Radar

I

Introduction

Radar, electronic system, used to locate objects beyond the range of vision, and to determine their distance by projecting radio waves against them. The term radar is derived from the phrase “radio detection and ranging”, and this name was used by Allied forces during World War II for a variety of devices concerned with radio detection and position finding. Such devices not only indicate the presence and range of a distant object, called the target, but also determine its position in space, its size and shape, and its velocity and direction of motion. Although originally developed as an instrument of war, radar today is used extensively in many peacetime pursuits, such as navigation, controlling air traffic, detecting weather patterns, and tracking spacecraft.

II

Development

All radar systems employ a high-frequency radio transmitter to send out a beam of electromagnetic radiation, ranging in wavelength frequency from a few centimetres to about 1 m (3 ft). Objects in the path of the beam reflect these waves back to the transmitter. The basic concepts of radar are based on the laws of radio-wave reflection, which are inherent in the equations governing the behaviour of electromagnetic waves developed by the British physicist James Clerk Maxwell in 1864. These principles were first demonstrated in 1886 in experiments by the German physicist Heinrich Hertz. The German engineer Christian Hülsmeyer was the first to propose the use of radio echoes in a detecting device designed to avoid collisions in marine navigation. A similar device was suggested in 1922 by the Italian inventor Guglielmo Marconi.

The first successful radio range-finding experiment occurred in 1924, when the British physicist Edward Victor Appleton used radio echoes to determine the height of the ionosphere—an ionized layer of the upper atmosphere that reflects longer radio waves. In the following year the American physicists Gregory Breit and Merle Antony Tuve achieved independently the same measurements of the ionosphere—using the radio-pulse technique that was subsequently adopted in most radar systems. Development of the latter was impossible until electronic techniques and equipment were improved in the 1930s.

The first practical radar system was produced in 1935 by the British physicist Robert Watson-Watt. His work gave Britain a head start in this vitally important technology, and by 1939 they had established a chain of radar stations along the south and east coasts of England to detect aggressors in the air or at sea. In that same year two British scientists were responsible for the most important advance made in the technology of radar during World War II. The physicist Henry Boot and biophysicist John T. Randall invented an electron tube called the resonant-cavity magnetron. This type of tube is capable of generating high-frequency radio pulses with a large amount of power, thus permitting the development of microwave radar, which operates in the very short wavelength band of less than 1 cm, using lasers. Microwave radar, also called LIDAR (light detection and ranging), is used today for communications and to measure atmospheric pollution.

The advanced radar systems developed in the 1930s played an essential role in the Battle of Britain, which raged from August through October 1940, and in which Adolf Hitler's Luftwaffe (airforce) failed to win control of the skies over England. Although the Germans had their own radar systems, throughout the rest of the war the British and the Americans were able to maintain technical superiority.

III

Operation

Radio waves travel at about 300,000 km/sec (186,000 mi/sec), or at the speed of light. Radar equipment consists of a transmitter, an antenna, a receiver, and an indicator. Unlike radio broadcasting, in which a transmitter sends out radio waves and receivers intercept them, radar transmitters and receivers are usually located in the same place. The transmitter broadcasts a beam of electromagnetic waves by means of an antenna, which concentrates the waves into a shaped beam pointing in the desired direction. When these waves strike an object in the path of the beam, some are reflected from the object, forming an echo signal. The antenna collects the energy contained in the echo signal and delivers it to the receiver. Through an amplification process and computer processing, the radar receiver produces a visual signal on the screen of the indicator, essentially a computer display monitor.

A

Transmitters

To operate radar successfully, the transmitter must emit a large burst of energy and receive, detect, and measure a tiny fraction (about a billionth of a billionth) of the total radio energy, returned in the form of an echo. One way to solve the problem of detecting the tiny echo in the presence of the enormously strong searching signal is by using the pulse system. A pulse of energy is transmitted for 0.1 to 5 microseconds; thereafter the transmitter is silent for a period of hundreds or thousands of microseconds. During the pulse, or broadcast, phase the receiver is isolated from the antenna by means of a TR (transmit-receive) switch; during the period between pulses the transmitter is disconnected from the antenna by means of an ATR (anti-TR) switch.

Continuous-wave radar broadcasts a continuous signal rather than pulses. Doppler radar, which is often used to measure the speed of an object, such as a car or a ball, transmits at a constant frequency. Signals reflected from objects that are moving relative to the antenna will be of different frequencies because of the Doppler effect. The difference in frequency bears the same ratio to the transmitted frequency as the target velocity bears to the speed of light. Thus, a target moving towards the radar at 179 km/hr (111 mph) shifts the frequency of 10-cm (3,000-MHz) radar by exactly 1 kHz.

If a radar receiver is so arranged that it rejects echoes that have the same frequency as the transmitter and amplifies only those echoes that have different frequencies, it shows only moving targets. Such a receiver can pick out vehicles moving over terrain in darkness. In this way, police measure the speed of cars.

Frequency-modulated (FM) radar broadcasts a continuous signal of uniformly changing frequency. During the time it takes a signal to be transmitted, reflected, and received, the transmitting frequency changes. The difference between the echo frequency and the transmitter frequency at the instant of echo reception is then measured and translated into the distance between transmitter and object. These systems are more accurate than the pulse type, although they operate over a shorter range.