of empty orbital slots in 1991 was calculated for each 30° sector by simply subtracting the number of satellites in that sector from the total number of available slots (100 for 0.3° separation, 60 for 0.5° separation, etc.). Separate calculations were carried out for the three distribution cases, and also for four satellite separation conditions — 0.3°, 0.5°, 1° and 2°. The full results are presented in (6). One factor to be allowed for in the calculations is longitude-sharing of satellites — i.e., the use of the same basic longitudinal position, but with small orbital inclinations or eccentricities. Analysis reveals that 11 of the 73 present GEOSATs share longitude locations, and the surprisingly high total of 92 is reached if planned satellites are included. It is suggested that this latter figure is rather inflated due to the fact that many of the future satellites will simply be replacements for present versions, and will thus have the same position. In addition, many of these future satellites are several years distant, and some may well have their positions altered when consideration of other satellites in their vicinity (particularly foreign) is taken into account. A compromise figure of 25% was suggested for the proportion of satellites sharing longitude locations in 1991. This would reduce the total number of filled slots by 12*/2%, and the number of available slots for SPS would thus show a corresponding increase. This assumes a maximum of two satellites per orbital slot, which is not always the case, as up to five have been planned for some slots. Longitude-sharing would not be practical for SPSs, due to their complex nature and vast sizes. Complications are caused by increased collision possibilities, twice-daily eclipsing, and the need to continually readjust the microwave beam to allow for orbital eccentricities. In addition, the microwave beam would need to be turned off twice a day, when the sharing satellite appears in the firing line. Adding up the empty slot numbers for each of the 12 longitudinal sectors, and assuming a 25% longitude-sharing figure, the following figures were arrived at for the total numbers of global orbital slots potentially available for the SPS (6): 988-995 slots for 0.3° spacing 508-515 slots for 0.5° spacing 148-155 slots for 1° spacing (—32)—(—25) slots for 2° spacing. The negative numbers indicate that it is predicted that there will be more satellites than available slots for the 2° spacing condition. In these and all calculations, the geostationary satellite numbers have been assumed to stay at the predicted 1991 level for the duration of the SPS programme. Further orbital demands after this date have thus been assumed to be met by increased satellite capabilities, rather than increased numbers. U.S.A. SLOT AVAILABILITY It is predicted (7), that SPSs to serve the U.S.A, will need to be placed over
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