The Boeing Company

Reynold E Wilbert

Charles R Pond

Patrick D Texeira

C Ray Pond

An electromagnetic (e.g., laser) beam acquisition and tracking system that is particularly useful onboard high-speed aircraft (10) to acquire and track a target (12) supporting a retroreflector (13) is disclosed. The acquisition and tracking system includes two deflection systems--a mechanical deflection system (19) and an acousto-optic deflection system (17)--for controlling the direction of the electromagnetic beam (14). The two deflection systems are serial, i.e., first one system deflects the beam and then the other system deflects the beam. The deflection systems are controlled in different manners, depending upon the mode of operation of the acquisition and tracking system, which comprise a search mode, a spiral search mode and a tracking mode. In the search mode of operation the mechanical deflection system (19), which includes one or more servo-driven scanning mirrors (54 and 56), relatively slowly sweeps the electromagnetic beam in either a line scan (18) or an area scan (16) search pattern. As the beam is swept, the acousto-optic deflection system (17) relatively rapidly moves the beam in a predetermined pattern over a relatively small field of view. When a target is acquired during the search mode, the acquisition and tracking system passes through the spiral mode of operation and enters the track mode of operation. In the track mode of operation, the acousto-optic deflection system (17) dithers the beam (14) around its central axis. If the target moves from a position lying along the central axis, the scanning mirrors (54 and 56) are repositioned such that the beam impinges on the target (12) when the beam lies along the central axis of the acousto-optic deflection system (17). In the event the target is lost, the acquisition and tracking system shifts to the spiral mode of operation. In the spiral mode of operation the beam (14) is moved in a spiral pattern by the acousto-optic deflection system (17) until the target is reacquired. If the target is not reacquired, the acquisition and tracking system returns to the search mode of operation.

Electromagnetic beam acquisition and tracking system

A filter and beam combiner is comprised of a small glass plate having a microscopic two sided elliptical mirror on one side thereof. The signal beam is spatially filtered on one side of the mirror and then directed to a target. On the other side of the mirror there is a compensating glass plate through which the reference beam passes, to correct for refraction relative to its passing through the first plate, before it enters the other glass plate and is directed onto said other side. The reference beam is spatially filtered by the mirror for combination with the signal beam, which returns from the target substantially enlarged by diffraction so as to pass around the mirror and through the plate to be combined with the reference beam. The signal beam is passed through acousto-optic modulator (AOM). The reference beam is passed through an 81 MHz AOM. The AOM's produce a doppler shift of the coherent light source frequency as the light is diffracted by a moving sound wave in the modulator. When the two beams are combined, the fringe position is proportional to the phase of the 1 MHz frequency modulation difference frequency. The signal beam and the reference beam are combined as the former passes around the mirror and the latter is reflected thereby, forming a composite beam to form interference patterns, and from which can be extracted, with extremely high resolution, information concerning the physical characteristics of the target.

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Seattle, WA, United States of America

Reynold E Wilbert