conjugation circuits, and each circuit responds only to the carrier frequency of the pilot signal transmitted by one of the n stations. Information modulated on the carrier of the pilot signal from one of the stations, call it Station A, is demodulated and remodulated onto the carriers of downlink signals retrodirected to one or more of the other ground stations. Similarly, information from any of these other stations can be simultaneously modulated onto the downlink to A. The average power available for each downlink must, of course, decrease with n, but the full gain of the array is available to each downlink independently of n. If time division instead of frequency multiplex is used, only one phase conjugation circuit and one receiver is required for each element. However, the bandwidth of that receiver must be n times greater than that of each of the n receivers required in the frequency multiplex case. Depending on the dimensions of the array, the required bandwidth, and the scan angles required to point beams at different ground stations, time delay compensation may be required in order to properly synchronize the data streams transmitted by the various elements [7] Deep Space Probes ARA’s may also be useful as deep space probe antennas. As the (distance) x (data rate) product increases, it will eventually become necessary to use spacecraft apertures too large to be mechanically pointed. Here, however, certain errors proportional to the velocity of the spacecraft relative to the ground station may become important. These errors are discussed in detail in Sect. II below. They are unimportant in geosynchronous satellite, such as the SPS and most communication satellites, because of their small relative velocities, but deep space probes may experience much higher velocities in the course of a
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