PART VII High Resolution Ground Mapping and Imaging
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1. Hypothetical operational situation for a SAR radar. With real
antenna trained at fixed azimuth angle of 90
˚ to flight path,
radar maps a 1-mile wide swath at range of 8 miles.
2. Points representing positions of center of antenna when succes-
sive pulses are transmitted. Each point constitutes one “element”
of synthetic array.
3. Returns received by successive elements of the synthetic array are
summed in bank of range bins spanning the range interval being
mapped.
Rudimentary Example (Unfocused Array). Just how the
array is synthesized is perhaps most easily visualized by
considering an extremely simple SAR system in a hypotheti-
cal operational situation.
An aircraft carrying an X-band radar is flying in a straight
line at constant speed and altitude. The radar antenna is
pointed downward slightly and aligned at a fixed angle of
90˚ relative to the flight path (Fig. 1).
As the aircraft progresses, the beam sweeps across a
broad swath of ground parallel to the flight path. Only a rel-
atively narrow portion of this swath, however, is of immedi-
ate interest. That portion, we’ll say, is a strip 1 nautical mile
wide, offset from the flight path by about 8 nautical miles.
The aircraft’s mission requires that the ground within this
strip be mapped with a resolution of about 50 feet. As will
be explained shortly (page 414), to provide 50-foot resolu-
tion at a range of 8 miles, our hypothetical SAR radar must
synthesize an array roughly 50 feet long.
The aircraft’s ground speed, let’s say, is 1000 feet per sec-
ond (600 knots) and the PRF is 1000 pulses per second.
Consequently, every time the radar transmits a pulse, the
center of the radar antenna is one foot farther along the
flight path. The synthetic array can thus be thought of as
consisting of a line of elemental radiators one foot apart
(Fig. 2). To synthesize the required 50-foot long array, 50
such elements are required. In other words, the returns
from 50 consecutive transmitted pulses must be summed.
Typically summing is done after the receiver’s output has
been digitized. A bank of range bins is provided which just
spans the 1 mile range interval being mapped (Fig. 3).
Following every transmission, the return from each resolv-
able range increment within this interval is added to the
contents of the appropriate bin.
This operation corresponds to the summing performed
by the feed structure that interconnects the radiating ele-
ments of a real array antenna. The fundamental difference is
that, with the real array, the return from each range incre-
ment is received simultaneously by all array elements every
time a pulse is transmitted; whereas, with the synthetic
array, the return is collected by the individual elements seri-
ally over the period of time the radar takes to traverse the
array.
The return from the first pulse is received entirely by ele-
ment number one; the return from the second pulse is
received entirely by element number two; and so on.
The result, however, is substantially the same. Provided
that the range is long compared to the array length, the dis-
tance from a patch of ground on the boresight line (perpen-
dicular to the flight path) to each array element is essential-
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