Indium phosphide was discussed, as well as the heteroepitaxial InP approaches such as InP/Ge and InP/Si. The cost is high today, but it was agreed that ultimately the cost of InP/Si or InP/Ge could be made competitive. It was agreed that these cells may have an application in orbits which see high radiation environments. The concentrator approach was discussed. This is not a direct replacement into existing arrays, and may find some resistance from program managers due to pointing requirements. However, they have the potential for high efficiency and good radiation tolerance [7], Finally, it was mentioned that new generation silicon cells with efficiency of 20% have performance as good as GaAs cells. However, the radiation tolerance of these cells is yet to be determined, and they are not yet space qualified. Thermophotovoltaic (TPV) cells were mentioned as a promising new use for photovoltaics, but since there was a separate workshop on these concepts, they were not discussed in detail. Discussion Session In the summary session, Geoffrey Landis took issue with the consensus that future systems will be primarily LEO and GEO. He suggested that the most significant commercial space system in the next decade will be the emplacement of a worldwide communications satellite network for portable telephone systems, with an investment of tens of billions of dollars, and that these satellite systems may be significantly different from currently operated GEO satellites. He presented results from an unpublished study [8] that shows that the number of satellites required to provide global phone coverage can be reduced by a factor of four if an intermediate orbit of 3200 kilometers is chosen instead of the low Earth orbit proposed. A page from these results is shown in figure 1. Andrew Meulenberg agreed with this conclusion. He said that a study done by Comsat on behalf of Inmarsat concluded that Inmarsat could save nearly a billion dollars on their worldwide telephone satellite constellation "Inmarsat-P" due to the reduced number of satellites required if they went to intermediate orbits instead of low orbit. According to articles in Space News [9], the price of the Inmarsat-P system reduces from 3 billion dollars to 2 billion, and the number of satellites is reduced from 54 to 12- 15, if intermediate orbit of 10,300 km is chosen instead of LEO. (This data point is shown to the right of the curve shown in figure 1). A recent study published in Space News indicated that there will be a market for as many as four of these worldwide communications satellites systems, and that these will produce a revenue of $9 billion per year [10], These intermediate orbits see an intense radiation environment. At 3200 km, the radiation dose received from trapped protons in one day is approximately the same as that seen in geosynchronous orbit in a year! This implies that radiation tolerant solar cells may be critical components of future communications satellite networks, and thus could have considerable commercial value. Data presented by Landis showed that InP
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