Both situations must be considered. The important space research band at 2.3 GHz is close to the planned SPS frequency, and since interplanetary space vehicles must be tracked continuously along the ecliptic plane, the harmful interference criteria are stringent. A power flux spectral density of 2 -256 dB W/m -Hz must not be exceeded by the noise radiated by the SPS at this frequency. The radio astronomy bands at 1400-1427 MHz and at 2690-2700 MHz are the next closest bands to the planned SPS frequency, and the harmful flux levels for these bands, of -255 and -247 dB W/m -Hz respectively, must not be exceeded. Presently available data make it clear that these criteria will be difficult to meet; the uncertainties in the available data make it clear that noise measurements for any planned system must be made at an early stage. The strongest harmonic, the second at 4.9 GHz, does not fall into a pres- entlyyallocated band of these services, but does fall well within the principal operating band of the VLA, a major national radio astronomy facility in Socorro, New Mexico, and at the shortest usable wavelengths of the Arecibo telescope, the world's largest radio/radar telescope. These facilities are unique in the world and could be seriously compromised if not protected against the consequences of such interference. The available data are insufficient to conclude that the SPS will not cause such interference, and we recommend that the problem be given a careful engineering study. The SPS is unique in the impressively large quantities of radio frequency radiation that will be generated, especially at the fundamental frequency. Radio observatories have been carefully situated at remote sites to protect against such adjacent channel interference effects from earth-based transmitters, but location cannot protect against such effects from transmitters in synchronous orbits. The field strengths in grating side lobes are of particular concern, and in view of the large number of SPS ground-receiving stations, it is not clear that grating side lobes will miss the active radio observatories, or that observatory operation within a side lobe will be practical. The National Radio Astronomy Observatory at Green Bank, West Virginia is in a National Radio Quiet Zone, as specified in the National Rules (Vol. Ill, § 73.1030), and the effects of the SPS side-lobe levels on the operation of this facility should be investigated carefully. Furthermore, major
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