The radar altimeter, also known as a radio altimeter, measures an aircraft's height above the terrain. A properly calibrated radar altimeter will read zero just as the aircraft touches ground. Radar altimeters operate in the C-band at 4.3 gigahertz and are generally used when an aircraft is flying low, within 2,500 feet of the terrain. The radar altimeter system consists of a transceiver, transmitting and receiving antennas and an altitude indicator.
In use, a radar altimeter's transmitting antenna emits a radio signal that travels to the ground and bounces back to the receiving antenna. Processing circuits measure the time interval between signal transmission and reception and translate that interval into an altitude reading. Radar altimeters can include audible alerts to the pilot when the aircraft reaches a preset altitude. They also can be interconnected with other aircraft systems such as the automatic pilot or ground proximity warning systems.
The barometric altimeter measures an aircraft's height above sea level, based on mathematical models of how air pressure and temperature decrease with altitude. The device consists of a barometric capsule that measures air pressure. The capsule is connected to a mechanical or electronic processor that translates the readings into an altitude reading on an indicator. The models are based on a standard sea level pressure of 29.92 inches of mercury and standard temperature of 59 degrees Fahrenheit.
Because air pressure varies from place to place, the pilot must adjust the barometric altimeter to the barometric pressure at the departure airport several times en route, and again when approaching the arrival airport. Flight rules require altimeter adjustment every 100 miles of flight when flying below 18,000 feet. The pilot can't adjust the barometric altimeter readings for temperature even though temperature will affect the readings. In most cases, temperature errors are insignificant, but when air temperature at the arrival airport drops below freezing, the aircraft will be lower than the indicated altitude. This can be dangerous unless the pilot makes an appropriate altitude adjustment.
