A device for determining the radial velocity of an object by measuring the frequency change in the echo of a radio wave reflected from the object.

Radar, an acronym derived from “radio detection and ranging,” uses radio waves of constant frequency reflected from a target to determine its position, distance, direction, and speed. When radio waves emitted from a transmitting antenna are interrupted by a solid object, a portion of the energy, called an echo, is reflected back toward the transmitter, which can also function as a receiver. The distance to the object is determined by the time required for the radio wave to travel from the transmitter to the object and back to the receiver. Since radio waves, like light, travel at the constant speed of 186,000 miles per second, the measured time delay is proportional to the distance. If the transmitter were operating continuously, however, reflected signals would also be continuous, making it impossible to disassociate the emitted signal from the returning signals of different objects. The emitted signal is therefore labeled by emitting the signal in short, high-powered pulses rather than continuously. During the short interval between pulses, the transmitter is operated as a receiver. If the transmitter is not emitting when the return signal arrives, the received signal can be associated with a specific transmitted pulse.

The Doppler effect, discovered by Austrian physicist Christian Johann Doppler in 1842, is the change in the observed frequency of a wave due to relative motion between the observer and the wave source. When observer and source approach one another, the emitted frequency of the waves is measured to be higher due to the velocity of approach; the greater the relative speed, the greater the frequency shift. When the source and observer are receding from each other, the emitted frequency is measured to be lower, in direct proportion to the velocity of recession. The effect is particularly noticeable for sound waves; when an ambulance speeds past, the pitch (frequency) of its siren drops noticeably.

Although the Doppler effect has been well-known for sound and light since the mid-nineteenth century, radar systems utilizing this effect were not developed until after World War II. By detecting the frequency shift in the reflected signal caused by an object having a component of velocity toward or away from the observer, the object’s speed along the line of sight can be calculated from the Doppler equation. Unlike the pulsed systems, Doppler radar is a continuous wave system. A single antenna can be utilized because the reflected signal from a moving object returns at a different frequency, hence the outgoing and incoming signals are not confused.

Radar was developed by the military for its own use and still finds its major applications in the military arena. It is used to detect aircraft, missiles, artillery projectiles, ships, land vehicles, and satellites. Civilian applications include the surveillance of aircraft and weather in the vicinity of airports. Air route surveillance tracks aircraft between airports up to 200 miles away. Radar is also used as a surface detector at airports to give the controller the location and movement of ground-based vehicles within the airport.

A Doppler navigator is a simple continuous-wave system used to determine a plane’s ground speed. The plane’s radar has an antenna that directs a beam forward and down toward the ground at a 45-degree angle to the direction of flight. The plane’s velocity can then be determined from the relative radial motion. A radar altimeter gives the height of an airplane above the ground by reflecting signals straight down.

The National Weather Service (NWS) and the Federal Aviation Administration (FAA) now deploy a network of Doppler radar systems to monitor potential weather hazards to aircraft. By measuring the radial velocity of precipitation in conjunction with the strength of reflected signals, the severity of storms up to 250 miles away can be accurately gauged. The intensity of the echoes from raindrops and ice particles reveals the type of approaching storm and enables forecasters to predict when violent storms will reach specific regions. Doppler radar can also be used to pinpoint hazardous wind conditions such as downbursts, strong blasts of air associated with storm systems and a major cause of aircraft accidents.

Since Doppler radar is the only remote sensing instrument that can detect and measure the radial velocity of wind inside areas veiled by clouds, it is also used to probe the internal motions and structure of tornadoes or other potentially hazardous weather systems. In addition to aiding researchers better to comprehend the dynamics and life cycles of severe storms, this unique capability provides improved early warnings of impending weather hazardous to human communities.

• Blake, Bernard, ed. Jane’s Radar and Electronic Warfare Systems. 6th ed. London: Jane’s Publishing, 1994. Complete description of the various types of radar systems and their military applications.
• Doviak, R. J., and Dusan Zrnic. Doppler Radar and Weather Observations. 2d ed. New York: Academic Press, 1993. A comprehensive summary which introduces basic theory enhanced with numerous observations and measurements not available in other texts. Although the presentation is often technical, there is a wealth of information accessible to the general reader.
• Hitzeroth, Deborah. Radar: The Silent Detector. Murray Hill, N.J.: Lucent Books, 1990. A clear and concise treatment of the principles and practice of radar and its applications.
• Skolnik, Merrill, ed. Radar Handbook. 2d ed. New York: McGraw-Hill, 1990. A complete handbook detailing all aspects of radar, including a wealth of technical information.
• Strong, W., and G. R. Plitnik. Music, Speech, Audio. Provo, Utah: Soundprint, 1992. An easy-to-read introduction to the science of acoustics, containing a complete explanation of the physics of the Doppler effect in descriptive terms easily understood by the general reader.

Air traffic control

Airports

Federal Aviation Administration

Instrumentation

Military flight

Radar

Weather conditions

World War II