A radar altimeter measures an aircraft’s absolute altitude above the ground by directing radio waves at the ground and reading the reflected signals. It is an essential flight aid and part of the ground proximity warning system, providing accurate ground clearance and runway clearance readings. Radar altimeters have limited range but are critical for early warning of unexpected low ground conditions. They can be built into glass cockpit display units and serve as a good backup option in the event of failure of other systems.
A radar altimeter is a limited-scale aircraft altitude measuring instrument that indicates the absolute altitude or exact height of the aircraft above the ground. The instrument accomplishes this by directing the radio waves directly at the ground and reading the reflected signals. The time lapse between the transmitted and received signals is used to calculate the height above ground level. Conventional altimeters do not give absolute readings and instead measure barometric altitude, or the aircraft’s height above sea level. This makes the radar altimeter an extremely valuable piece of equipment used as a flight aid and as part of the ground proximity warning system (GPWS) of many aircraft.
Instruments used as primary altitude indicators in most aircraft depend on barometric pressure readings for their operation and only provide an accurate measurement above a pressure datum or gradient. This is typically sea level; Pilots should pay particular attention to the relationship between the known heights of terrain features and the barometric altitude returned by altimeters. When situations such as loss of orientation occur, the pilots’ lack of knowledge regarding their exact location is extremely dangerous as they have no way of knowing how high the plane is above the ground. Radar altimeters serve as essential reference aids that give pilots accurate ground clearance readings. They are also useful during landing operations where they provide accurate runway clearance readings, allowing pilots to properly widen or raise the aircraft’s nose prior to landing.
Radar is an acronym for Radio Detection and Ranging, and altimeter radar technology is based on focused radio waves transmitted vertically down to the ground. The reflected signals are recovered by a receiver on the aircraft, and the time lag between the transmitted and received signals extrapolates the absolute altitude readings. Also known as radio altimeters, these instruments return limited-scale readings with typical maximum range capabilities of 2,500 feet or 762 meters. Despite their limited range, radar altimeters are a critical part of GPWS systems and can provide pilots with early warning of unexpected low ground conditions.
Radar altimeter formats differ considerably according to the age of the system and with examples of the round analogue or tape-type type typical of older installations. Those found in modern avionics suites are usually built into glass cockpit display units and feature sophisticated GPWS functionality. Due to the lack of reliance on the ram and air-static systems in the aircraft, the radar altimeter is also a good backup option in the event of failure of those systems.
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