Radar

C

1

Air Defense

A typical surface-based air defense system relies upon several radar systems. First, a lower frequency radar with a high-powered transmitter and a large antenna searches the airspace for all aircraft, both friend and foe. A secondary radar system reads the transponder signals sent by each aircraft to distinguish between allies and enemies. After enemy aircraft are detected, operators track them more precisely by using high-frequency waves from special fire control radar systems. The air defense system may attempt to shoot down threatening aircraft with gunfire or missiles, and radar sometimes guides both gunfire and missiles (see Guided Missiles).

Longer-range air defense systems use missiles with internal guidance. These systems track a target using data from a radar system on the missile. Such missile-borne radar systems are called seekers. The seeker uses radar signals from the missile or radar signals from a transmitter on the ground to determine the position of the target relative to the missile, then passes the information to the missile’s guidance system.

The military uses surface-to-air systems for defense against ballistic missiles as well as aircraft (see Defense Systems). During the Cold War both the United States and the Union of Soviet Socialist Republics (USSR) did a great deal of research into defense against intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs). The United States and the USSR signed the Anti-Ballistic Missile (ABM) treaty in 1972. This treaty limited each of the superpowers to a single, limited capability system. The U.S. system consisted of a low-frequency (UHF) phased-array radar around the perimeter of the country, another phased-array radar to track incoming missiles more accurately, and several very high speed missiles to intercept the incoming ballistic missiles. The second radar guided the interceptor missiles.

Airborne air defense systems incorporate the same functions as ground-based air defense, but special aircraft carry the large area search radar systems. This is necessary because it is difficult for high-performance fighter aircraft to carry both large radar systems and weapons.

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Modern warfare uses air-to-ground radar to detect targets on the ground and to monitor the movement of troops. Advanced Doppler techniques and synthetic aperture radar have greatly increased the accuracy and usefulness of air-to-ground radar since their introduction in the 1960s and 1970s. Military forces around the world use air-to-ground radar for weapon aiming and for battlefield surveillance. The United States used the Joint Surveillance and Tracking Radar System (JSTARS) in the Persian Gulf War (1991), demonstrating modern radar’s ability to provide information about enemy troop concentrations and movements during the day or night, regardless of weather conditions.

C

2

Countermeasures

The military uses several techniques to attempt to avoid detection by enemy radar. One common technique is jamming—that is, sending deceptive signals to the enemy’s radar system. During World War II (1939-1945), flyers under attack jammed enemy radar by dropping large clouds of chaff—small pieces of aluminum foil or some other material that reflects radar well. “False” returns from the chaff hid the aircraft’s exact location from the enemy’s air defense radar. Modern jamming uses sophisticated electronic systems that analyze enemy radar, then send out false radar echoes that mask the actual target echoes or deceive the radar about a target’s location.

Stealth technology is a collection of methods that reduce the radar echoes from aircraft and other radar targets (see Stealth Aircraft). Special paint can absorb radar signals and sharp angles in the aircraft design can reflect radar signals in deceiving directions. Improvements in jamming and stealth technology force the continual development of high-power transmitters, antennas good at detecting weak signals, and very sensitive receivers, as well as techniques for improved clutter rejection.

D

Traffic safety

Since the 1950s, police have used radar to detect motorists who are exceeding the speed limit. Most older police radar “guns” use Doppler technology to determine the target vehicle’s speed. Such systems were simple, but they sometimes produced false results. The radar beam of such systems was relatively wide, which meant that stray radar signals could be detected by motorists with radar detectors. Newer police radar systems, developed in the 1980s and 1990s, use laser light to form a narrow, highly selective radar beam. The narrow beam helps insure that the radar returns signals from a single, selected car and reduces the chance of false results. Instead of relying on the Doppler effect to measure speed, these systems use pulse radar to measure the distance to the car many times, then calculate the speed by dividing the change in distance by the change in time. Laser radar is also more reliable than normal radar for the detection of speeding motorists because its narrow beam is more difficult to detect by motorists with radar detectors.

E

Meteorology

Meteorologists use radar to learn about the weather. Networks of radar systems installed across many countries throughout the world detect and display areas of rain, snow, and other precipitation. Weather radar systems use Doppler radar to determine the speed of the wind within the storm. The radar signals bounce off of water droplets or ice crystals in the atmosphere. Gaseous water vapor does not reflect radar waves as well as the liquid droplets of water or solid ice crystals, so radar returns from rain or snow are stronger than that from clouds. Dust in the atmosphere also reflects radar, but the returns are only significant when the concentration of dust is much higher than usual. The Terminal Doppler Weather Radar can detect small, localized, but hazardous wind conditions, especially if precipitation or a large amount of dust accompanies the storm. Many airports use this advanced radar to make landing safer.

F

Scientific Applications

Scientists use radar in several space-related applications. The Spacetrack system is a cooperative effort of the United States, Canada, and the United Kingdom. It uses data from several large surveillance and tracking radar systems (including the Ballistic Missile Early Warning System) to detect and track all objects in orbit around the earth. This helps scientists and engineers keep an eye on space junk—abandoned satellites, discarded pieces of rockets, and other unused fragments of spacecraft that could pose a threat to operating spacecraft. Other special-purpose radar systems track specific satellites that emit a beacon signal. One of the most important of these systems is the Global Positioning System (GPS), operated by the U.S. Department of Defense. GPS provides highly accurate navigational data for the U.S. military and for anyone who owns a GPS receiver.

During space flights, radar gives precise measurements of the distances between the spacecraft and other objects. In the U.S. Surveyor missions to the moon in the 1960s, radar measured the altitude of the probe above the moon’s surface to help the probe control its descent. In the Apollo missions, which landed astronauts on the moon during the 1960s and 1970s, radar measured the altitude of the Lunar Module, the part of the Apollo spacecraft that carried two astronauts from orbit around the moon down to the moon’s surface, above the surface of the moon. Apollo also used radar to measure the distance between the Lunar Module and the Command and Service Module, the part of the spacecraft that remained in orbit around the moon.

Astronomers have used ground-based radar to observe the moon, some of the larger asteroids in our solar system, and a few of the planets and their moons. Radar observations provide information about the orbit and surface features of the object.

The U.S. Magellan space probe mapped the surface of the planet Venus with radar from 1990 to 1994. Magellan’s radar was able to penetrate the dense cloud layer of the Venusian atmosphere and provide images of much better quality than radar measurements from Earth.

Many nations have used satellite-based radar to map portions of the earth’s surface. Radar can show conditions on the surface of the earth and can help determine the location of various resources such as oil, water for irrigation, and mineral deposits. In 1995 the Canadian Space Agency launched a satellite called RADARsat to provide radar imagery to commercial, government, and scientific users.

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