Tercom/TERCOM

A navigational technique in which a vehicle compares the profile of the terrain below with the one stored in its memory. Tercom stands for terrain contour matching or terrain comparison. There are two types of systems. In a terrain contour navigation system, the terrain profile is measured by a radio altimeter and is matched with the stored terrain profile in the vicinity of the aircraft’s position as previously estimated by the DR (dead reckoning) system. In a terrain characteristic matching system, a number of sensors such as radar and radiometric sensors sense changes in the terrain characteristics below the aircraft. The data can then be processed to detect the edges or boundaries where the terrain characteristics change abruptly. These are matched with stored terrain features. Digital terrain maps of the area of interest are prepared beforehand, and the data of the desired route are put into memory. The system provides a very high accuracy of navigation.

Aircraft/missile carries a three-dimensional map of its route. TERCOM compares a picture of the ground to one in its memory and adjusts its course accordingly. Theoretically, this allows it to maintain high speeds at low altitude, minimizing radar detection. In case of aircraft, TERCOM compares the elevation of the ground at regular intervals with digital maps and helps pinpoint the position of the aircraft. The vertical motion of the aircraft is measured by baro systems, relative horizontal location of radar, altimeter reading, dead reckoning system, and ground clearance is provided by radio altimeter.