Radar

:This article is about the device. For the fictional character in M*A*S*H (movie) and M*A*S*H (TV series), see Corporal Walter (Radar) O'Reilly.

Position measurement

Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located.

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Early systems

:main article History of radar

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Early systems tended to use omni-directional broadcast antennas, with directional receiver antennas which were pointed in various directions. For instance the first system to be deployed, Chain Home, used two straight antennas at right angles for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by rotating the antenna so one display showed a maximum while the other shows a minimum.

Related Topics:
Omni-directional broadcast - Chain Home - Right angle - Rotating

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One serious limitation with this type of solution is that the broadcast is sent out in all directions, so the amount of energy in the direction being examined is subject to the inverse-square law. To get a reasonable amount of power on the "target", the broadcast should also be steered. More modern systems used a steerable parabolic "dish" to create a tight broadcast beam, typically using the same dish as the receiver. Such systems often combined two radar frequencies in the same antenna in order to allow automatic steering, or radar lock.

Related Topics:
Inverse-square law - Parabolic

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Phased array

Another method of steering is used in a phased array radar, which uses the radio signal's interference with itself. If one were to broadcast a single signal from a large number of antennas, the result will be a single beam with the waves in the rest of space cancelling each other. If the phase of the signal is changed before broadcast, the direction of the beam can be moved because the point of constructive interference will move. Instead of constructing a single large antenna, such a system has a number of small omni-directional antennas referred to as elements, usually arranged in a flat plane.

Related Topics:
Phased array - Space

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Phased array radars require no physical movement. The beam can be steered by electronically controlling devices called phase-shifters that drive the phase of each small radiating element. This means that the beam can scan at thousands of degrees per second, fast enough to irradiate many individual targets, and still run a wide-ranging search periodically. By simply turning some of the antennas on or off, the beam can be spread for searching, narrowed for tracking, or even split into two or more virtual radars.

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Phased array radars have been in use since the earliest years of radar use in World War II, but limitations of the electronics led to fairly poor accuracy. Phased array radars were originally used for missile defence. They are the heart of the ship-bourne Aegis combat system, and the Patriot Missile System, and are increasingly used in other areas because the lack of moving parts makes them more reliable, and sometimes permits a much larger effective antenna.

Related Topics:
World War II - Missile - Defence - Aegis combat system - Patriot Missile System

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As the price of electronics has fallen, phased array radars have become more and more common. Almost all modern military radar systems are based on phased arrays, where the small additional cost is far offset by the improved reliability of a system with no moving parts. Traditional moving-antenna designs are now limited to roles where cost is the main factor such as weather radars and similar systems.

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Phased array radars are also valued for use in aircraft, since they can track multiple targets. The first aircraft to use phased array radar was the Mikoyan MiG-31. The MiG-31M's SBI-16 Zaslon phased array radar is considered to be the world's most powerful fighter radar.

Related Topics:
Mikoyan MiG-31 - Zaslon

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