Radar
Radar is an acronym for radio detection and ranging. It is a system used to detect, range and map various arial objects such as aircraft and rain. Strong radio signals tuned to be reflected from the source of interest are sent out into the air, and another antenna listens for their echos being reflected back. Through various analisys of the reflected signal, details of the source can be established.
History
The effect that radar relies on is the reflection of radio signals by metallic objects. This was first
Radar was invented by Sir Robert Watson-Watt in the early 1930s and its use in meteorology was patented by him in 1935. However its use in detecting approaching aircraft was also foreseen and both England and Germany started projects to deploy radar.
Shortly before the outbreak of World War II several radar stations known as Chain Home were constructed in the south of England. They proved highly effective during the Battle of Britain, and are often credited with allowing the RAF to defeat the much larger Luftwaffe forces. Whereas the Luftwaffe had to hunt all over to find the RAF fighters, the RAF on the other hand knew exactly where the Luftwaffe bombers where, and could converge all of their fighters on them.
As one might expect from the first radar to be deployed, CH was a simple system. The broadcast side was formed from two 300' tall steel towers formed with a series of cables of varying lengths that were strung between them. The output of a powerful 50MHz radio of about 200kW (up to 800kW) was fed into these cables, pulsed at about 50 times a second. A second set of 240' tall wooden towers were used for reception, with a series of crossed antennas at various heights up to 215'. In fact most stations had more than one set of each antenna, tuned to operate at different frequencies.
The radar was operated with an oscilloscope. When a pulse was sent out into the broadcast towers, the scope was triggered to start its beam moving across the screen very rapidly. The output from the receiver was fed into the scope, and a reflection from an aircraft would deflect the beam upward. This formed a spike on the display, and the distance from the left side - measured with a small scale on the bottom of the screen - would give the distance to the target. By rotating the receiver antennas to make the display "go away" the operator could determine the direction (this is the reason for the cross shaped antennas), the size of the vertical displacement indicated something of the number of aircraft involved, and by comparing the strengths returned from the various antennas up the tower, you could determine the height.
The next major development in the history of radar was the invention of the cavity magnetron by Randall and Boot of the Birmingham University. This was a small device which generated much greater amounts of microwaves than previous devices, which in turn allowed for the detection of much smaller objects and the use of much smaller antennas. The secrecy of the device was so high that it was decided in 1940 to move production to the USA, which resulted in the creation of the MIT Radiation Lab to develop the device further.
The magnetron had the size and power to be fitted into the nose of an aircraft, and vigorous efforts were made to develop an AI (airborne interception) set. Initial sets were available in 1941 and fitted to Bristol Blenheim aircraft, replaced quickly with the better performing Bristol Beaufighter, which quickly put an end to German night and bad weather bombing over England.
Description
It uses electromagnetic waves with wavelengths which vary from the centimeter to the decimeter range.
To find the distance of an object, the time taken between sending out the wave and receiving a radio echo from that object is measured.
To measure the speed of an object, the Doppler effect is used.
When extremely high sensitivity is needed, the pulse is extended, and given a changing frequency. Reflections are then correlated by frequency to develop the same information as if a much shorter pulse delivered the same energy as the extended pulse. This is called "chirped" radar.
Specific radar systems
See also: