LARGE ACTIVE RETRODIRECTIVE ARRAYS FOR SPACE APPLICATIONS I. INTRODUCTION A. Definition and Principles An active retrodirective array (ARA) transmits a beam towards the apparent source of an illuminating signal called the pilot. ’'Active” implies that the array produces, not merely reflects, RF power. Retrodirectivity is achieved by retransmitting from each element of the array a signal whose phase is the "conjugate” of that received by the element. Assuming that the phase of the pilot signal of angular frequency co received by the kth element of the array at time t is where [] is the distance from the kth element to the source of the pilot signal, we define the conjugate of [] to be where [] is the angular frequency of the conjugate signal, and [] is an arbitrary phase offset which must, however, be constant over the entire array. In order to do this, each element of the array must be equipped with a phase conjugation circuit (PCC) as shown in Figure 1. The phase of the signal received from the kth element by a receiver located at the pilot source (r = o) is, at time t, Thus the contributions to the field at r = o from the various elements of the array are all in phase at that point, which means that the beam (or a major lobe if the array is too thin a form a single beam) points toward the pilot source.
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