| An introduction to SAR
A radar system illuminates an area with microwaves, and records the
strength and travel-time of the returned signals. This allows the range
(or distance) of the reflecting objects to be determined.
In an optical instrument, the resolution is determined by the range,
frequency and the size of the aperture: a larger aperture gives a finer
resolution. It is possible to obtain good high-resolution optical and
infrared images from spaceborne satellites. However, scaling up to radar
wavelengths produces requirements for an antenna that is far too large to
be carried and kept stable. For example, to achieve an (azimuth)
resolution of 100m with a 5cm radar from a range of 800km requires an
aperture some 400m long.
If the radar is attached to a moving platform, either a satellite or an
aircraft, then it is possible to combine reflected signals from along the
flight path to synthesise a very long antenna. The aperture, or area used
to receive signals, is created artificially during the signal processing.
This is a Synthetic Aperture Radar (SAR).
The synthetic aperture gives the radar a high resolution in the azimuth
(or along-track) direction - the line of the flightpath. The resolution in
the range direction is determined by the duration of the transmitted
pulses. In practice, to achieve a fine resolution, the pulse width would
be too short to contain sufficient power. Therefore, longer,
frequency-modulated pulses (linear chirps) and transmitted. This
complicates the image formation process. However, with the use of parallel
computing, SAR images can be formed in real-time.
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