Terrain-following radar (TFR) is a military aerospace technology that allows a very-low-flying aircraft to automatically maintain a relatively constant altitude above ground level and therefore make detection by enemy radar more difficult. 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. TFR systems work by scanning a radar beam vertically in front of the aircraft and comparing the range and angle of the radar reflections to a pre-computed ideal manoeuvring curve. By comparing the distance between the terrain and the ideal curve, the system calculates a manoeuvre that will make the aircraft clear the terrain by a pre-selected distance, often on the order of 100 metres (330 ft). Using TFR allows an aircraft to automatically follow terrain at very low levels and high speeds. Terrain-following radars differ from the similar-sounding terrain avoidance radars; terrain avoidance systems scan horizontally to produce a map-like display that the navigator then uses to plot a route that avoids higher terrain features. The two techniques are often combined in a single radar system: the navigator uses the terrain avoidance mode to choose an ideal route through lower-altitude terrain features like valleys, and then switches to TFR mode which then flies over the chosen route at a minimum altitude. Another related technology is a terrain awareness and warning system, which works similarly to a TFR but uses a database of known terrain features and GPS locations instead of radar measurements. The concept was initially developed at the Cornell Aeronautical Laboratory in the 1950s. It was first built in production form starting in 1959 by Ferranti for use with the TSR-2 aircraft, flying for the first time in an English Electric Canberra testbed in 1962. While the TSR-2 project was ultimately abandoned, the concept was widely deployed in 1960s and 70s strike aircraft and interdictors, including the General Dynamics F-111, Panavia Tornado and Sukhoi Su-24 "Fencer". The wider introduction of stealth aircraft technologies through the 1990s has led to a reduction in low-altitude flight as a solution to the problem of avoiding anti-aircraft weapons and the technique is no longer common. Most aircraft of this class have since retired although the Su-24 and Tornado remain in use in some numbers.
Technology The system works by transmitting a pencil beam radar signal towards the ground area in front of the aircraft while the radar scans up and down. The signal is sent as a series of brief pulses and the reflections of these pulses off the ground produces very powerful returns. The time the pulse takes to travel to and from the terrain produces a range measurement to the terrain in front of the aircraft. The angle relative to the aircraft is returned by a sensor on the vertical gimbal that returns a calibrated voltage.
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