0191 -9067/81/040347-07$02.00/0 Copyright £ 1981 SUNSAT Energy Council PILOT SIGNALS FOR LARGE ACTIVE RETRODIRECTIVE ARRAYS C. H. CHAN* University of Alabama at Huntsville Huntsville, Alabama 35807, USA Abstract — It has been suggested that for large active retrodirective arrays, as in the solar power system, a two-tone uplink pilot signal with frequencies symmetrically situated around the downlink frequency be used in order to reduce ionospheric biases and to lower the cost since a two-tone receiver is much cheaper than a single-tone phase-locked receiver. Unfortunately, such a system now faces the following well-known difficulties: (i) the rr-ambiguity, (ii) a phase difference between the downlink and uplink signals. We show in this paper how the rr-ambiguity can be easily removed by using a two-tone uplink signal with both frequencies situated at one side of a downlink frequency, and the phase difference can be greatly reduced with a three-tone or a four-tone uplink pilot signal. I. INTRODUCTION It has been suggested (1) that active retrodirective arrays (2,3) would be particularly suitable as solar power satellite antennas (4) because they are inherently failsafe. The active retrodirective array works on the phase-conjugation principle. It electronically points a microwave beam back at the apparent source of an incident pilot signal. Retrodirectivity is achieved by retransmitting from each element of the array a signal whose phase is the conjugate of that received by the element. In the satellite power system, the pilot source on ground may be situated at the center of a large rectenna and the retrodirective array is the space antenna in geosynchronous orbit. Retrodirectivity can be most easily achieved if the uplink signal and the downlink beam have the same frequency. But due to input-output isolation problems, the uplink frequency is either upshifted or downshifted from the downlink frequency, a phase-locked receiver is used to achieve phase conjugation. When the uplink and downlink frequencies are different and because the ionosphere and transmission lines are dispersive, the conjugated uplink phase is no longer exactly equal to the downlink phase and the beam coherence at the rectenna can be lost. The downlink beam then points to a wrong direction and this is known as beam squint. A two-tone pilot uplink signal with frequencies symmetrically situated around the downlink frequency has subsequently been suggested. The two-tone uplink signal circumvents the beam squint problem. It reduces ionospheric biases and biases due to the dispersion of the transmission line. It also lowers the cost since a two-tone receiver is much cheaper than a single-tone phase-locked receiver. But it introduces a new problem, known as the rr-ambiguity. Raytheon (5), Boeing (6), and Rockwell ♦Part of this work was done while the author was a NASA/ASEE Summer Faculty Fellow at MSFC.
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