A radar altimeter is a limited scale aircraft altitude measurement instrument that indicates absolute altitude or the aircraft's exact height above the ground. The instrument achieves this by directing radio waves directly down at the ground and reading the reflected signals. The time lapse between transmitted and received signals is then used to calculate the height above ground level. Conventional altimeters do not give absolute readings and instead measure barometric altitude or the height of the aircraft above sea level. This makes the radar altimeter an extremely valuable piece of equipment utilized both as a flight aid and as part of many aircrafts' ground proximity warning system (GPWS).
The instruments used as primary altitude indicators in most aircraft are dependent on barometric pressure readings for their operation and only give an exact measurement above a given datum or pressure gradient. This is typically sea level; pilots have to pay special attention to the relationship between the known heights of ground features and the barometric altitude returned by the 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 aircraft is above the ground. Radar altimeters serve as essential referential aids giving the pilots exact ground clearance readings. They are also useful during landing operations where they supply accurate runway clearance readings, thereby allowing the pilots to flare or lift the aircraft's nose correctly prior to touchdown.
Radar is an acronym for Radio Detection and Ranging, and radar altimeter technology relies on focused radio waves transmitted vertically down towards the ground. The reflected signals are retrieved by a receiver in the aircraft, and the time lapse between transmitted and received signals extrapolates 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. Notwithstanding their limited range, radar altimeters are a critical part of GPWS systems and can supply pilots with lifesaving early warning of unexpected ground proximity situations.
Radar altimeter formats differ considerably according to the age of the system and with analog type round or ribbon dial examples typical of older installations. Those found in modern avionic suites are usually integrated into glass cockpit display units and feature sophisticated GPWS functionality. Due to the lack of reliance on ram and static air systems in the aircraft, the radar altimeter is also an good backup option in the case of failures of those systems.