Calculations indicate that one complete 10 GW SPS in geostationary orbit would suffer approximately 10 collisions from 100 hypothetical inactive satellites above 3 m square during its 30-year operational lifetime (4). It is thought that this quoted figure of 100 inactive satellites in geostationary orbit could well be a realistic estimate in the light of planned missions and predicted launch rates. The current reference system SPS is sized at 5 GW, so the number of collisions would be significantly reduced. However, the reference system also assumes construction in geostationary orbit, which would involve additional collisions during the construction operations. There is a possibility, if the number of predicted collisions becomes too large, or if the collisions are likely to cause significant damage, that the orbit may need to be “cleared” of inactive satellites before implementation of the SPS programme. SHADOWING AND ECLIPSES At certain times of the year, all geostationary spacecraft pass through the Earth's shadow for short periods of time. Fortunately, these eclipses occur at local midnight when power demand is small, so the impact on supplies from an SPS will be at a minimum. Nevertheless, reserve capacity must be available. These eclipses occur during two 45-day periods each year centered around the equinoxes. The maximum time in shadow is 70-72 min, and this decreases rapidly on each side of the equinox. The overall effect, it is estimated, will be an annual reduction of less than 1% of the total power produced by the SPS. Eclipse conditions can also occur if a series of satellites are closely spaced in geostationary orbit. Calculations performed for the 10 GW SPS suggest that, for a 0.5° separation between neighbouring SPSs, mutual shadowing will occur twice per day over a total orbital angle of 12.48°. Each shadow period will last 25 min, with a depth of shadow varying between 0% and 77%. The total loss of power due to shadowing over any orbit is estimated at 1.2%. However, if the variation in the sun's latitude is taken into account, it becomes apparent that successive satellites can only be shadowed for 13 days per year. Consequently, the effect is very small indeed. GLOBAL SLOT AVAILABILITY As mentioned previously, the author has found evidence of 73 active geostationary satellites at the beginning of 1980, together with 103 documented planned missions (5). The functions and positions of these satellites, where known accurately, are summarised in Fig. 1. In order to gain information about the longitudinal distribution, the satellite numbers were related to 12 equal sections of 30° in the geostationary orbit. In addition to this, 170 additional launches were postulated for 1980-91, to bring total launch numbers for this period up to the figure mentioned in the Introduction. These 170 additional launches were longitudinally distributed in three different ways in order to yield a wider data base. The first case allows for an additional distribution
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