with the efficiency dependent only on the conversion factors for the solar collector and transmitter in space and the collector on Earth. Substantial encouragement has now been given for the initiation of R & D on the systemv ' and in this paper we draw attention to the effects which an operational system might have on important areas of astronomical research. 2. The present proposals for a Solar Power Satellite System (SPS) In a 1975 presentation to NASA of advanced space concepts for the epoch (3) 1980-2000 AD, Bekeyv ' outlined parameters for a system around which most of the subsequent discussion has centred. The scheme invisages a number of satellites in synchronous equatorial orbits. Each satellite would have solar collecting arrays with dimensions of 13.5 x 4.8 km feeding a 10 GW transmitter beamed to Earth on a frequency of 2.45 GHz through a 1 km aerial array. The beam would be directed to an antenna on Earth covering 10 km x 10 km and it is computed that each system would ultimately deliver 5 to 10 GW into the terrestrial distribution system. Each satellite would weigh over 11 million kg and require 20 billion photocells, and at the turn of the century it has been estimated that 8 satellites of this type could supply the UK electrical needs and that 100 could provide one-third of the U.S. electricity requirements. The power density at Earth in the 10 GW, 2.45 GHz beam transmitted from each -2 satellite is computed to be 200 W m and we first compare this flux with that received from celestial sources by radio telescopes. 3. Interference with radio telescopes Terrestrial radio telescopes normally operate over the radio wavebands from a few centimetres to several metres. Research in the millimetre waveband generally demands special high altitude sites and at the long wave end the ionosphere becomes the limiting factor. By normal standards the signals received from the galaxy and extragalactic objects are extremely weak. The achievable sensitivity varies over the waveband but as a guide for present purposes it may be assumed that most radio astronomical systems today work at sensitivity limits of a few millijansky (1 mJy =10 W m Hz ). Internationally regulated bands for terrestrial transmitters screen a few frequencies for use in radio astronomy within bandwidths of the order 4 MHz. Thus the incident flux on the antennae at limiting sensitivity is of the order
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