A radar altimeter, radio altimeter, low range radio altimeter (LRRA) or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft. This type of altimeter provides the distance between the plane and the ground directly below it, as opposed to a barometric altimeter which provides the distance above a pre-determined datum, usually sea level.
- Main article: Radar signal processing
As the name implies, radar (radio detection and ranging) is the underpinning principle of the system. Radio waves are transmitted towards the ground and the time it takes them to be reflected back and return to the aircraft is timed. Because speed, distance and time are all related to each other, the distance from the surface providing the reflection can be calculated as the speed of the radio wave and therefore the time it takes to travel a distance are known quantities.
Alternatively, Frequency Modulated Continuous-wave radar can be used. The greater the frequency shift the further the distance travelled. This method can achieve much better accuracy than the aforementioned for the same outlay and radar altimeters that use frequency modulation are industry standard.
In 1924, American engineer Lloyd Espenschied invented the radio altimeter. However, it took 14 years before Bell Labs was able to put Espenschied's device in a form that was adaptable for aircraft use. In 1938 in co-operation with Bell Labs, United Air Lines fitted a radar type device to some of its airliners as a terrain avoidance device.
Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions (see instrument flight rules) and also automatic landings (autoland), allowing the autopilot to know when to begin the flare maneuver.
In civil aviation applications, radio altimeters generally only give readings up to 2,500 feet (760 m) above ground level (AGL).
Today, almost all airliners are equipped with at least one and usually several radar altimeters, as they are essential to autoland capabilities (determining height through other methods such as GPS(Global Positioning System) is not permissible under current legislation). Even older airliners from the 1960s, such as Concorde and the British Aircraft Corporation BAC 1-11 were so equipped and today even smaller airliners in the sub-50 seat class are supplied with them (such as the ATR 42 and BAe Jetstream series).
Radio altimeters are an essential part in ground proximity warning systems (GPWS), warning the pilot if the aircraft is flying too low or descending too quickly. However, radar altimeters cannot see terrain directly ahead of the aircraft, only that directly below it; such functionality requires either knowledge of position and the terrain at that position or a forward looking terrain radar which uses technology similar to a radio altimeter. Radio altimeter antennas have a fairly large main lobe of about 80° so that up to bank angles of about 40°, the radio altimeter detects the range from the aircraft down to the ground, specifically to the nearest large reflecting object. This is because range is calculated based on the first signal return from each sampling period. It does not detect slant range until beyond about 40° of bank or pitch. This is not an issue for landing as pitch and roll do not normally exceed 20° or so during approach and landing.
It is interesting to note that the altitude specified by the device would not match the altitude read from the standard altimeter the pilot uses. This is because aviation is centered around True altitude, the height above Mean Sea Level (MSL), and the radio altimeter measures Absolute altitude, the height Above Ground Level (AGL). Absolute altitude is sometimes referred to as height since it is the height above the terrain directly below the aircraft, that which is provided from a radio altimeter.
Radar altimeters normally work in the E band, or Ka band or S bands for more advanced sea-level measurement. Radar altimeters also provide a reliable and accurate method of measuring height above water, when flying long sea-tracks. These are critical for use when operating to and from oil rigs.
Radar altimeters are also used in military aircraft to fly quite low over the land and the sea to avoid radar detection and targeting by anti-aircraft guns or surface-to-air missiles. A related use of radar altimeter technology is terrain-following radar, which allows fighter bombers to fly at extremely low altitudes.
As an example of their use, the F-111s of the Royal Australian Air Force and the U.S. Air Force had a forward-looking, terrain-following radar (TFR) system connected via digital computer to their automatic pilots. Beneath the nose radome, were two separate TFR antennae, each providing individual information to the dual-channel TFR system. In case of a failure in that system, the F-111s also had a back-up radar altimeter system built in, and also connected to the plane's automatic pilot. This radar altimeter was programmed with a preset minimum altitude (for example, 15 meters). Then, if the F-111 ever dipped below this altitude for any reason, its automatic pilot would be commanded to put the F-111 into a 2G fly-up, a steep nose-up climb to avoid crashing into the ground or into the sea. Even in combat, the fatal hazard to the aircrew of a collision with the ground or with the sea is far greater than the danger of being detected by the enemy.
Similar systems are now used by the F-15E Strike Eagle and F/A-18 Super Hornet warplanes flown by the Americans and by the Australians.
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