DSMAC

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

The Goodyear Aircraft Corporation ATRAN (Automatic Terrain Recognition And Navigation) system for the MGM-13 Mace was the earliest known TERCOM system. In August 1952, Air Materiel Command initiated the mating of the Goodyear ATRAN with the MGM-1 Matador. This mating resulted in a production contract in June 1954. ATRAN was difficult to jam and was not range-limited by line-of sight, but its range was restricted by the availability of radar maps. In time, it became possible to construct radar maps from topographic maps.

Preparation of the maps required the route to be flown by an aircraft. A radar on the aircraft was set to a fixed angle and made horizontal scans of the land in front. The timing of the return signal indicated the range to the landform and produced an amplitude modulated (AM) signal. This was sent to a light source and recorded on 35 mm film, advancing the film and taking a picture at indicated times. The film could then be processed and copied for use in multiple missiles.

In the missile, a similar radar produced the same signal. A second system scanned the frames of film against a and produced a similar AM signal. By comparing the points along the scan where the brightness changed rapidly, which could be picked out easily by simple electronics, the system could compare the left-right path of the missile compared to that of the pathfinding aircraft. Errors between the two signals drove corrections in the autopilot needed to bring the missile back onto its programmed flight path.

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Wikipedia