| definition |
-
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 aircraft 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.
[Source: ESA]
|
