Page 161 - Intro to Space Sciences Spacecraft Applications

140 Introduction to Space Sciences and Spacecraft Applications

Synthetic-aperture radar systems employ a smaller antenna that transmits a
relatively broad azimuth or along-track beam as shown in Figure 6-6.
A SAR “synthetically” obtains its along-track resolution by moving the
real antenna beam in relation to the target. In the most basic sense, there
must be a translation of either the target through the real beam, the real
beam through the target, or a combination of both processes. As shown in
the figure, as‘the radar beam translates the target, the radar sends a string
of carefully prescribed pulses illuminating and receiving backscatter
responses from the target area. The SAR preserves and saves the ampli-
tude and phase histories of each of these responses. A special image sig-
nal processing system performs a target specific weighting, shifting, and
summing process focusing on one resolution element at a time and, as a
result, achieves an along-track resolution that is independent of range and
wavelength, becoming equal to one-half of the actual antenna length in
the along-track direction, as shown in the figure.

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Figure 6-6. Aperture synthesis. A synthetic aperture radar (SAR) uses signal
processing over many radar returns to obtain resolutions comparable with
normal radars of much larger apertures.