Radar

Radar

 Radar (ra dio d etection a nd r anging) is a device that obtains information about an object of interest (eg, distance, position) by emitting an electronic signal and observing the echo returned from the object. The key elements of radar systems are the transmitter, the receiver and the data-processing unit. Radars use radio-frequency transmissions, first described theoretically by J.C. Maxwell in 1864.

Radio waves are part of the electromagnetic spectrum, as is visible light. H.R. Hertz provided practical demonstration of Maxwell's theory and, in 1888, actually performed radio-wave-reflection experiments. In 1900 Nikola Tesla suggested that moving targets should be observable through radio-frequency shifts (predicted by C.J. Doppler in 1842 for waves in general). The first patent for a rudimentary radar was issued in Germany in 1904. It required the threat of World War II to motivate real progress.

In 1934 Robert Watson-Watt proposed to the British government the principle of aircraft detection by ground-based pulsed radar. By spring 1935 an experimental system had proved successful, leading to construction of the British Home Chain. Five stations facing Europe were operational in 1938. The system is credited with making the decisive difference in the Battle of BRITAIN.

Development of radar exploded during WWII, aided by high-level technical exchange agreements among the Allies. Scientists at the NATIONAL RESEARCH COUNCIL OF CANADA participated, and A.E. COVINGTON in 1946 used surplus radar research equipment to construct Canada's first radio telescope (see OBSERVATORY). The range and sensitivity of radars were constantly improved and smaller units were built for shipborne and airborne use. In 1935 radars worked at wavelengths of 25 m, and by 1940 at 10 cm, thus initiating microwave radar.

The critical development was that of the cavity magnetron by John T. Randal and Henry A. Boot of Birmingham University (1939-41). The device was capable of generating microwave pulses at up to 500 kW and permitted construction of small but, for the time, very accurate radars. Development of radar by the Allies was considered classified information during WWII. Therefore, wavelengths received code names, still recognized today: L-band (25 cm), S-band (10 cm), C-band (5 cm), X-band (3 cm) and K-band (1 cm). Shorter waves are used by "millimetre radars."

Operation

Radar devices emit energy within a beam shaped by the radiating antenna. The beam-limited radiated energy propagates in range at the speed of light. When it strikes an object, a small fraction of the energy is reflected back, to be received by the radar. The echo arrives at a time delay after transmission; delays are usually very short (for terrestrial radars) and are measured in microseconds. Since the speed of light is known, each microsecond (0.000001 second) of delay corresponds to 150 m in range. The ranging pulse must be short and powerful to detect reliably several small, closely spaced objects, such as aircraft in formation. These properties are specified by the range resolution (usually given in metres) and the sensitivity (usually given in m²); both factors are "better" for smaller numbers. A good radar may be described as "high resolution" because it is able to separate targets only a few metres apart.

Types of Radars

Today there are many radar systems, from the speed-measuring units used by police forces (costing a few thousand dollars each) to RADARSAT II (costing approximately $600 million). RADARSAT II, built by a consortium of companies led by Canada's Spar Aerospace, is the next generation of remote-sensing SATELLITE; it will be a "super-camera" capable of taking photographs of small objects (about 3 m in length, or the size of a car) any place on Earth from space, through clouds or at night. Like the earlier RADARSAT I, it will be used for ecological surveys, mapping remote areas, oil exploration and monitoring ice floes. RADARSAT II is scheduled to be launched in 2002. Military applications, especially surveillance, targeting, guidance, navigation and EARLY-WARNING, are the most important uses. Canada and the US continue to co-operate in the area of ballistic missile defence. For example, BMEWS (Ballistic Missile Early Warning System) has major radar electronics and antennas sited in Canada's North, yet the system's data-processing centre is an underground control centre in Colorado, which has access to the information from many other systems, including AWACS (Airborne Warning and Control System).

In the civilian area, the most important application is in air-traffic control. Ship navigation and maritime vessel traffic-management radars are essential for safe operations on Canada's waterways. Other applications include weather and storm-centre tracking, atmospheric sounding and radar ASTRONOMY.

Radar Remote Sensing

Since 1976 Canada has developed a world-recognized reputation in certain radar techniques, taking a lead in the application of radar to the observation and monitoring of vast areas (see REMOTE SENSING). In addition to routine mapping of ice in the Arctic and North Atlantic by airborne radar, Canada benefited from the US Seasat, an experimental satellite (July-October 1978) that carried the first imaging radar into Earth orbit. This was an L-band, high-resolution system, capable of sweeping out a continuous image 100 km wide. The instrument was a synthetic-aperture radar (SAR), the operating principles of which are closely related to holography. The data could be processed optically from the image.

Canada developed the first digital processor for such data; MacDonald, Dettwiler and Associates of Vancouver published the first image in November 1978. This technology continues to set the world standard. Airborne and spaceborne SAR radar technologies are areas of active development in Canada and promise to contribute to RESOURCE MANAGEMENT. The Canada Centre for Remote Sensing has developed airborne X- and C-band SARs, and the STAR 1 and 2 radars operated by Intera of Calgary are the best civilian imaging radars in the world, used to monitor Canada's Arctic ice, map cloud-covered tropical regions, such as Indonesia, and for many other applications. See also SONAR.