Also known as a radar altimeter, a radio altimeter is a device that is used to measure the distance beneath the craft in which it is mounted, usually an airplane or helicopter, and the ground. Unlike a barometric altimeter, which measures altitude from a given height, typically sea level, a radio altimeter measures the distance between the aircraft and the terrain directly below it. Invented in the early 1920, radio altimeters did not see widespread use until much later, but are now common in commercial and amateur flight for landing maneuvers and low-altitude navigation.
Functionally, a radio altimeter works in one of two ways. The original and underlying method is by transmitting radio waves downward, and interpreting the time it takes for the waves to reflect back as a function of distance. The second way, which is the industry standard, is via frequency modulated continuous-wave (FMCW) radar, which sends out a continuous transmission wave and measures distance by the size of the shift in the signal's frequency. FMCW is considered a much more accurate and therefore safer technology.
Typically, most radio altimeter units operate between 4.2 and 4.4GHz in frequency, but only use 150 megahertz within that range. A radio altimeter consists of two separate transmit and receive antennas, as the time between the transmission and reception of a wave is insufficient to allow a single antenna to perform both functions. Commercially available radio altimeters can be accurate up to 2 feet (0.6 meters) or less at extremely low altitudes, and are generally set to provide readings up to an altitude of 2,500 feet (762 meters).
Despite the proliferation of Global Positioning Satellite (GPS) technology, almost all civil aircraft still carry and use at least one radio altimeter due to legislative restrictions on the use of GPS. Radio altimeters in this capacity remain essential for landing on autopilot, but are also useful in low-visibility conditions. Additionally, radio altimeters are employed when landing manually to help alert a pilot when to engage in a maneuver known as a "flare," which is performed just before touchdown to lessen the impact of the plane on the ground.
A radio altimeter also functions, generally in conjunction with forward looking radar and other sensors, as part of an aircraft's ground proximity warning system, providing warning when a plane descends beneath a certain point or too close to the ground. Radio altimeter technology is also employed in military applications, most commonly among helicopters and other low-flying craft to avoid radar detection. It also functions as a component in terrain-following radar, allowing a craft to fly at high speeds over varied topography.