The hallmark of a cruise missile is its incredible accuracy. A common statement made about the cruise missile is, "It can fly 1,000 miles and hit a target the size of a single-car garage." Cruise missiles are also very effective at evading detection by the enemy because they fly very low to the ground (out of the view of most radar systems).
Four different systems help guide a cruise missile to its target:
- IGS - Inertial Guidance System
- Tercom - Terrain Contour Matching
- GPS - Global Positioning System
- DSMAC - Digital Scene Matching Area Correlation
The IGS is a standard acceleration-based system that can roughly keep track of where the missile is located based on the accelerations it detects in the missile's motion (click here for a good introduction). Tercom uses an on-board 3-D database of the terrain the missile will be flying over. The Tercom system "sees" the terrain it is flying over using its radar system and matches this to the 3-D map stored in memory. The Tercom system is responsible for a cruise missile's ability to "hug the ground" during flight. The GPS system uses the military's network of GPS satellites and an onboard GPS receiver to detect its position with very high accuracy.
Once it is close to the target, the missile switches to a "terminal guidance system" to choose the point of impact. The point of impact could be pre-programmed by the GPS or Tercom system. The DSMAC system uses a camera and an image correlator to find the target, and is especially useful if the target is moving. A cruise missile can also be equipped with thermal imaging or illumination sensors (as used in smart bombs).