Since the flux from the SPS is of the order 10 times that of the natural radio signals from astronomical objects it is necessary to enquire whether the operational conditions of the SPS and of radio telescopes lead to any measure of compatibility. 4. Operational circumstances The feasibility of operating radio telescopes when the 2.45 GHz beams are incident on Earth depends (a) on the polar diagram and the sidelobe level of the solar power beam, (b) on the polar diagram and the sidelobe level of the radio telescope and (c) on the undesired ambient power levels at which the receiver of the telescope can operate. On these points the following information is available. (a) The polar diagram of the 2.45 GHz array The 1 km diameter SPS array on 2.45 GHz will have a beam width of approximately 0.02 deg. The flux in the main beam collected by a 10 km "rec- _ tenna" on Earth will be 200 W m . At the edge of the rectenna the polar diagram of the SPS must be such that the radiation safety standards must be met. These allowable limits at present extend from 100 W m in the -2 USA to 0.1 W m in the USSR. We assume an intermediate value of 1 W m at the edge of the collector and that by skilful design and control of the array elements in space the far-out sidelobes could be reduced to -50dB with respect to the main beam. At a level of 200 W m in the main beam the intensity of the far-out sidelobes would then be of the order 2 x - 10 W m . However, Kassing and Reinhertzv ' quote a figure 50 times _ greater of 0.1 W m as the achievable diminution of flux from the SPS in the far out-zones. In the present calculations these values of 10"^ and 2 x 10 W m are taken as the likely maximum and minimum values of the SPS flux in the far-out zones. (b) The polar diagram of the terrestrial radio telescope We are concerned with the power delivered to the focus of the radio telescope from the 2.45 GHz transmissions from the SPS, notwithstanding the actual frequency on which the telescope is receiving the signals from celestial objects. The relevant characteristics of the radio
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