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The meaning of «tercom»

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 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 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 photocell 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 with 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.

Modern TERCOM systems use a different concept, based on the altitude of the ground the missile flies over and comparing that to measurements made by a radar altimeter. TERCOM "maps" consist of a series of squares of a selected size. Using a smaller number of larger squares saves memory, at the cost of decreasing accuracy. A series of such maps are produced, typically from data from radar mapping satellites. When flying over water, contour maps are replaced by magnetic field maps.

As a radar altimeter measures the distance between the missile and the terrain, not the absolute altitude compared to sea level, the important measure in the data is the change in altitude from square to square. The missile's radar altimeter feeds measurements into a small buffer that periodically "gates" the measurements over a period of time and averages them out to produce a single measurement. The series of such numbers held in the buffer produce a strip of measurements similar to those held in the maps. The series of changes in the buffer is then compared with the values in the map, looking for areas where the changes in altitude are identical. This produces a location and direction. The guidance system can then use this information to correct the flight path of the missile.

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