Radar altimeter
A radar altimeter, radio altimeter 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.
Principle
For more information, please see: Radar#Radar signal processing
As the name implies, radar (radio detection and ranging) is the underpinning principle of the system. Radiowaves 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 radiowave and the time it takes to travel are known quantities.
Alternatively, the change in frequency of the wave can be measured, the greater the 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.
Civil applications
Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions (see instrument flight rules, Autoland) and also automatic landings, allowing the autopilot to know when to begin the flare manoeuvre.
In aviation applications, radio altimeters generally only give readings up 5,000', allowing TCAS to receive inputs for IFF outputs. Today, almost all airliners are equipped with at least one and usually several radio altimeters, as they are essential to autoland capabilities (determining height through other methods such as GPS is not permissible under current legislation). Even older airliners from the 1960s, such as Concorde and the 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 essential 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.
It is interesting to note that whilst they are called altimeters, the information they provide is not called altitude in aviation; altitude is specifically height above sea level which is usually obtained from a pressure altimeter. The term height when used in aviation refers to the height above the terrain directly below the aircraft, that from a radio altimeter, in order to avoid confusion. "Radar-altitude" is used on some instrumentation but for communication purposes, the term height is always used.
Radar altimeters normally work in the E-band, or Ka 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.
Military applications
Radar altimeters are also used in military aircraft flying extremely low over terrain to avoid radar detection and targeting by anti-aircraft artillery or Surface-to-air Missiles. Radar altimeter technology is also used in terrain-following radar allowing fighter aircraft to fly at very low altitude.