Nap-of-the-earth
Encyclopedia
Nap-of-the-earth is a type of very low-altitude flight
course used by military aircraft to avoid enemy detection and attack in a high-threat environment.
During NOE flight, geographical features are used as cover, exploiting valleys and folds in the terrain by flying in, rather than over, them. This keeps below enemy radar coverage, avoiding "skylining". Other terms are also used, including "ground-hugging", "terrain masking
", or "flying under the radar
".
A high-flying aircraft can be detected by defense systems at long range, giving an air defense system time to react, alerting SAM
and AAA missile systems and fighter aircraft. Using NOE flight, the approach may be undetected; the aircraft "pops up" to attack the target and then turns to escape before the enemy can respond. Doppler radar
has the potential to detect NOE flight, but the incoming aircraft has to be within radar range in the first place, and low flight minimizes this possibility due to the effect of terrain masking
.
or terrain-following radar
system is also used, the latter enabling low flying in adverse weather where it would not be possible by visual reference and manual pilot control. At night, a night-vision device may be used.
s because they have lower speeds and more maneuverability than fixed-wing aircraft, particularly fast-jets. Helicopters can fly at treetop levels or even below the height of surrounding trees where there are clear areas (such as in river gullies), flying under wires (such as electricity cables) rather than over them. Attack helicopter
s can hide behind trees or buildings, "popping up" just enough to use their (rotor mast-mounted) radar or other sensors and then minimally exposing themselves to launch weapons. Escape can then be made by further NOE flying. The altitude is different dependent on the area of operations, and is related to understory canopy layer (<15m/50ft) or low building, tree canopy layer of less than 45m (147ft), presence of tall buildings or concrete pylons with heights of up to 25 metres (76ft) international standard, or emergent tree layer canopy and lattice steel electricity pylon to 100m (328ft) being present in the area. Over populated areas an altitude of 500 feet AGL applies where transmission towers of 500ft are in use, these first installed in the 1940s in the northern hemisphere.
In May 2011, four U.S. military helicopters evaded the Pakistani air defense system during the Osama bin Laden mission
. Officials in Pakistan attributed the failure to detect the aircraft to their use of nap-of-the-earth flying techniques.
. Special maps are produced that plot the routes of these wires but these are difficult to keep up-to-date, especially for foreign/enemy countries. Pilots are trained to scan for the pylons or power-poles that support these wires, because they can be seen at a distance where the wires themselves cannot.
Flight
Flight is the process by which an object moves either through an atmosphere or beyond it by generating lift or propulsive thrust, or aerostatically using buoyancy, or by simple ballistic movement....
course used by military aircraft to avoid enemy detection and attack in a high-threat environment.
During NOE flight, geographical features are used as cover, exploiting valleys and folds in the terrain by flying in, rather than over, them. This keeps below enemy radar coverage, avoiding "skylining". Other terms are also used, including "ground-hugging", "terrain masking
Terrain mask
Terrain mask is a term used in both aviation and GPS equipment.A terrain mask refers to the natural curvature of the earth. It is important as a means of avoiding active radar by positioning the aircraft so there is natural earth hiding it from the radio waves sent from the radar system...
", or "flying under the radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
".
Purpose
NOE is used to minimize detection by hostile aircraft, AWACS surveillance and control systems, ground-based radar, or attack targets.A high-flying aircraft can be detected by defense systems at long range, giving an air defense system time to react, alerting SAM
Surface-to-air missile
A surface-to-air missile or ground-to-air missile is a missile designed to be launched from the ground to destroy aircraft or other missiles...
and AAA missile systems and fighter aircraft. Using NOE flight, the approach may be undetected; the aircraft "pops up" to attack the target and then turns to escape before the enemy can respond. Doppler radar
Doppler radar
A Doppler radar is a specialized radar that makes use of the Doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the frequency of the returned signal has been...
has the potential to detect NOE flight, but the incoming aircraft has to be within radar range in the first place, and low flight minimizes this possibility due to the effect of terrain masking
Terrain mask
Terrain mask is a term used in both aviation and GPS equipment.A terrain mask refers to the natural curvature of the earth. It is important as a means of avoiding active radar by positioning the aircraft so there is natural earth hiding it from the radio waves sent from the radar system...
.
Sensors for NOE
Most NOE flying is done during the day using visual reference by pilots who are experienced in low flying. Data from a radar altimeterRadar altimeter
A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...
or terrain-following radar
Terrain-following radar
Terrain-following radar 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...
system is also used, the latter enabling low flying in adverse weather where it would not be possible by visual reference and manual pilot control. At night, a night-vision device may be used.
Helicopter NOE flying
The lowest NOE flying is by helicopterHelicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
s because they have lower speeds and more maneuverability than fixed-wing aircraft, particularly fast-jets. Helicopters can fly at treetop levels or even below the height of surrounding trees where there are clear areas (such as in river gullies), flying under wires (such as electricity cables) rather than over them. Attack helicopter
Attack helicopter
An attack helicopter is a military helicopter with the primary role of an attack aircraft, with the capability of engaging targets on the ground, such as enemy infantry and armored vehicles...
s can hide behind trees or buildings, "popping up" just enough to use their (rotor mast-mounted) radar or other sensors and then minimally exposing themselves to launch weapons. Escape can then be made by further NOE flying. The altitude is different dependent on the area of operations, and is related to understory canopy layer (<15m/50ft) or low building, tree canopy layer of less than 45m (147ft), presence of tall buildings or concrete pylons with heights of up to 25 metres (76ft) international standard, or emergent tree layer canopy and lattice steel electricity pylon to 100m (328ft) being present in the area. Over populated areas an altitude of 500 feet AGL applies where transmission towers of 500ft are in use, these first installed in the 1940s in the northern hemisphere.
In May 2011, four U.S. military helicopters evaded the Pakistani air defense system during the Osama bin Laden mission
Death of Osama bin Laden
Osama bin Laden, then head of the Islamist militant group al-Qaeda, was killed in Pakistan on May 2, 2011, shortly after 1 a.m. local time by a United States special forces military unit....
. Officials in Pakistan attributed the failure to detect the aircraft to their use of nap-of-the-earth flying techniques.
Height Above Ground Level
Height Above Ground Level (AGL) in NOE and low flying generally vary with the aircraft speed, aircraft maneuverability and the ruggedness of the terrain. Helicopters are capable of flying down to a few feet below the skids or wheels. Fast jets are more constrained and at a typical low-flying speed of 450 knots (800 km/h), 200 feet (60 m) is not unusual and 50 feet (15 m) is possible in relatively flat terrain. Power wires are a danger to all aircraft flying at low level and "wire strikes" are not uncommon, such as the Cavalese cable car disaster (1998)Cavalese cable car disaster
Two cable car disasters have occurred at Cavalese:* Cavalese cable car disaster * Cavalese cable car disaster...
. Special maps are produced that plot the routes of these wires but these are difficult to keep up-to-date, especially for foreign/enemy countries. Pilots are trained to scan for the pylons or power-poles that support these wires, because they can be seen at a distance where the wires themselves cannot.