Editing Doppler radar (section)

== History ==
[[File:AN-APN-81 Doppler Radar Navigation system, General Precision Laboratory, mid 1950s - National Electronics Museum - DSC00313.JPG|thumb|right|AN/APN-81 Doppler radar navigation system, mid-1950s]]

Doppler radar tends to be lightweight because it eliminates heavy pulse hardware. The associated filtering removes stationary reflections while integrating signals over a longer time span, which improves range performance while reducing power. The military applied these advantages during the 1940s.

Continuous-broadcast, or FM, radar was developed during [[World War II]] for [[United States Navy]] aircraft, to support night combat operation. Most used the [[Ultra high frequency|UHF]] spectrum and had a transmit [[Yagi antenna]] on the [[Port (nautical)|port]] wing and a receiver Yagi antenna on the [[starboard]] wing. This enabled [[bomber]]s to fly an optimum speed when approaching ship targets, and let escort fighter aircraft train guns on enemy aircraft during night operation. These strategies were adapted to [[semi-active radar homing]].

In 1951, [[Carl A. Wiley]] [[History of synthetic-aperture radar|invented synthetic-aperture radar]], which, though distinct from mainstream Doppler radar, was based on Doppler principles, and originally patented as "Pulsed Doppler Radar Methods and Means," #3,196,436.

Modern Doppler systems are light enough for mobile ground surveillance associated with infantry and surface ships. These detect motion from vehicles and personnel for night and all weather combat operation. Modern police radar guns are a smaller, more portable version of these systems.<ref>{{cite web| title = Ground Surveillance Radar Section| url = http://www.ichiban1.org/html/cs_radar.htm| publisher = 1st Battalion 50th Infantry Association}}</ref><ref>{{cite web|url= http://articles.janes.com/articles/Janes-Radar-and-Electronic-Warfare-Systems/AN-SPG-51-gun-and-missile-fire-control-radar-United-States.html|title= AN/SPG-51 Gun and Missile Fire Control Radar|publisher= Jane's Information Group|access-date= 2012-08-15|archive-date= 2013-01-27|archive-url= https://archive.today/20130127014829/http://articles.janes.com/articles/Janes-Radar-and-Electronic-Warfare-Systems/AN-SPG-51-gun-and-missile-fire-control-radar-United-States.html|url-status= dead}}</ref>

Early Doppler radar sets relied on large analog filters to achieve acceptable performance. Analog filters, waveguide, and amplifiers pick up vibration like microphones, so bulky vibration damping is required. That extra weight imposed unacceptable kinematic performance limitations that restricted aircraft use to night operation, heavy weather, and heavy jamming environments until the 1970s.

Digital [[fast Fourier transform]] (FFT) filtering became practical when modern [[microprocessor]]s became available during the 1970s. This was immediately connected to coherent pulsed radars, where velocity information was extracted. This proved useful in both weather and [[air traffic control]] radars. The velocity information provided another input to the software tracker, and improved computer tracking. Because of the low [[pulse repetition frequency]] (PRF) of most coherent pulsed radars, which maximizes the coverage in range, the amount of Doppler processing is limited. The Doppler processor can only process velocities up to ±{{sfrac|1|2}} the PRF of the radar. This is not a problem for weather radars. Velocity information for aircraft cannot be extracted directly from [[Pulse repetition frequency#Low PRF|low-PRF radar]] because sampling restricts measurements to about 75 miles per hour.

Specialized radars quickly were developed when digital techniques became lightweight and more affordable. [[Pulse-Doppler radar]]s combine all the benefits of long range and high velocity capability. Pulse-Doppler radars use a medium to high PRF (on the order of 3 to 30&nbsp;kHz), which allows for the detection of either high-speed targets or high-resolution velocity measurements. Normally it is one or the other; a radar designed for detecting targets from zero to [[Mach number|Mach]] 2 does not have a high resolution in speed, while a radar designed for high-resolution velocity measurements does not have a wide range of speeds. Weather radars are high-resolution velocity radars, while [[air defense]] radars have a large range of velocity detection, but the accuracy in velocity is in the tens of [[Knot (unit)|knots]].

Antenna designs for the CW and FM-CW started out as separate transmit and receive antennas before the advent of affordable microwave designs.  In the late 1960s, traffic radars began being produced which used a single antenna. This was made possible by the use of circular polarization and a multi-port waveguide section operating at X band. By the late 1970s this changed to linear polarization and the use of ferrite [[circulator]]s at both X and K bands. PD radars operate at too high a PRF to use a transmit-receive gas filled switch, and most use [[Solid-state (electronics)|solid-state]] devices to protect the receiver low-noise amplifier when the transmitter is fired.