Optical contour matching[[url=https://en.wikipedia.org/w/index.php?title=TERCOM&action=edit§ion=2]edit]
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 Commandinitiated 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.
Altitude matching[[url=https://en.wikipedia.org/w/index.php?title=TERCOM&action=edit§ion=3]edit]
Modern TERCOM systems use a different concept, based on the altitude of the ground the missile files over and comparing that to measurements made by a radar altimeter. TERCOM "maps" consist of a series of squares of a given size. Using a smaller number of 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, 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 which 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.
During the flight to the target the accuracy of the system has to be enough only to avoid terrain features. This allows the maps to be relatively low resolution in these areas. Only the portion of the map for the terminal approach has to be higher resolution, and would normally be encoded at the highest resolutions available to the satellite mapping system.
TAINS[[url=https://en.wikipedia.org/w/index.php?title=TERCOM&action=edit§ion=4]edit]
Due to the limited amount of memory available in mass storage devices of the 1960s and 70s, and their slow access times, the amount of terrain data that could be stored in a missile-sized package was far too small to encompass the entire flight. Instead, small patches of terrain information were stored and periodically used to update a conventional
inertial platform. These systems, combining TERCOM and inertial navigation, are sometimes known as
TAINS, for TERCOM-Aided Inertial Navigation System.
Advantages[[url=https://en.wikipedia.org/w/index.php?title=TERCOM&action=edit§ion=5]edit]
TERCOM systems have the advantage of offering accuracy that is not based on the length of the flight; an inertial system slowly drifts after a "fix", and its accuracy is lower for longer distances. TERCOM systems receive constant fixes during the flight, and thus do not have any drift. Their absolute accuracy, however, is based on the accuracy of the radar mapping information, which is typically in the range of meters, and the ability of the processor to compare the altimeter data to the map quickly enough as the resolution increases. This generally limits first generation TERCOM systems to targets on the order of hundreds of meters, limiting them to the use of
nuclear warheads. Use of conventional warheads requires further accuracy, which in turn demands additional terminal guidance systems.
Disadvantages[[url=https://en.wikipedia.org/w/index.php?title=TERCOM&action=edit§ion=6]edit]
One disadvantage of early TERCOM systems was that, due to the limitations of storage and computing systems of the time, the entire route had to be pre-planned, including its launch point. If the missile was launched from an unexpected location or flew too far off-course, it would never fly over the features included in the maps, and become lost. The INS system can help in this regard, allowing it to fly to the general area of the first patch, but gross errors simply cannot be corrected. This made early TERCOM based systems much less flexible than more modern systems like
GPS, which can be set to attack any location from any location, and do not require any sort of pre-recorded information which means they can be targeted immediately prior to launch.
The availability of compact fast computers and high-capacity storage, combined with the availability of global
digital elevation maps, has mitigated this problem, as TERCOM data is no longer limited to small patches, and the availability of
side-looking radar allows much larger areas of landscape contour data to be acquired for comparison with the stored contour data.