Editing Doppler radar (section)

=== Technology ===
There are four ways of producing the Doppler effect. Radars may be:
* [[Continuous-wave radar#Interruption|Coherent pulsed]] (CP),
* [[Pulse-Doppler radar]],
* [[Continuous wave]] (CW), or
* [[Frequency modulation]] (FM).

Doppler allows the use of narrow band receiver filters that reduce or eliminate signals from slow moving and stationary objects. This effectively eliminates false signals produced by trees, clouds, insects, birds, wind, and other environmental influences but various inexpensive [[Pocket Radar|hand held]] Doppler radar devices not using this may produce erroneous measurements.

[[Continuous-wave radar|CW Doppler radar]] only provides a velocity output as the received signal from the target is compared in frequency with the original signal. Early Doppler radars included CW, but these quickly led to the development of frequency modulated continuous wave ([[Continuous-wave radar#Modulated continuous-wave|FMCW]]) radar, which sweeps the transmitter frequency to encode and determine range.

With the advent of digital techniques, [[Pulse-Doppler radar]]s (PD) became light enough for aircraft use, and Doppler processors for coherent pulse radars became more common. That provides [[Look-down/shoot-down]] capability. The advantage of combining Doppler processing with pulse radars is to provide accurate velocity information.  This velocity is called [[Range rate|range-rate]].  It describes the rate that a target moves toward or away from the radar. A target with no range-rate reflects a frequency near the transmitter frequency and cannot be detected. The classic zero doppler target is one which is on a heading that is tangential to the radar antenna beam. Basically, any target that is heading 90 degrees in relation to the antenna beam cannot be detected by its velocity (only by its conventional [[reflectivity]]).

[[Ultra-wideband]] waveforms have been investigated by the [[United States Army Research Laboratory|U.S. Army Research Laboratory (ARL)]] as a potential approach to Doppler processing due to its low average power, high resolution, and object-penetrating ability. While investigating the feasibility of whether UWB radar technology can incorporate Doppler processing to estimate the velocity of a moving target when the platform is stationary, a 2013 ARL report highlighted issues related to target range migration.<ref>{{Cite journal|last=Dogaru|first=Traian|date=March 2013|title=Doppler Processing with Ultra-wideband (UWB) Impulse Radar|url=https://apps.dtic.mil/sti/pdfs/ADA595731.pdf|journal=U.S. Army Research Laboratory}}</ref> However, researchers have suggested that these issues can be alleviated if the correct [[matched filter]] is used.<ref>{{Cite journal|last=Dogaru|first=Traian|date=January 1, 2018|title=Doppler Processing with Ultra-Wideband (UWB) Radar Revisited|url=https://apps.dtic.mil/sti/pdfs/AD1047118.pdf|journal=U.S. Army Research Laboratory|via=Defense Technical Information Center}}</ref>

In military airborne applications, the Doppler effect has 2 main advantages. Firstly, the radar is more robust against counter-measure. Return signals from weather, terrain, and countermeasures like [[chaff (countermeasure)|chaff]] are filtered out before detection, which reduces computer and operator loading in hostile environments. Secondly, against a low altitude target, filtering on the radial speed is a very effective way to eliminate the [[ground clutter]] that always has a null speed. Low-flying military plane with countermeasure alert for hostile radar track acquisition can turn perpendicular to the hostile radar to nullify its Doppler frequency, which usually breaks the [[radar lock|lock]] and drives the radar off by hiding against the ground return which is much larger.