For the last century, much of the world's scientific research has been conducted at the request of governments, and most often under the supervision of the military. The technological advances resulting from such research--from the development of the computer chip to the invention of flame-retardant fabrics--have affected nearly every sector of society. During the 1930s one such government-sponsored project was Britain's top-secret study of radio detection and ranging, now familiarly abbreviated as radar.
The properties of radio waves had been studied for many years before the idea of radio ranging emerged. In 1904, Christian Hullsmeyer, a German engineer, used radio waves in his telomobiloskop, a crude radar device. The telomobiloskop was invented to prevent collisions between ships. In 1922 Italian physicist Guglielmo Marconi noted that metallic objects reflect these waves, and in 1924 English physicist Edward Appleton (1892-1965) and Miles Barnett used radio signals to find the "mirror in the sky" known as the ionosphere (a layer of ionized particles that often reflects radio waves). The British government became intrigued with the military potential of radio in 1925, when Gregory Breit and American physicist Merle Tuve (1901-1982) introduced the pulse signal. The British Air Ministry subsequently funded a major project to develop a radio "death ray" that would immobilize enemy aircraft and would send them tumbling to earth. This concept was dismissed when Sir Robert Watson Watt took over the project; he instead suggested the use of radio for echolocation.
To find an object with echolocation, a signal must be transmitted. Since the velocity of radio waves (equivalent to the speed of light) is a constant, it is easy to calculate the distance to an object by measuring the time it takes for a radio signal to travel that distance. When one of the transmitted signals returns to the source, the back-and-forth time is recorded, as well as the direction from which it is reflected. This information is collated and displayed on a cathode-ray screen, pinpointing the exact position of the object.
The concept of echolocation was not new at the time of early radar research; Paul Langevin had just perfected his sonar system in the years following World War I. However, the rise of Nazism in Germany and the attendant threat of invasion by air motivated the British government to accelerate their research. In order to adapt sonar technology for use as radar, a chain of radiolocation stations had to be constructed which would throw an invisible net of radio waves over Britain with the ability to detect the approach and precise location of any aircraft. By 1938 Britain had built a network of radio stations along their eastern border--the direction from which Nazi bombers would arrive. When Harry Boot and John Randall invented the multicavity magnetron (the first practical microwave transmitter), the system was complete, just in time for its most important test.
In 1940 Hitler ordered a massive air strike against England. The Nazi air force, which greatly outnumbered the Royal Air Force, would certainly have defeated the British had it not been for the radar shield which foiled the Nazis' usual stealthy tactics. Soundly beaten in the daytime bombings of the Battle of Britain, Hitler's forces concentrated their efforts upon night bombings. This strategy failed as well; the radar's eye could easily locate the Nazi planes, even in darkness and poor weather. The military use of radar continued throughout World War II. Compact transmitters were developed that could be mounted on the underside of a plane to scan the ground far below for targets. Bombs and shells equipped with radar tracking systems were designed that could "look" for their targets, exploding at just the right moment.
Though radar use was confined to the military for years, radar devices began to trickle into everyday use soon after the end of World War II. In 1947 a young engineer named John Barker, while attempting to use radar to regulate traffic lights, noticed that a passing automobile would reflect a radio pulse, and that the velocity of the vehicle could then be determined by examining the returning signal. Much to the dismay of chronic speeders, Barker had devised the first radar speed-gun, now used by police and highway patrolmen worldwide. Indeed, Robert Watson-Watt himself was caught speeding in Canada in 1954 with a radar speed gun. Subsequently radar detectors for drivers, often called fuzzbusters, were invented in 1968 and became widely used in the 1980s. They detect when a police radar transmitter is working within a certain area, and communicate the distance in a series of beeps. Other radar detector, such as the Safety Warning System, can also inform drivers of any highway hazards such as accidents or construction.
The civilian use of radar is also widespread: marine navigators, surveyors, meteorologists, and astronomers all have found use for radiolocation technology. A continuous-wave version called Doppler radar is often used to track storms and hurricanes. Microwave Early Warning radar is used by air traffic controllers to help guide airplanes. Probes launched into space have also used radar to map the surfaces of other planets including Venus in the early 1960s. Radar has also been used to map Earth from space. On a space shuttle mission in 1994, an experimental radar used microwave frequencies to map a segment of Earth's vegetation in color. This radar uses three different frequencies to end up with one color picture.
The future of radar lies in the micropower impulse radar, or MIR. Invented by Tom McEwan, an electronics engineer at California's Lawrence Livermore National Laboratory, the battery-operated MIR can fit on a 1.5 inch (3.81 centimeters) small circuit board. Working within a 200 feet (about 610 meters) radius, the MIR uses a rapid fire sampler to analyze very short radio pulses. It has amazing accuracy at low power. Because it is also unaffected by noise or temperature, and is inexpensive, the MIR has potentially numerous uses. It can be placed in a car to detect if anything in its path when it backs out of a parking space or change lanes. The signal from MIR is nearly undetectable making it useful in such items as burglar alarms. The MIR also has potential applications in medical diagnostics and, of course, the military.
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