A doppler radar is a radar using the doppler effect of the returned echoes from targets to measure their radial velocity. To be more specific the microwave signal sent by the radar antenna's directional beam is reflected toward the radar and compared in frequency, up or down from the original signal, allowing for the direct and highly accurate measurement of target velocity component in the direction of the beam. Doppler radars are used in air defense, air traffic control, sounding satellites, police speed guns, and radiology.
Recent weather radars process velocities of precipitations by Pulse-Doppler radar technique, on top of their intensities. This is a slightly different treatment of Doppler data that has been publicized so much in the United States that the term Doppler radar is often wrongly used by the public to mean weather radar.
The phenomenon known as the Doppler Effect is named after Christian Andreas Doppler. Doppler was an Austrian physicist who first described in 1842 how the observed frequency of light and sound waves was affected by the relative motion of the source and the detector.
This is most often demonstrated by the change in the sound wave of a passing train. The sound of the train whistle will become "higher" in pitch as it approaches and "lower" in pitch as it moves away. This is explained as follows: the number of sound waves reaching the ear in a given amount of time (this is called the frequency) determines the tone, or pitch, perceived. The tone remains the same as long as you and the train are not moving relative to each other. As the train moves closer to you the number of sound waves reaching your ear in a given amount of time increases. Thus, the pitch increases. As the train moves away from you the opposite happens.
A Doppler radar is a radar that produces a velocity measurement as one of its outputs. Doppler radars may be Coherent Pulsed, Continuous Wave, or Frequency Modulated. A continuous wave (CW) doppler radar is a special case that only provides a velocity output. Early doppler radars were CW, and it quickly led to the development of Frequency Modulated (FM-CW) radar, which sweeps the transmitter frequency to encode and determine range. The CW and FM-CW radars can only process one target normally, which limits their use. With the advent of digital techniques Pulse-Doppler (PD) radars were introduced, and doppler processors for coherent pulse radars were developed at the same time.
The advantage of combining doppler processing to pulse radars is to provide accurate velocity information. This velocity is called Range-Rate. It describes the rate that a target moves towards 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).
FM radar was highly developed during World War II for the use by US Navy aircraft. Most used the UHF spectrum, and had a transmit yagi antenna on the port wing, and a receiver yagi antenna on the starboard wing. This allowed bombers to fly an optimum speed when approaching ship targets. Later when magnetrons and microwaves became available, the use of FM radar fell into disuse.
When the Fast Fourier transform became available digitally, it was immediately connected to Coherent Pulsed radars, where velocity information was extracted. This quickly proved useful in both weather and air traffic control radars. The velocity information provided another input to the software tracker, and improved computer tracking. Due to 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 ±1/2 the PRF of the radar. This was not a problem for weather radars.
Specialized radars quickly were mechanized when digital techniques became affordable. Pulse-Doppler radars combine all the benefits of long range, and high velocity capability. Pulse-Doppler radars use a medium to high PRF (on the order of 30 kHz). This high PRF allows for the detection of either high speed targets, or high resolution velocity measurements. Normally it is one or the other, that is, a radar designed for detecting targets from zero to 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 10's of 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 circulators 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 devices to protect the receiver Low Noise Amplifier when the transmitter is fired.