Doppler radar

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Doppler effect

Doppler radar is radar that makes use of the doppler effect in order to produce data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the original signal has been altered by the object(s) that reflected it. Variations in the frequency of the signal give direct and highly accurate measurments of a target's velocity relative to the radar source and the direction of the microwave beam. Doppler radars are used in air defense, air traffic control, sounding satellites, police speed guns, and radiology.

The specific term "Doppler Radar", due in part to its extremely common use by television meteorologists, has erroneously become nearly synonomous with the type of radar used by weather stations. It is correct that most modern weather radar use the pulse-doppler technique to examine the motion of precipitation inside the clouds, but it is only a part of the processing of their data. So, while these radars use a highly specialized form of doppler radar, the term is much broader in its meaning and its applications.

Doppler Effect

The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave for an observer moving relative to the source of the waves. It is commonly heard when a vehicle sounding a siren approaches, passes and recedes from an observer. The received frequency is increased (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is decreased during the recession. Consider the following analogy: A pitcher throws one ball every second in a person's direction (a frequency of 1 ball per second). Assume that the balls travel at a constant velocity. If the pitcher is stationary, the man will catch one ball every second. However, if the pitcher is jogging towards the man, he will catch balls more frequently because the balls will be less spaced out (the frequency increases). The inverse is true if the pitcher is moving away from the man; he will catch balls less frequently due to the pitcher's backward motion (the frequency decreases). If the pitcher shoots microwaves instead of throwing balls, the analogy demonstrates that the frequency of the microwaves is affected by the movement of the pitcher; as he runs towards the man, the frequency of the microwaves the man receives increases, and as he runs away from the man the frequency of the microwaves the man receives decreases. Note that, from the point of view of the pitcher, the frequency remains constant (whether he's throwing balls or transmitting microwaves). Since with electromagnetic radiation like microwaves frequency is inversely proportional to wavelength, the wavelength of the waves is also affected. Thus, the relative difference in velocity between a source and an observer is what gives rise to the doppler effect.

Basic concept

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.

Bibliography

• David G. C. Luck, Frequency Modulated Radar, published by McGraw-Hill, New York, 1949, 466 pages.

See also

• Radar
  • Continuous wave radar
  • Fm-cw radar
  • Weather radar, including the Doppler weather radar
  • Pulse-Doppler radar
  • Radar gun

External links

• Description of Doppler shift used in Continuous wave Doppler radar
• Listen to X-Band Doppler of person walking and golf ball rolled