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obtain the azimuth (cross-range) coordinate by processing it coher-
ently over several returns. The range-compression signal processing
involves integration in space, while the azimuth-compression signal
processing involves integration in time.
Figure 6.23 shows the optical and electronic subsystems that per-
form the space and time integrations. The radar return signal is het-
erodyned to the middle frequency of an acoustooptical sensor and
added electronically to a reference sinusoid to capture the history of
the phase of the return signal interferometrically for compression in
azimuth. The resulting signal is applied to the acoustooptical sensor
via a piezoelectric transducer. The acoustooptical sensor thus becomes
a cell that encodes the evolving SAR return.
Meanwhile, pulses of light a few tens of nanoseconds long are
generated by a laser diode in synch with the transmitted pulses and

FIGURE 6.23 Acoustooptical synthetic-aperture radar.