automatic pilot. Then, if the F-111 ever dips below the preset minimum altitude (for example, 15 meters) for any reason, its automatic pilot is commanded to put the F-111 into a 2G fly-up (a steep nose-up climb) to avoid crashing into terrain or water. Even in combat, the hazard of a collision is far greater than the danger of being detected by an enemy. Similar systems are used by F/A-18 Super Hornet aircraft operated by Australia and the United States. International regulation The International Telecommunication Union (ITU) defines radio altimeters 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" in article 1.108 of the ITU Radio Regulations (RR). Radionavigation equipment shall be classified by the radiocommunication service in which it operates permanently or temporarily. The use of radio altimeter equipment is categorised as a safety-of-life service, must be protected for interferences, and is an essential part of navigation. See also Laser altimeter Satellite altimetry Notes References Citations Bibliography Navigation Radio navigation Navigational aids Air traffic control Measuring instruments Navigational flight instruments