under observation. The receiving system is set up to compensate for phase changes resulting from the expected motion, and the output from the unwanted signal becomes a quasi-sinusoidal waveform which is reduced in amplitude by time-averaging in the data processing. This effect will be referred to as fringe-frequency averaging. Second, the variable delays within the receiving system are adjusted to compensate for the difference in space transmission paths for radiation from the desired direction, but the time delays for interfering signals are generally unequal. If the interference is broadband, it is therefore partially decorrelated. Calculations have been made to estimate the quantitative effects of the fringe-frequency averaging and the decorrelation. The fringe-frequency averaging depends on the size of the synthesized field of view, and hence on the configuration and observing frequency. The most compact configuration, for which the interference reduction is least effective, will be used in considering the tolerable interference levels. For this case the reduction is approximately 10 dB for the 18 to 21 cm band and 17 dB for 1.3 cm. These figures refer to declinations south of +70°, and the fring-frequency averaging becomes less effective toward the pole. The decorrelation effect depends mainly on the antenna configuration and the bandwidth, and again the most compact configuration will be considered. The interference reduction is approximately 17 dB for a 25-MHz bandwidth and 17 dB for a 50-MHz bandwidth. These figures apply to a source of interference at the declination of the satellite, -5.5°, and for an observing declination north of +20°. As the observing declination approaches that of the satellite, the interference reduction becomes much less effective. Harmful interference levels, which are regarded as the general criteria for radio astronomy observations, are given in CCIR Report 224-4. These levels are derived by considering observations using a single large antenna, for which the fringe-frequency averaging and decorrelation effects do not apply. Thus, under certain circumstances, the array should be able 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°
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