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 . Technology and usage . 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 . This allows the pilot to focus on other aspects of the flight besides the extremely intensive task of low flying itself . 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 . 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 . 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 . Advantages and disadvantages . By flying at very low altitude , the aircraft can take advantage of terrain mask ing and avoid detection by enemy radar systems . The system is also largely automated and can take some of the workload off the pilot . 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 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 . Alternatives . There are very few alternatives to using terrain - following radar for high - speed , low altitude flight . TERPROM , a terrain - referenced navigation system provides a limited but passive terrain - following functionality .