Under certain circumstances the above effects should allow the array to operate with no decrease in performance in the presence of interference levels above those indicated in CCIR Report 224-4. The CCIR levels refer to interference received in the far side lobes of the antennas, for which the gain is equivalent to that of an isotropic radiator. With this condition, and for declinations from +20° to +75°, levels of interference about 20 dB to 30 dB higher than the CCIR levels should be tolerable. However, as the declination of the satellite is approached, this margin falls to about 13 dB. Figure 1 in A. R. Thompson's paper shows the levels corresponding to the VLA bands from CCIR Report 224-4, the maximum sensitivity levels of the VLA and the predicted flux density levels from the satellites, including both thermal and transmitter-generated radiation. For the thermal radiation unit emissivity is assumed. The transmitter noise at the second harmonic is arbitrarily assumed to be 40 dB below that at the fundamental frequency. The 20-30 dB margin above the CCIR levels should allow the array to operate in the presence of the satellites for those declinations for which the required 8 hours of tracking can be achieved without presenting to the satellites an antenna response much greater than isotropic. The resolution of the 0.5 x 1.0 arcminute collector arrays by the spaced antennas will have the effect of further reducing interference from the thermal radiation. This effect, incidentally, enables the array to operate in the daytime in the presence of the quiet sun, which is, of course, much more highly resolved than the satellites would be. Since the line of satellites is essentially constant in declination, at -5.5° as viewed from the VLA site, there will be a zone of declinations centered on -5.5° in which the array cannot operate without receiving interference above a tolerable level. To avoid such interference, the range of hour angle tracking would have to be reduced by something more than the 60° longitude range of the satellites, and satisfactory mapping of celestial objects in this zone would not be possible. SITING CONSIDERATIONS In addition to the specific bands allocated to the Radio Astronomy Service, passive radio astronomy research is conducted at many frequencies
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