With the end of the Cold War, a lot of previously classified military technology is making its way into civilian hands. Spy satellites whose very existence was top secret, for instance, are now being used to survey crops.

This post-war transfer of technology from military to civilian use is not new: it happened after the Second World War, too. One of the military technologies that moved into widespread use was radar.

“Radar” is derived from “radio detection and ranging.” A high-frequency transmitter sends out a beam of radio waves. When these waves hit a distant object, a portion of them bounce back toward the transmitter. These returning waves are intercepted by an antenna (traditionally a very large, dish-shaped antenna, because the returning signal is extremely weak). Radio waves travel at a known speed, the speed of light (300,000 kilometres per second), so the time elapsed between the emission of the initial pulse of radio energy and the return of the echo tells you the reflecting object’s distance.

The first practical radar system was produced in 1935 by the British physicist Sir Robert Watson-Watt. By the outbreak of the Second World War radar had also been developed independently by the U.S., Germany and France, but the British, thanks to the 1939 invention of the “resonant cavity magnetron” by Henry Boot and John T. Randall, had microwave radar, a more powerful form of radar that probably saved the country in the Battle of Britain.

With the end of the war, radar found numerous uses in civilian life, such as air traffic control (where it was welcomed) and police traffic control (where it was not). Police radar measures, not just the distance to an object, but how fast that object is moving.

It does that by measuring what’s called the “Doppler shift” in frequency between the initial signal and the echo. If an object is moving toward the radar gun, the returning echo is at a slightly higher frequency because the echo’s waves have been squashed closer together by the approach of the object. The echo of an object moving away from the transmitter is at a lower frequency than the original beam because its waves have been elongated by the object’s retreat. (It’s the same effect that causes the sound of a train to deepen as it roars past you and away.)

By measuring the change in frequency, you can precisely determine the speed of the object. If you take your own speed into account (which modern police radar guns do), you can even do so from a moving vehicle.

Police radar has been refined over the years in the never-ending quest to capture speeders. There are now two main kinds: the older “down-the-road” type, which projects a wide radar beam into oncoming traffic, and the newer “across-the-road” type, which projects a narrow beam across the road. Down-the-road radar has difficulty targetting a single vehicle and is more vulnerable to interference. Across-the-road radar’s narrower beam permits more accurate targetting of vehicles.

Radar guns gave the police an advantage over ever-in-a-hurry motorists, so naturally, someone came up with a radar detector.

There is a “window” within the range of any radar where the echoes from objects struck by the outgoing beam are too weak to be detected. Detect the outgoing beam within that window, and you might just have time to slow down before your echo becomes strong enough to register on the police radar. (Of course, tit for tat, there are now radar detector detectors for police; radar detectors put out their own distinctive radio emissions.)

The latest technologies in police hands are photo radar and lasers. Photo radar devices consist of an unmanned radar gun and a camera. When the radar gun detects a speeding vehicle, it activates a camera, which snaps a picture of the car’s license plate as it zooms past. A ticket is automatically generated and sent to the owner of the car.

The laser gun uses light signals instead of radio waves, but otherwise works the same as radar. The very tight beam of the laser allows for very precise targetting of vehicles–and since it doesn’t emit radio waves, it’s undetectable by standard radar detectors.

Radar has found many other civilian uses in the post-war world than the pursuit of speeders, but it’s safe to say that, even in this time of peace, radar will continue to be used in war–the ongoing war between those who would drive fast and those who would stop them.

Permanent link to this article: https://edwardwillett.com/1995/08/radar/

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