A radar (radio detection and ranging) system that detects long-range objects and determines their position by measuring the time taken for radio waves to travel to the objects, be reflected, and return. Radar is used for navigation, air control, fire control, storm detection, in radar astronomy, and for catching speeding drivers. It developed out of experiments in the 1920s measuring the distance of the ionosphere by radio pulses. Robert Watson-Watt showed that the technique could be applied to detecting aircraft, and from 1935 Britain installed a series of radar stations which were a major factor in winning the Battle of Britain in World War II. From 1940 the UK and the US collaborated to develop radar.
There are two main types of radar: continuous-wave radar, which transmits continuously, the frequency being varied sinusoidally, and detects the signals received by their instantaneously different frequency; and the more common pulsed radar. This latter has a highly directional antenna which scans the area systematically or tracks an object. A cavity magnetron or klystron emits pulses, typically 400 per second, 1 μs across, and at a frequency of 3 GHz. A duplexer switches the antenna automatically from transmitter to receiver and back as appropriate. The receiver converts the echo pulses to an intermediate frequency of about 30 MHz, and they are then amplified, converted to a video signal, and displayed on a cathode-ray tube. A synchronizer measures the time-lag between transmission and reception, and this is represented by the position of the pulse on the screen. Various display modes are used: commonest is the plan-position indicator (PPI), showing horizontal position in polar coordinates.
Related entry LORAN
Related category INSTRUMENTATION
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