Page 9 - Spec Tech Vol 1 Issue 10

SPACE SENSORS
Image Credit: DLR Portal
A Synthetic Aperture Radar (SAR), or SAR,
is a coherent mostly airborne or
spaceborne sidelooking radar system which
utilizes the flight path of the platform to
simulate an extremely large antenna or
aperture electronically, and that generates
high-resolution remote sensing imagery.
Over time, individual transmit/receive cycles
(PRT's) are completed with the data from
each cycle being stored electronically. The
signal processing uses magnitude and
phase of the received signals over
successive pulses from elements of a
synthetic aperture. After a given number of
cycles, the stored data is recombined
(taking into account the Doppler effects
inherent in the different transmitter to target
geometry in each succeeding cycle) to
create a high-resolution image of the terrain
being over flown. The SAR works similar of
a phased array, but contrary of a large
number of the parallel antenna elements of
a phased array, SAR uses one antenna in
time-multiplex. The different geometric
positions of the antenna elements are result
of the moving platform now.
The SAR-processor stores all the radar
Space Explorer 2022
returned signals, as amplitudes and phases,
for the time period T from position A to D.
Now it is possible to reconstruct the signal
which would have been obtained by an
antenna of length v T, where v is the
platform speed. As the line of sight direction
changes along the radar platform trajectory,
a synthetic aperture is produced by signal
processing that has the effect of lengthening
the antenna. Making T large makes the
„synthetic aperture” large and hence a high-
er resolution can be achieved.
As a target (like a ship) first enters the radar
beam, the backscattered echoes from each
transmitted pulse begin to be recorded. As
the platform continues to move forward, all
echoes from the target for each pulse are
recorded during the entire time that the
target is within the beam. The point at which
the target leaves the view of the radar beam
some time later, determines the length of the
simulated or synthesized antenna. The
synthesized expanding beamwidth,
combined with the increased time a target is
within the beam as ground range increases,
balance each other, such that the resolution
remains constant across the entire swath.
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