Civil Aviation Applications
Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions (see instrument flight rules) and automatic landings, allowing the autopilot to know when to begin the flare maneuver. Radar altimeters give data to the autothrottle which is a part of the Flight Computer.
Radar altimeters generally only give readings up to 2,500 feet (760 m) above ground level (AGL). Frequently, the weather radar can be directed downwards to give a reading from a longer range, up to 60,000 feet (18,000 m) above ground level (AGL). As of 2012, almost all airliners are equipped with at least one and usually several radar altimeters, as they are essential to autoland capabilities. (As of 2012, determining height through other methods such as GPS is not permitted by regulations.) Older airliners from the 1960s (such as the British Aircraft Corporation BAC 1-11) and smaller airliners in the sub-50 seat class (such as the ATR 42 and BAe Jetstream series) are equipped with them.
Radar 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 below it; such functionality requires either knowledge of position and the terrain at that position or a forward looking terrain radar. Radar altimeter antennas have a fairly large main lobe of about 80° so that at bank angles up to about 40°, the radar detects the range from the aircraft 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°.
The altitude specified by the device is not the indicated altitude of the standard barometric altimeter. Aviation is primarily referenced to true altitude - the height above Mean Sea Level (MSL). A radar altimeter measures absolute altitude - the height Above Ground Level (AGL). Absolute altitude is sometimes referred to as height because it is the height above the underlying terrain.
Read more about this topic: Radar Altimeter
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