Radar altimeter

Radar altimeter

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A radar altimeter, radio altimeter, low range radio altimeter (LRRA) or simply RA measures altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...

 above the terrain
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...

 presently beneath an aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 or spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

. This type of altimeter
Altimeter
An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.-Pressure altimeter:...

 provides the distance between the plane and the ground directly below it, as opposed to a barometric altimeter which provides the distance above a pre-determined datum, usually sea level.

Principle

Main article: Radar signal processing


As the name implies, radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 (radio detection and ranging) is the underpinning principle of the system. Radio waves are transmitted towards the ground and the time it takes them to be reflected back and return to the aircraft is timed. Because speed, distance and time are all related to each other, the distance from the surface providing the reflection can be calculated as the speed of the radio wave
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

 and therefore the time it takes to travel a distance are known quantities.

Alternatively, Frequency Modulated Continuous-wave radar
FMCW
Frequency Modulated Continuous-wave radar is a short range measuring radar set. This kind of radar is often used as “radar altimeter” to measure the exact height during the landing procedure of aircraft.- Technical introduction :...

 can be used. The greater the frequency shift the further the distance travelled. This method can achieve much better accuracy than the aforementioned for the same outlay and radar altimeters that use frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...

 are industry standard.

Invention


In 1924, American engineer Lloyd Espenschied
Lloyd Espenschied
Lloyd Espenschied was an American electrical engineer who invented the modern coaxial cable with Herman Andrew Affel.-Biography:He was born in St. Louis, Missouri on April 27, 1889....

 invented the radio altimeter. However, it took 14 years before Bell Labs was able to put Espenschied's device in a form that was adaptable for aircraft use. In 1938 in co-operation with Bell Labs, United Air Lines fitted a radar type device to some of its airliners as a terrain avoidance device.

Civil applications


Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions (see instrument flight rules
Instrument flight rules
Instrument flight rules are one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual flight rules ....

) and also automatic landings (autoland
Autoland
In aviation, autoland describes a system that fully automates the landing phase of an aircraft's flight, with the human crew merely supervising the process.-Description:...

), allowing the autopilot to know when to begin the flare maneuver.

In civil aviation applications, radio altimeters generally only give readings up to 2500 feet (762 m) above ground level
Above ground level
In aviation and atmospheric sciences, an altitude is said to be above ground level when it is measured with respect to the underlying ground surface. This is as opposed to above mean sea level , or in broadcast engineering, height above average terrain...

 (AGL).

Today, almost all airliners are equipped with at least one and usually several radar altimeters, as they are essential to autoland capabilities (determining height through other methods such as GPS(Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

) is not permissible under current legislation). Even older airliners from the 1960s, such as Concorde
Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...

 and the British Aircraft Corporation
British Aircraft Corporation
The British Aircraft Corporation was a British aircraft manufacturer formed from the government-pressured merger of English Electric Aviation Ltd., Vickers-Armstrongs , the Bristol Aeroplane Company and Hunting Aircraft in 1960. Bristol, English Electric and Vickers became "parents" of BAC with...

 BAC 1-11 were so equipped and today even smaller airliners in the sub-50 seat class are supplied with them (such as the ATR 42
ATR 42
-Civil operators:The largest operators of the ATR-42 are FedEx Express, Airlinair, TRIP Linhas Aéreas,and Mexico City-based Aeromar respectively. Number of aircraft as of 2010:Some 70 other airlines operate smaller numbers of the type....

 and BAe Jetstream series).

Radio 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. However, radar altimeters cannot see terrain directly ahead of the aircraft, only that directly below it; such functionality requires either knowledge of position and the terrain at that position or a forward looking terrain radar which uses technology similar to a radio altimeter. Radio altimeter antennas have a fairly large main lobe of about 80° so that up to bank angles of about 40°, the radio altimeter detects the range from the aircraft down to the ground, specifically to the nearest large reflecting object. This is because range is calculated based on the first signal return from each sampling period. It does not detect slant range until beyond about 40° of bank or pitch. This is not an issue for landing as pitch and roll do not normally exceed 20° or so during approach and landing.

It is interesting to note that the altitude specified by the device would not match the altitude read from the standard altimeter the pilot uses. This is because aviation is centered around True altitude, the height above Mean Sea Level (MSL), and the radio altimeter measures Absolute altitude, the height Above Ground Level
Above ground level
In aviation and atmospheric sciences, an altitude is said to be above ground level when it is measured with respect to the underlying ground surface. This is as opposed to above mean sea level , or in broadcast engineering, height above average terrain...

 (AGL). Absolute altitude is sometimes referred to as height since it is the height above the terrain directly below the aircraft, that which is provided from a radio altimeter.

Radar altimeters normally work in the E band
E band
The NATO E band is the range of radio frequencies from 2 GHz to 3 GHz in the electromagnetic spectrum. This is equal to wave lengths between 15 cm and 10 cm. The E band is in the upper UHF range of the radio spectrum...

, or Ka band
Ka band
The Ka band covers the frequencies of 26.5–40 GHz. The Ka band is part of the K band of the microwave band of the electromagnetic spectrum. This symbol refers to "K-above" — in other words, the band directly above the K-band...

 or S band
S band
The S band is defined by an IEEE standard for radio waves with frequencies that range from 2 to 4 GHz, crossing the conventional boundary between UHF and SHF at 3.0 GHz. It is part of the microwave band of the electromagnetic spectrum...

s for more advanced sea-level measurement. Radar altimeters also provide a reliable and accurate method of measuring height above water, when flying long sea-tracks. These are critical for use when operating to and from oil rigs.

Military applications


Radar altimeters are also used in military aircraft
Military aircraft
A military aircraft is any fixed-wing or rotary-wing aircraft that is operated by a legal or insurrectionary armed service of any type. Military aircraft can be either combat or non-combat:...

 to fly quite low over the land and the sea to avoid radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 detection and targeting by anti-aircraft gun
Anti-aircraft warfare
NATO defines air defence as "all measures designed to nullify or reduce the effectiveness of hostile air action." They include ground and air based weapon systems, associated sensor systems, command and control arrangements and passive measures. It may be to protect naval, ground and air forces...

s or surface-to-air missile
Surface-to-air missile
A surface-to-air missile or ground-to-air missile is a missile designed to be launched from the ground to destroy aircraft or other missiles...

s. A related use of radar altimeter technology is terrain-following radar
Terrain-following radar
Terrain-following radar is an aerospace technology that allows a very-low-flying aircraft to automatically maintain a relatively constant altitude above ground level. It is sometimes referred-to as ground hugging or terrain hugging flight...

, which allows fighter bomber
Strike fighter
In a current military parlance, a strike fighter is a multi-role combat aircraft designed to operate primarily in the air-to-surface attack role while also incorporating certain performance characteristics of a fighter aircraft. As a category, it is distinct from fighter-bombers...

s to fly at extremely low altitudes.

As an example of their use, the F-111s of the Royal Australian Air Force
Royal Australian Air Force
The Royal Australian Air Force is the air force branch of the Australian Defence Force. The RAAF was formed in March 1921. It continues the traditions of the Australian Flying Corps , which was formed on 22 October 1912. The RAAF has taken part in many of the 20th century's major conflicts...

 and the U.S. Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...

 had a forward-looking, terrain-following radar (TFR) system connected via digital computer to their automatic pilots. Beneath the nose radome, were two separate TFR antennae, each providing individual information to the dual-channel TFR system. In case of a failure in that system, the F-111s also had a back-up radar altimeter system built in, and also connected to the plane's automatic pilot
Autopilot
An autopilot is a mechanical, electrical, or hydraulic system used to guide a vehicle without assistance from a human being. An autopilot can refer specifically to aircraft, self-steering gear for boats, or auto guidance of space craft and missiles...

. This radar altimeter was programmed with a preset minimum altitude (for example, 15 meters). Then, if the F-111 ever dipped below this altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...

 for any reason, its automatic pilot would be commanded to put the F-111 into a 2G fly-up, a steep nose-up climb
Climb
thumb|right|An [[Embraer ERJ 145]] climbingIn aviation, the term climb refers both to the actual operation of increasing the altitude of an aircraft and to the logical phase of a typical flight following take-off and preceding the cruise, during which an increase in altitude to a predetermined...

 to avoid crashing into the ground or into the sea. Even in combat, the fatal hazard to the aircrew
Aircrew
Aircrew are the personnel who operate an aircraft while in flight. The composition of the crew depends on the type of aircraft as well as the purpose of the flight.-Civilian:*Aviator** Pilot-in-command** First officer** Second officer** Third officer...

 of a collision with the ground or with the sea is far greater than the danger of being detected by the enemy.

Similar systems are now used by the F-15E Strike Eagle and F/A-18 Super Hornet warplanes flown by the Americans and by the Australians.