Introduction
RADAR is an acronym made from RAdio Detection And Ranging. The name to this electronic system was given during World War II. Its basic principle comprises emitted radio waves which bounce off a target in order to detect its presence and allow locating its position.
Like many modern technical achievements military use was the initial spark to radar. The development of the earliest practical radar system is credited to Sir Robert Alexander Watson-Watt, although a large number of scientists contributed numerous technical details.
Technical Principle
A radio transmitter generates radio waves, which are then radiated from an antenna. A target in this area scatters a small portion of this radio energy back to a receiving antenna. This weak signal is amplified by an electronic amplifier and displayed on a cathode-ray tube (CRT), where it can be studied by a radar operator. By this the presence of a target has been detected, but to determine its position the target's distance (range) and bearing must be measured. Because radio waves travel at a known constant velocity - the speed of light, which is 300,000 km/sec - the range may be found by measuring the time taken for a radio wave to travel from transmitter to target and back to the receiver. For example, if the range were 300 kilometers, the time for the round trip would be (2 X 300 km) 600 km : 300,000 km/sec = two-thousandths of a second, or 2,000 microseconds.
Historical Development
The roots of radar can be traced back to the year 1886 when Heinrich Hertz, a German, verified Maxwell's electromagnetic theory by showing that shortwave radiation (60 centimeters in length) could be reflected from metallic and dielectric bodies. Nearly two decades later a fellow German, Christian Huelsmeyer, attempted to develop a proximity warning system for ships, so that these maintained knowledge of each other's location during bad weather and night. Still later the the technology generated by these experiments was found to be applicable to weather forecasting by Britain's Sir Robert Alexander Watson-Watt. He discovered that the electro-magnetic acitivity in storms could be detected by basically the same equipment. Heinrich Hertz conceived the idea of a directional loop antenna to take advantage of the phenomenon, and thus permit the relatively precise location of bad weather and its general direction of movement.
During 1940-41 the long-wave radar systems used in Britain against German aircraft measured range well but were much less accurate when it came to detecting direction, because the radiated beams were very wide. So the "net of radio beams" consisted of a fairly loose mesh. The key to this problem's solution was found in reducing the wavelength of radio waves. It became possible to build antennas to form narrow beams that could be rotated like a lighthouse beam. Only when the targeted aircraft lay within the beam would a radar echo be received; thus with such a narrow-beam system the bearing could be observed directly. Another advance in radar was duplexing - switching methods that allowed the same antenna to be used for both transmission and reception. In addition, the echo signals were now displayed on a CRT that used a radial time-base that rotated in synchronism with the aerial; therefore, the scattering targets appeared in their correct plan positions relative to the radar station, and this form of display was named the plan position indicator (PPI). The most important advance made during this phase of radar development, however, was the invention of the cavity magnetron, a device for generating high-power microwave pulses, by Sir John T. Randal and Henry A. Boot in 1940.
The great operational advantage of microwave radars during World War II was that they were relatively free from electronic counter measures (ECM) by the enemy. Electronic warfare has now become a major threat to military radar systems, and modern radars have to be designed to reduce the effects of ECM. For example, antennas have been developed with increased resolving power but with very low side lobes so that active jamming cannot penetrate into the receiver as readily as with earlier systems. Simultaneously, the effect of passive jamming is reduced: the observation of false targets because of backscatter from "chaff" - falling clouds of scattered tinfoil strips - is reduced.
Advances in Radar
Modern radar also provides excellent moving-target indication (MTI) by use of the Doppler shift in frequency that a radio wave undergoes when it is reflected from a moving target. Target detection is hindered by "clutter" echoes arising from backscatter from the ground or raindrops. The modern radar, with its higher transmitter power and more sensitive receiver, causes clutter to be even more pronounced so that even flocks of birds may show up on the screen. Antenna design can reduce these effects, and the use of circularly polarized waves reduces rain echoes. The wartime radar operator interpreted the mass of data displayed on his PPI. The tracing of the histories of many targets simultaneously, however, which is what is needed in modern civil or military air-traffic control, requires that the incoming radar data be electronically processed to make it more accessible to the controller for the task of airspace management. Progress toward satisfying this need had to await the arrival of large-scale integrated circuits and charge-coupled devices and the development of the technology for processing digital signals. Another important advance has been the development of computerized handling of video data, as in automatic plot extraction and track formation.
Applications
Although radar was first developed as a military aid, it has proved to be a very effective sensing and measuring device for use in many civil systems and in many fields of scientific research. It is employed for the blind landing of aircraft and for airport surface surveillance, and it is used in aircraft for cloud and collision warning. Other civil uses include merchant-ship navigation and docking radar, highway traffic control, and security systems. Scientific applications include lunar and planetary studies; meteorological instrumentation for studying clouds and precipitation; measurement of the thickness of ice sheets from aircraft; satellite surveys of the Earth's surface; and ionospheric and magnetospheric investigations. Radar is now also making a major contribution to certain behavioral studies in biology, for example, the migration and flight behavior of birds, observations of the swarming of insects, crop protection in agriculture, and investigations relating to the acoustic echolocation system of bats.
Under construction, will be completed soon!
This page is hosted by GeoCities.