Radar altimeter

radio altimeterradar altimetryelectronic altimeterradaraltimeteraltimetrydelay-Doppler radarelectromagnetic sensorsRadar Altimeter (RA)radar altimeters
A radar altimeter, electronic altimeter, reflection altimeter, radio altimeter (RADALT), low range radio altimeter (LRRA) or simply RA, used on aircraft, measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to reflect from the ground and return to the plane.wikipedia
179 Related Articles

Altitude

high altitudealtitudeshigh-altitude
A radar altimeter, electronic altimeter, reflection altimeter, radio altimeter (RADALT), low range radio altimeter (LRRA) or simply RA, used on aircraft, measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to reflect from the ground and return to the plane.
Absolute altitude is the vertical distance of the aircraft above the terrain over which it is flying. It can be measured using a radar altimeter (or "absolute altimeter"). Also referred to as "radar height" or feet/metres above ground level (AGL).

Radar

radar stationradarsradar system
As the name implies, radar (radio detection and ranging) is the underpinning principle of the system.
Some of these make the use of radar altimeters possible.

Altimeter

altimetrybarometric altimeterpressure altimeter
This type of altimeter provides the distance between the antenna and the ground directly below it, in contrast to a barometric altimeter which provides the distance above a defined datum, usually mean sea level.
A radar altimeter measures altitude more directly, using the time taken for a radio signal to reflect from the surface back to the aircraft.

Autoland

automatic landingautomatic landing systemautolanding
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.
Autoland requires the use of a radar altimeter to determine the aircraft's height above the ground very precisely so as to initiate the landing flare at the correct height (usually about 50 ft).

Continuous-wave radar

continuous wave radarcontinuous wave illuminating radarscontinuous-wave
Alternatively, Frequency Modulated Continuous-wave radar can be used.
This kind of radar is often used as "radar altimeter" to measure the exact height during the landing procedure of aircraft.

Autothrottle

auto-thrustautomatic throttle systemautothrust
Radar altimeters give data to the autothrottle which is a part of the Flight Computer.
A radar altimeter feeds data to the autothrottle mostly in this mode.

Ground proximity warning system

GPWSenhanced ground proximity warning systemEGPWS
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.
The system monitors an aircraft's height above ground as determined by a radar altimeter.

Lloyd Espenschied

Espenschied, LloydLloyd Espenshied
In 1924, American engineer Lloyd Espenschied invented the radio altimeter.
In 1930, he applied for a patent on a device based on a mathematical analysis used by the radio altimeter invented by William Littell Everitt at The Ohio State University.

Satellite geodesy

satellite altimetrygeodeticAltimetry
Satellite altimetry
A radar altimeter uses the round-trip flight-time of a microwave pulse between the satellite and the Earth's surface to determine the distance between the spacecraft and the surface.

Terrain

relieftopographic reliefland surface
A radar altimeter, electronic altimeter, reflection altimeter, radio altimeter (RADALT), low range radio altimeter (LRRA) or simply RA, used on aircraft, measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to reflect from the ground and return to the plane.

Aircraft

heavier-than-airflying machineheavier-than-air flight
A radar altimeter, electronic altimeter, reflection altimeter, radio altimeter (RADALT), low range radio altimeter (LRRA) or simply RA, used on aircraft, measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to reflect from the ground and return to the plane.

Spacecraft

spaceshipspaceshipsspace ship
A radar altimeter, electronic altimeter, reflection altimeter, radio altimeter (RADALT), low range radio altimeter (LRRA) or simply RA, used on aircraft, measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to reflect from the ground and return to the plane.

International Telecommunication Union

ITUInternational Telecommunication Union´sInternational Telecommunications Union
From the legal point of view, a radio altimeter is – according to article 1.108 of the International Telecommunication Union´s (ITU) ITU Radio Regulations (RR) – defined as «Radionavigation equipment, on board an aircraft or spacecraft, used to determine the height of the aircraft or the spacecraft above the Earth's surface or another surface.» Radionavigation equipment shall be classified by the radiocommunication service in which it operates permanently or temporarily.

ITU Radio Regulations

Radio RegulationsRRInternational Radio Regulations
From the legal point of view, a radio altimeter is – according to article 1.108 of the International Telecommunication Union´s (ITU) ITU Radio Regulations (RR) – defined as «Radionavigation equipment, on board an aircraft or spacecraft, used to determine the height of the aircraft or the spacecraft above the Earth's surface or another surface.» Radionavigation equipment shall be classified by the radiocommunication service in which it operates permanently or temporarily.

Safety service

safety-of-life service
The utilization of radio altimeter equipment is categorised as so-called safety-of-life service, must be protected for Interferences, and is an essential part of Navigation.

Interference (communication)

interferenceInterferencesinterfere
The utilization of radio altimeter equipment is categorised as so-called safety-of-life service, must be protected for Interferences, and is an essential part of Navigation.

Navigation

nauticalnavigatemarine navigation
The utilization of radio altimeter equipment is categorised as so-called safety-of-life service, must be protected for Interferences, and is an essential part of Navigation.

Speed of light

clight speedvelocity of light
The altitude above the ground is calculated from the radio waves' travel time and the speed of light.

E band (NATO)

EE bandE-band
Radar altimeters normally work in the E band, or, for more advanced sea-level measurement, S band.

S band

S-bandSS-
Radar altimeters normally work in the E band, or, for more advanced sea-level measurement, S band.

Pythagorean theorem

Pythagoras' theoremPythagoras47th Problem
From the previous illustration of the radar pulse vertical cross-section, the radius of the outer edge can be found using the Pythagorean theorem:

Frequency modulation

FMfrequency modulatedfrequency-modulated
This method can achieve much better accuracy than the pulsed radar for the same outlay and radar altimeters that use frequency modulation are industry standard.

Synthetic-aperture radar

synthetic aperture radarSARsynthetic aperture
The primary difference between delay-doppler (or Synthetic Aperture Radar) and pulse-limited altimetry is that delay-doppler altimetry looks at a smaller section of the pulse-limited radar footprint, but emits far more pulse signals to give the effect of covering the same footprint as pulse-limited but with better resolution.

Ice cap

ice-cappedice capsice dome
Icecap - There have been cases of hazard caused to airplanes flying over an icecap, by their radar altimeters displaying the height above the underlying bedrock instead of the height above the ice.

Airliner

airlinerspassenger aircraftcommercial aircraft
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.