Editing Radar (section)

===Pulse-Doppler signal processing===
{{main|Pulse-Doppler signal processing}}
[[File:Pulse doppler signal processing.png|thumb|Pulse-Doppler signal processing. The ''Range Sample'' axis represents individual samples taken in between each transmit pulse. The ''Range Interval'' axis represents each successive transmit pulse interval during which samples are taken. The Fast Fourier Transform process converts time-domain samples into frequency domain spectra. This is sometimes called the ''bed of nails''.]]

Pulse-Doppler signal processing includes frequency filtering in the detection process. The space between each transmit pulse is divided into range cells or range gates. Each cell is filtered independently much like the process used by a [[spectrum analyzer]] to produce the display showing different frequencies. Each different distance produces a different spectrum. These spectra are used to perform the detection process. This is required to achieve acceptable performance in hostile environments involving weather, terrain, and electronic countermeasures.

The primary purpose is to measure both the amplitude and frequency of the aggregate reflected signal from multiple distances. This is used with [[weather radar]] to measure radial wind velocity and precipitation rate in each different volume of air. This is linked with computing systems to produce a real-time electronic weather map. Aircraft safety depends upon continuous access to accurate weather radar information that is used to prevent injuries and accidents. Weather radar uses a [[Pulse repetition frequency#Low PRF|low PRF]]. Coherency requirements are not as strict as those for military systems because individual signals ordinarily do not need to be separated. Less sophisticated filtering is required, and range ambiguity processing is not normally needed with weather radar in comparison with military radar intended to track air vehicles.

The alternate purpose is "[[look-down/shoot-down]]" capability required to improve military air combat survivability. Pulse-Doppler is also used for ground based surveillance radar required to defend personnel and vehicles.<ref name="Syracuse Research Corporation; Massachusetts Institute of Technology">{{cite web|url=https://www.mit.edu/~lrv/cornell/publications/Ground%20Surveillance%20Radars%20and%20Military%20Intelligence.pdf|title=Ground Surveillance Radars and Military Intelligence|publisher=Syracuse Research Corporation; Massachusetts Institute of Technology|url-status=dead|archive-url=https://web.archive.org/web/20100922174712/http://www.mit.edu/~lrv/cornell/publications/Ground%20Surveillance%20Radars%20and%20Military%20Intelligence.pdf|archive-date=22 September 2010}}</ref><ref>{{cite web|url=https://www.youtube.com/watch?v=B0q1Pgz6Cm8| archive-url=https://ghostarchive.org/varchive/youtube/20211030/B0q1Pgz6Cm8| archive-date=30 October 2021|title=AN/PPS-5 Ground Surveillance Radar| date=29 December 2009|via=YouTube; jaglavaksoldier's Channel}}{{cbignore}}</ref> Pulse-doppler signal processing increases the maximum detection distance using less radiation close to aircraft pilots, shipboard personnel, infantry, and artillery. Reflections from terrain, water, and weather produce signals much larger than aircraft and missiles, which allows fast moving vehicles to hide using [[nap-of-the-earth]] flying techniques and [[stealth technology]] to avoid detection until an attack vehicle is too close to destroy. Pulse-Doppler signal processing incorporates more sophisticated electronic filtering that safely eliminates this kind of weakness. This requires the use of medium pulse-repetition frequency with phase coherent hardware that has a large dynamic range. Military applications require [[Pulse repetition frequency#Medium PRF|medium PRF]] which prevents range from being determined directly, and [[range ambiguity resolution]] processing is required to identify the true range of all reflected signals. Radial movement is usually linked with Doppler frequency to produce a lock signal that cannot be produced by radar jamming signals. Pulse-Doppler signal processing also produces audible signals that can be used for threat identification.<ref name="Syracuse Research Corporation; Massachusetts Institute of Technology"/>