Large active retrodirective arrays for space applications

An active retrodirective array (ARA) electronically points a microwave beam back at the apparent source of an incident pilot signal. Retrodirectivity is the result of the phase conjugation of the pilot signal received by each element of the array. The problem of supplying the correct phase reference to the phase conjugation circuit (PCC) associated with each element of the array is solved by "central phasing." By eliminating the need for structural rigidity central phasing confers a decisive advantage on ARA's as large spaceborne antennas. A new form of central phasing suitable for very large arrays is described. ARA's may easily be modified to serve both as transmitting and receiving arrays simultaneously. ARA's are particularly suitable as solar power satellite antennas because they are inherently failsafe. Communication satellites and deep space probes are other suggested applications. A new kind of exact frequency-translating PCC is described. Such PCC's provide the ARA with input-output isolation and freedom from squint. The pointing errors caused by the radial and transverse components of the ARA's velocity, by the propagation medium, and by multipath are discussed. As part of a NASA funded program a two-element ARA breadboard has been built and tested at the Jet Propulsion Lab. Its performance is limited primarily by multipath-induced errors.