TERCOM, which is an acronym for TER
atching, is a guidance
technique used on the U.S. Tomahawk
Land Attack Missile (TLAM
). It consists of a radar altimeter
on the missile, coupled with a digital elevation map of the general flight plan of the missile and a fairly simple computer. In order to navigate, the missile takes frequent measurements of its altitude AGL
. It compares this to its barometric altitude in order to determine the elevation of the terrain passing beneath it; it then runs a nav algorithm on that data to compare it to the contour data
of its route stored in memory. Thus it can tell where it is with reasonable accuracy. This is usually used with an INS
onboard for general location, and is coupled with DSMAC
for terminal area guidance
One major disadvantage of this method is the massive amount of effort required to generate mission plans. In order to do so, you need to transmit a massive amount of information to the missile (in relative terms) to cover all the terrain you might pass over, and still retain enough storage for DSMAC to operate.
This method has been replaced by GPS navigation on newer (and retrofitted) Tomahawk TLAM-C and TLAM-D missiles (Conventional unitary and conventional dispensing submunition warhead versions). The GPS missiles can be simply given a precise geolocation in one line of coordinates; a series of them strung together designates a route through wypoints. This allows the missile to evade detection by following the terrain and hiding behind landscape features.