General purpose of radar and applications

Metal and water have been known to be excellent reflectors of radio signals since the late 1800s,and attempts have been made over the years to build ships,trains and iceberg detectors since then.Most of these have obvious practical limitations,especially with low frequency signals that require large antennas to provide reasonable performance.The Bell installation operated at a base frequency of 450 MHz,one of the highest frequency systems of its time.In Canada,the National Research Council began research on altimeter-based airborne radar systems.This came as quite a surprise to British researchers who visited in October 1940 as part of the Tizard mission,because at the time the British thought they were the only ones working on the concept.However,the Canadian design was eventually abandoned in favor of building a fully developed British ASV Mark II design,which operated at a higher power level.In France,researchers at IT&T France were conducting similar experiments as the German invasion loomed on the Paris lab.The lab was intentionally destroyed to prevent the research from falling into German hands, but the German team found the antenna in the rubble and demanded an explanation.IT&T research chief deflects suspicion by showing them contraption on magazine cover and warning them not up to date with latest navigation technology. 

Applications

In civil aviation.radar altimeters are often used by commercial aircraft for approach and landing,especially in low visibility conditions and for autolanding,to let the autopilot know when to initiate a flare maneuver.The radar altimeter provides data to the autothrottle which is part of the flight computer.Radar altimeters typically only provide readings up to 2,500 feet (760 meters) above ground level (AGL).Typically,weather radars can be directed downward to provide readings at longer ranges,up to 60,000 feet (18,000 meters) above ground level (AGL). As of 2012,all airliners are equipped with at least two or more radar altimeters,as they are critical to autoland capability.(As of 2012, regulations do not allow other methods such as GPS to determine altitude.) Older airliners from the 1960s (such as the British Aircraft Corporation BAC 1-11) and smaller airliners with less than 50 seats (such as the ATR 42 and BAe Jetstream series) are equipped with they.Radar altimeters are an essential part of the Ground Proximity Warning System (GPWS), which warns pilots if the plane is flying too low or descending too quickly.However, a radar altimeter cannot see the terrain directly in front of the aircraft,only the terrain below it; this functionality requires knowledge of the location and the terrain at that location, or a forward-looking terrain radar.The radar altimeter antenna has a fairly large main lobe of about 80°,so at tilt angles up to about 40° the radar detects the range from the aircraft to the ground (especially to the nearest large reflective object).This is because the range is calculated from the first signal returned each sample period.It does not detect a tilt range until approximately 40° of bank or pitch is exceeded.This is not a problem for landing,since pitch and roll are usually not more than 20°.Radio altimeters used in civil aviation operate in the IEEE C-band between 4.2 and 4.4 GHz.Potential interference from 5G cell towers causing some flight delays and some cancellations in the U.S. in early 2022.

in military aviation,military aircraft also use radar altimeters to fly very low over land and sea to avoid radar detection and targeting by anti-aircraft guns or surface-to-air missiles.A related use of radar altimeter technology is terrain following radar,which allows fighter bombers to fly at very low altitudes.F-111 aircraft of the Royal Australian Air Force and the United States Air Force have a forward-looking terrain-following radar (TFR) system linked to their autopilots via a digital computer.Below the nose radome are two separate TFR antennas,each providing separate information for the dual-channel TFR system.If this system fails,the F-111 has a backup radar altimeter system,also connected to the autopilot.Then,if the F-111 for any reason drops below a preset minimum altitude (say 15 meters),its autopilot is commanded to put the F-111 into 2G flight (steep nose-up climb) to avoid hitting onto terrain or water. Even in combat,the danger of collision is far greater than the danger of being detected by the enemy. F/A-18 Super Hornet aircraft operated by Australia and the US use a similar system.

International regulation

The International Telecommunication Union (ITU) defines a radio altimeter in Article 1.108 of the ITU Radio Regulations as "a radio navigation device on board an aircraft or spacecraft for determining the altitude of the aircraft or spacecraft above the surface of the Earth or another surface" ( RR).Radionavigation aids should be classified according to the radiocommunication service in which they operate permanently or temporarily.The use of radio altimeter equipment is classified as a safety-of-life service,must be protected from interference,and is an essential part of navigation.