TERCOM is an automated navigation system used primarily by an unmanned aerial vehicle such as a long-range cruise missile. The system uses a predefined contour map of the flight path which acts as a comparison master image. The missile is equipped with a sophisticated radar altimeter which constantly reads the terrain it is crossing and compares the readings to the master image. When deviations are detected, the missile's guidance system makes the necessary corrections to its flight path. This makes extremely accurate navigation and collision avoidance possible, thereby allowing the missile to fly closer to the ground and avoiding detection by radar.
Terrain contour mapping, or TERCOM as it is more commonly referred to, is one of the most accurate and reliable referential navigation systems for long-range, unmanned aerial vehicles such as cruise missiles. In the initial stages of long-range missile development, several methods of navigation were used such as the graphic comparison systems in early intercontinental ballistic missiles (ICBMs). These systems employed surveillance photographs as a master while an onboard camera took a series of photographs en route as comparison material. The master and en route photos were scanned by a computer for high contrast similarities, and the missile navigated according to the results. This process was extremely slow and laborious and resulted in a navigation validation process based on very few reference points.
The advent of reliable satellite mapping allowed system designers to incorporate highly accurate digital terrain maps as comparison masters. Equally sophisticated radar altimeters in the missile supply accurate, real time ground clearance readings which form a running terrain profile for comparison with the master file. This allows the missile's steering system to make instantaneous, pinpoint course corrections to maintain the programmed track throughout the flight with none of the “drift” experienced with older inertial reference systems (INS). TERCOM master terrain maps consist of a strip of terrain detail which represents the ideal course as well as an additional area on either side of the central track. This allows for diversions to be made without the missile "wandering off" into uncharted territory.
The high degree of accuracy possible in terms of exact altitude above all terrain profiles allows TERCOM-equipped missiles to maintain low altitude flight paths while avoiding obstacles. This ground hugging ability confounds enemy ground radar systems, particularly during the final, pre-impact phase of the flight. The only real disadvantage of the TERCOM technology is a lack of pre-launch flexibility. The entire flight has to be planned from a specific launch location, and any unexpected changes to this constant can mean an aborted mission. In this sense, the latest global satellite positioning (GPS) systems are superior to terrain mapping technology by allowing a missile to be launched from any location.