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8. Side-looking radars

Most imaging radars used for remote sensing are side-looking airborne radars (SLARs). The antenna points to the side with a beam that is wide vertically and narrow horizontally. The image is produced by motion of the aircraft past the area being covered.

A short pulse is transmitted from the airborne radar, when the pulse strikes a target of some kind, a signal returns to the aircraft. The time delay
associated with this received signal, as with other pulse radars, gives the distance between target and radar.

When a single pulse is transmitted, the return signal can be displayed on an oscilloscope; however, this does not allow the production of an image. Hence,
in the imaging radar, the signal return is used to modulate the intensity of the beam on the oscilloscope, rather than to display it vertically in proportion to the signal strength.

Thus, a single intensity-modulated line appears on the oscilloscope, and is transferred by a lens to a film. The film is in the form of a strip that moves synchronously with the motion of the aircraft, so that as the aircraft moves forward the film also moves.

When the aircraft has moved one beamwidth forward, the return signals come from a different strip on the ground. These signals intensity-modulate the line on the cathode-ray tube and produce a different image on a line on the film adjacent to the original line. As the aircarft moves forward, a series of these lines is imaged onto the film, and the result is a two-dimensional picture of the radar return from the surface.

The speed of the film is adjusted so that the scales of the image in the directions perpendicular to and along the flight track are maintained as nearly
identical to each other as possible. Because the cross-track dimension in the image is determined by a time measurement, and the time measurement is
associated with the direct distance (slant range) from the radar to the point on the surface, the map is distorted somewhat by the difference between the slant range and the horizontal distance, or ground range.

In some radar systems, this distortion is removed by making the sweep on the cathode-ray tube nonlinear, so that the points are mapped in their proper ground range relationship. This, however, only applies exactly if the points all lie in a plane surface, and this modification can result in excessive distortion in mountainous areas.

Side-looking airborne radars normally are divided into two groups: the real-aperture systems that depend on the beamwidth determined by the actual antenna, and the synthetic aperture systems that depend upon signal processing to achieve a much narrower beamwidth in the along-track direction than that attainable with the real antenna.
The customary nomenclature used is "SLAR" for the real-aperture system and "SAR" for the synthetic aperture system, although the latter is also a side-looking airborne (or spaceborne) radar.