Airborne radars present unique design challenges, mainly in the severe nature of the ground echo received by the radar and in the installation constraints on the size of the radar. The peculiar clutter situation governs the nature of the signal processing, and the installation limitations influence the antenna design and the radio frequency to be used (the two being strongly related) as well as the packaging of the rest of the radar. Similar considerations influence the design of space-based radars as well.
A particularly valuable use of airborne radar is weather assessment. Radars generally operating in the C or X bands (around 6 GHz or around 10 GHz, respectively) permit both penetration of heavy precipitation, required for determining the extent of thunderstorms, and sufficient reflection from less intense precipitation. See also Meteorological radar; Radar meteorology.
Another basic and valuable airborne radar function is altimetry. The aircraft's altitude can be continuously measured, using (generally) C-band frequencies (around 6 GHz), low-power transmission, and a downward-oriented antenna beam. Sometimes, information from additional beams (looking somewhat forward, for example) is combined with measurements of the Doppler shift of the ground echo received to further aid in navigation. Another type of radar used in navigation is the radar beacon, in which a ground-based receiver detects an interrogation pulse from the aircraft and sends back a so-called reply on a different frequency, to which the receiver on the aircraft is tuned. See also Air-traffic control; Altimeter; Doppler effect; Surveillance radar.
Airborne radars are used effectively to provide high-resolution mapping of Earth's (or other planetary) surface, with a technique called synthetic aperture radar (SAR). The processing uses the fact that surface objects produce a Doppler shift (due to the aircraft's flight) unique to their position as the aircraft passes by; this Doppler history is indicative of the scatterer's lateral, or cross-range, position at the particular range determined by the usual echo timing. With very stable radars and well-measured flight characteristics (and other focusing methods), picture cells (pixels) of 1 ft × 1 ft (0.3 m × 0.3 m) can be formed in the processed images from radars tens or hundreds of miles away. The resolution is somewhat like that possible had the flight path itself been used as a huge antenna, the synthetic aperture.
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