mission, and this factor may limit the size of the ARA which can be used on such spacecraft. Deep Space Ground Receivers We noted above that an ARA can function as a receiving array. Such arrays may be useful for receiving weak signals from very distant deep space probes or as radio astronomy arrays. Since low noise front ends are fairly expensive, such an array would probably consist of a modest number of fairly large antennas rather than a very large number of small elements. Each of the large elements would be mechanically steered to keep the source within its beamwidth. As in communication satellite ARA’s, data processing may be required to remove time delay distortion. II. ARA DESIGN PROBLEMS AND SOLUTIONS A. Phase Reference Distribution From (1) and (2) we see that phase conjugation amounts to advancing the phase of an input signal by an amount equal to its delay. The phase conjugation circuit (PCC) must, therefore, be provided with a phase reference against which to measure that delay. If we locate each PCC at its associated ARA element as in Figure 1, then it is clear that we must transmit the phase reference to each PCC from some central source via transmission lines of equal phase delay modulo 2tt. But it may be difficult to do this if the transmission lines are very long. For example, consider the 1.0 km diameter SPS ARA described above operating at S-band (X = 12.5 cm). If the master phase reference is located at the center of the disk, the transmission lines to elements at the periphery will be 500 m long. If we wish to keep the phase delay in this line constant to within tt/10 radians, its length must not vary by more than [] cm, or a relative change no greater than
RkJQdWJsaXNoZXIy MTU5NjU0Mg==