When a mariner is approaching a rocky coast at night, he sometimes sets off a whistle blast and ticks off the seconds until he hears the faint echo of the blast come back from the shore. By noting the number of seconds that have elapsed, he can determine roughly how far off the coast is. This crude method of detection is based on the fact that sound travels in air at the rate of roughly 330 meters per second. Our mariner knows that the sound waves generated by the whistle have to go out to the shore and back. Hence the actual distance to the shore is 165 meters for every second of delay from the time the original blast was emitted to the time when the echo was received. A cliff 1,650 meters away would give back an echo about 10 seconds after setting off of the blast.

Like other sound waves, radio waves have echoes, reflected from metals, wood, glass, plastics, and other objects. The reflected radio energy will be scattered in many different directions including the direction from which the radio signals originally emanated. This phenomenon ha sbeen put to use in the device called radar (from RAdio Detection And Ranging).

A radar transmits radio-frequency energy. This is focused into a beam, which is made to scan a wide area. Reflections, or echoes, from a target in this area then make their way to a radio receiver. Since radio waves travel at a constant speed – about 300,000 kilometers per second – the distance the target can be determined by measuring the time that has elapsed from the transmission of the radio energy to the reception of the echo from the target. The latter’s position can be indicated by noting the position of the beam that yields the strongest echoes. In some cases, one can also calculate the velocity of the target.

How It Works

There are two basic types of radar – continuous wave, or c-w, pulsed radar. The continuous-wave radar transmits radio waves continuously, as the name indicates. In the tracking of moving targets, the radio signal that is emitted has a constant frequency. The frequency of the returning echo will depend upon the speed and the direction of the object that is being tracked. This is an example of the Doppler effect. In fact, the continuous-wave radar is sometimes called the Doppler radar. A part of the signal is fed, as it is transmitted, into the receiver. When the reflected signal arrives at the receiver, the differences in frequency between transmitted and reflected signals are continuously measured and are displayed or recorded. The operator can tell whether the object is moving toward him or away from him and what its velocity is.

For stationary targets, the transmitting frequency is varied regularly and continuously. The frequencies of the transmitted signal and of the target echo will differ by an amount proportional to 1) the rate of change of the transmitter frequency and 2) the time taken by the transmitted signal to reach that target and to return.

The pulsed radar is used far more widely that the continuous-wave variety. It transmits radio-frequency energy in short bursts, or pulses, emitted at the rate of from a few hundred to many thousands per second. After each pulse, there is a listening period, during which echoes from the target may be received with no interference from the radar’s transmitter.

Each pulsed radar generally has seven basic elements – the synchronizer, the modulator, the transmitter, the duplexer, the antenna, the receiver, and the indicator.

The synchronizer, an electronic oscillator, generates low-power pulses, setting the frequency of the pulses to be beamed later of the antenna. The modulator contains an electric-energy storage device. This is discharged to produce a high-power pulse every time a low-power pulse is received from the synchronizer. The transmitter, a high-frequency oscillator, converts the direct-current energy from the modulator into pulses of radio-frequency energy. These pulses are fed into an antenna, which beams them out into space.

The antenna serves a twofold purpose. Not only does it emit pulses but it also picks up return echoes from the target and sends them on to the receiver. This is made possible by the duplexer. It disconnects the receiver from the antenna while the latter is transmitting. After the pulse has been transmitted, the duplexer disconnects that transmitter from the antenna, so that the radio echoes may be channeled to the receiver.

There are various types of antennas. The most familiar one is a parabolic dish-shaped device, which keeps rotating while it is in operation. The dish is often cut away so as to send out a beam of a desired width, broader or narrower. Sometimes two antennas are set side by side, one sending out a broad beam, the other a pencil-thin beam. As an antenna rotates, generally many times a second, it scans a wide area.

The faint radio signals picked up from a target  by the antenna are fed to the receiver, where they are amplified, so that they may be displayed on the indicator. The basic component of the latter is a cathode ray tube. It is quite similar to the picture tube of a television receiver, though sometimes smaller. As in the case of the television picture tube, the inner face of the radar cathode-ray tube has an electroluminescent coating that glows when struck by a beam of electrons. In this way it produces the radar display on the outer face of the tube – the radar screen or scope, as it is often called.

Pulsed-radar information may be displayed in various ways. The A-display, or A-scope, is used to show the range or distance, of the target. In this device, the cathode-ray beam produces a trace of light running horizontally across the face of the tube from the viewer’s left to this right. The light trace actually moves back and forth, but it is not visible on the return sweep. The beam is deflected vertically by the receiver’s output, which includes random “noise,” as well as the radio echoes received from the targets. This noise is derived partly from unwanted targets. The targets produce wider deflections from the horizontal. These variations can be interpreted so as to indicate the range of the targets. In the B-display, or B-scope, the display shows both range and direction, as indicated on a screen scale laid out in squares. The C-scope does not indicate range. Instead, it displays direction and elevation.