Terrain-following radar

Terrain-following radar (TFR) 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. The term nap-of-the-earth flight may also apply but is more commonly used in relation to low-flying military helicopters, which typically do not use terrain-following radar. The technology was originally developed by Ferranti for use with the TSR-2 aircraft.

The system works by transmitting a radar signal towards the ground area in front of the aircraft. The radar returns can then be analysed to see how the terrain ahead varies, which can then be used by the aircraft's autopilot to maintain a reasonably constant height above the earth.

This technology is primarily used by military strike aircraft, to enable flight at very low altitudes (sometimes below 100 feet (30 metres)) and high speeds, avoiding detection by enemy radars and interception by anti-aircraft systems. Normally, this radar is used by the navigator and allows the pilot to focus on other aspects of the flight besides the extremely intensive task of low flying itself. It can also enable low-altitude flight at night and in other low-visibility conditions.

Some aircraft such as the Tornado IDS have two separate radars, with the smaller one used for terrain-following. However more modern aircraft such as the Rafale with phased array radars can look forward and at the ground simultaneously, by electronically steering one or more beams in milliseconds.

Most aircraft allow the pilot to select the ride "hardness", to choose between how closely the aircraft tries to keep itself close to the ground and the forces exerted on the pilot. The F-111 used a switch to select for a hard, medium or a soft ride.

The TFR computer will consider many factors in determining the flight path for the aircraft. These factors include, distance to the forward terrain, aircraft speed and velocity, angle of attack and quality of signal being returned.

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