TERCOM - Description

Description

TERCOM navigation "maps" consist of a series of strips of land that the missile is expected to fly over, encoded as a series of altitudes. Since a radar altimeter measures distances, height over the ground, and not an absolute altitude, the maps generally encode a series of changes in altitude, not the absolute altitude itself. Additionally, the strips of land on either side of the expected path are also stored. 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.

The missile's radar altimeter feeds measurements into a smaller 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 two are compared to overlay the buffer's strip on the known map, and the positioning of the strip within the map 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.

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.

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.

One disadvantage of TERCOM systems is that the entire route has to be pre-planned, including its launch point. If the missile is launched from an unexpected location or flies too far off-course, it will 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 makes TERCOM based systems much less flexible than more modern systems like GPS, which can be set to attack any location from any location, and does not require any sort of pre-recorded information which means they can be targeted immediately prior to launch.