Nap-of-the-earth
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Nap-of-the-earth (abbreviated NOE) is very low-level flight to avoid detection and attack by an enemy when in a high-threat environment.
Geographical features are used as cover, exploiting valleys and folds in the ground by flying in them rather than over them to avoid "skylining" to an enemy and to keep below radar cover. Other terms such as "ground-hugging", "terrain masking", or "flying under the radar" are also used.
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[edit] Purpose
NOE is used to minimise detection by hostile aircraft, by AWACS surveillance and control systems, or by ground-based radar. A high-flying aircraft would be detected by such systems at long range, allowing an air defense system time to react, alert SAM and AAA systems and scramble fighter (Air Defence) 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 flying minimises this possibility.
[edit] 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 altimeter 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, intensifier systems or infra-red can be used. "Night Vision Goggles" (NVGs) are visual intensifiers working in the near-IR area at a wavelength of about 1 micrometre and Forward-Looking Infra Red (FLIR, or Thermal Imaging (TI)) works at about 10 micrometres. NVGs require some ambient "light" such as from the moon or ground lights while FLIR is entirely passive and produces a picture based on the heat distribution of the area being observed.
[edit] Helicopter NOE flying
The lowest NOE flying is by helicopters because they have lower speeds and more maneuverability than fixed-wing aircraft, particularly fast-jets. Helicopters can fly at "tree-top" level, even below the height of surrounding trees where there are clear areas, flying under wires (such as electricity cables) rather than over them. Attack helicopters 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.
[edit] Heights Above Ground Level
Heights 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. Special maps are produced that plot the routes of these wires but these are difficult to keep up-to-date. 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. Cable car wires mountainous terrain and wires used in logging operations also prove hazardous when flying at low altitudes.
