Terrain mask
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Terrain mask is a term used in both aviation (primarily military) 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. For instance, suppose a radar transmitter is installed on a hill to monitor an area. If there was a sufficiently deep nearby valley, an aircraft flying near the bottom would not be detected because the radar would be absorbed by the top of the valley and not reach the aircraft.
When using GPS equipment, terrain masking relates to multipath effects of satellite signals.
[edit] Terrain masking
Terrain masking is most commonly used in aerial combat, is the act of flying at extremely low elevations upon normally hilly or mountainous terrain to minimize exposure time to anti-air weaponry.
Although this sounds counter-intuitive (an aircraft flying at lower altitudes will be easier to hear and see by those on the ground, not to mention the difficulty of navigating the aircraft through an otherwise complex course), if the terrain is hilly, the aircraft can simply maneuver around a hill and the anti-air weaponry will lose sight with the aircraft. If the aircraft was flying at a much higher altitude, the advantage of getting out of the weapon's line of sight could never be obtained.
