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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. This concept works even on flat terrain, although only at longer ranges, due to the curvature of the earth.
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.
"Terrain masking" is a means of avoiding active radar by positioning the aircraft so that there is natural earth hiding it from the radio waves sent by the radar system. This technique is used by military pilots to fly to their target stealthily, using the topography to mask their approach from prying radar microwaves.
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