This technology is primarily used by military strike aircraft, to enable flight at very low altitudes (sometimes below 100 feet (30 meters)) and high speeds, avoiding detection by enemy radars and interception by anti-aircraft systems. Under these conditions terrain-following radar is a necessity, since a human pilot cannot react quickly enough to changing terrain heights, and is much more likely to cause a crash than an automated system in the same circumstances.
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 forwards and at the ground simultaneously.
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 uses a switch to select for a hard or a soft ride.
Terrain-following radar is also sometimes used by civilian aircraft that map the ground and wish to maintain a constant height over it.
Military helicopters may also have terrain-following radar. Due to their lower speed and high maneuverability, helicopters are normally able to fly lower than fixed wing aircraft.
The radar emissions can be detected by enemy anti-aircraft systems with relative ease once there is no covering terrain, allowing the aircraft to be targeted. The use of terrain-following radar is therefore a compromise between the increased survivability due to terrain-masking and the ease with which the aircraft can be targeted if it is seen.
Since the radar cannot tell what is the beyond any immediate terrain, the flight path may suffer from "ballooning" over the top of high ground. Furthermore small obstacles such as radio antennas and electricity pylons may not show up on the radar at all. The objects present collision hazards when flying at very low level.
Even an automated system has limitations and all aircraft with terrain-following radars installed have limits on how low and fast they can fly. Factors such as system response-time, aircraft g-limits and the weather can all limit an aircraft.