A specialized type of radar uses the Doppler effect to detect the speed of an target. You have probably observed the Doppler effect hundreds of times without realizing it. The change in pitch as a vehicle approaches, then drives past you is an example of the Doppler frequency shift. The sound waves shift to a higher frequency as the vehicle comes toward you, raising the pitch, then as the vehicle pulls away the frequency of the sound is lowered, dropping the pitch. Doppler theory tells us that

where fd is the Doppler frequency shift, VR is the radial velocity of the target (i.e. velocity along the line-of-sight), and c is the speed of propagation of the radar pulse, known for pulses traveling in air. Doppler frequency shift is the difference between the frequency of the pulse transmitted to the target and the frequency of the return pulse. If this can be measured, then the radial speed, or speed along the line-of-sight can be determined. Note, however, that target velocity at right angles to the radar system line-of-sight does not cause Doppler shift. In such a case, the speed detector would register a target speed of zero. Similarly, if a target is moving at some angle to the direct line-of-sight, the system would only detect the radial component of its velocity. A cosine term can be added to the basic equation to account for non-radial motion. More sophisticated radar systems include this compensation, but typical law enforcement speed detectors do not, with the result that the measured velocity of the target is somewhat lower than the actual velocity.

A Doppler radar system consists of a continuously transmitting source, a mixer, and data and signal processing elements. The signal is sent out to the target continuously. When the return is received, it is "mixed" with a sample of the transmitted signal, and the frequency of the resultant output is the Doppler frequency shift caused by the radial velocity of the target. The Doppler shift is averaged over several samples and processed to yield target speed.

Effective operating range of a radar system is limited by antenna efficiency, transmitted power, the sensitivity of the detector, and the size of the target/energy it reflects. Reflection of electromagnetic waves from surfaces is fundamental to radar. All objects do not reflect radar waves equally well—the strength of the wave reflection depends on the size, shape, and composition of the object. Metal objects are the best reflectors, while wood and plastic produce weaker reflections. So-called stealth airplanes are based on this concept and are built from materials that produce a minimal reflection.

In recent years laser radar systems have been developed. Laser radar systems operate on essentially the same principle as conventional radar, but the significantly shorter wavelengths of visible light allow much higher resolution. Laser radar systems can be used for imaging and for measurement of reflectivity. They are used for vibration detection in automotive manufacturing and for mapping power lines. Because they are more difficult to detect than conventional radar systems, laser radar speed guns are increasingly being adopted by law enforcement agencies.

Radar has undergone considerable development since its introduction in the 1930s. It is a remarkably useful tool that touches our lives in a surprising number of ways, whether by the weather report that we listen to in the morning, or the guidance of the airplanes we ride in. It has given us a different way to see the world around us.

## Resources

### Books

Blake, Bernard, ed. Jane's Radar and Electronic Warfare Systems. Alexandria, VA: Jane's Information Group Inc., 1992.

Edde, Byron. Radar: Principles, Technology, Applications. Englewood Cliffs, NJ: Prentice-Hall, 1993.

Kristin Lewotsky Frank Lewotsky

## KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Bistatic

—A radar system with transmitting and receiving antennas in separate locations.

Duplexer

—In a monostatic system, the device that switches system operation between transmit and receive mode.

Modulation

—Variation, as in modulation of an electrical signal.

Monostatic

—A radar system in which a single antenna both transmits and receives; a system in which transmitting and receiving antennas are at the same location.

Transponder

—A beacon. In the case of an Air Traffic Control radar beacon system, a device that is capable of transmitting certain information when queried.