SATELLITE POWER SYSTEM EFFECTS ON VLBI B. F. Burke EFFECTS OF SPURIOUS NOISE EMISSIONS Very-Long Baseline Interferometry (VLBI) systems cross-correlate signals from two or more separately situated telescopes. The average cross-correlation is affected by noise of the receivers and by the presence of a coherent signal. Celestial sources move through the fringe system at a sidereal rate, and by determining the best value of the average amplitude and phase of the sidereal fringe rate signal, one obtains the desired data. In general, we find that VLBI is not as sensitive to interference, but since it involves collaboration of many expensive telescopes, the harmful effects of such interference have a major financial impact. The satellite power systems are not moving at a sidereal rate, so spurious noise signals would not mimic celestial sources except at predictable times. Furthermore, the coherence length of the received signals is the reciprocal of the bandwidth, which means that typical continuous measurements with megahertz bandwidths have coherence lengths on the order of hundreds of meters. The only coherent contribution, then, can come from aliasing of out-of-band signals, which are usually reduced by 80 dB or so. On both counts, therefore, the direct interference effects are minimal. However, spectral lines are narrower, with OH lines being only 1-kHz wide, so coherence lengths are on the order of hundreds of kilometers, and somewhat more careful consideration is needed. The direct effects of the spurious noise on the process of taking data must also be considered. A 10% increase in noise temperature in the receiver would be undesirable. Noise temperatures of the order of 10 K can be expected in the future, so a 1-K noise contribution would constitute harmful interference. The received noise power, P, with standard antennas, would be on the order of
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