TERCOM

Terrain contour matching, or TERCOM, is a navigation system used primarily by cruise missiles. It uses a contour map of the terrain that is compared with measurements made during flight by an on-board radar altimeter. A TERCOM system considerably increases the accuracy of a missile compared with inertial navigation systems (INS). The increased accuracy allows a TERCOM-equipped missile to fly closer to obstacles and at generally lower altitudes, making it harder to detect by ground radar.