Most ground-based air-surveillance radars that must see aircraft in the midst of clutter are generally MTI radars. When flown in an aircraft for purposes of air-surveillance, these are called AMTI, or airborne moving target indicator radar. AMTI radars are very good for airborne air-surveillance radars at UHF, but result in too many blind speeds when employed at the higher microwave frequencies. The high-prf and the medium-prf pulse doppler radars can both be used for airborne air-surveillance purposes at the higher frequencies. (The Sband AWACS, or AN/APY-1, airborne air-surveillance radar utilizes a high-prf pulse doppler waveform.) An X-band fighter/attack radar in a modern military aircraft might employ on a time-shared basis three widely differing prfs, depending on the operational situation. When no clutter is present (as when the antenna beam is looking up above the surface of the earth), a low prf waveform without any doppler processing might be employed. (The low prf waveform will provide the greatest range, if no clutter is present.) When searching for a target in clutter, the high prf and the medium-prf pulse doppler waveforms might be interleaved. The high prf would be looking for high-speed approaching targets at long range and the medium prf would be looking for slower-speed targets at shorter ranges.
The doppler frequency shift is important for many purposes in radar. In addition to the MTI, AMTI, and pulse doppler radars, the doppler frequency is the key to such radars as
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Doppler weather radars
Nexrad, whose output is regularly shown on TV weather broadcasts.
Terminal Doppler Weather Radar (TDWR). These are located in the vicinity of major airports to warn aircraft, that are landing or taking-off, of the presence of dangerous wind shear.
Wind profiler, that measures as a function of altitude the wind speed and direction for both weather prediction and the efficient routing of aircraft.
Airborne weather-avoidance doppler radar, that detects dangerous wind shear to warn the pilot of danger during take-off or landing.
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Synthetic aperture radar (SAR), (for mapping a scene on the surface of the earth, and the inverse synthetic aperture radar (ISAR), for imaging a target well enough to recognize it from other similar targets. (SAR is more usually thought of as an antenna synthesized in a digital processor, but it was originally invented as a doppler radar and can be described as depending on the doppler effect.)
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Doppler navigator, a multi-beam radar which can provide the vector velocity of an aircraft which carries it.
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HF over-the-horizon radar, which depends on the doppler frequency to detect aircraft and ships in the presence of large clutter echoes. The doppler spectrum of the sea echo obtained by an HF radar can also be used to extract the strength and direction of the winds over the ocean.
The continuous wave (CW) radar also depends on the doppler frequency shift to detect targets in clutter and/ or to measure relative velocity.