Cruise Missile Navigation has evolved in this manner
- At first it was INS (Inertial Navigation System)
- Then Came TERCOM (Terrain Contour Matching) along with INS
- GPS System
- DSMAC - Digital Scene Matching Area Correlation
The TERCOM system allowed pre recorded contour mapping along with onboard altimeter to enable terrain hugging features or low altitude flight. But due to limitations in data storage, all points are not normally mapped in the data.. Also, this is applicable for a pre planned route implying if the route is changed the data needs to be modified or else it cannot go terrain hugging at all.
See this for better understanding
View attachment 295622
As you see the mapped Terrain data is converted into Tercom map set and the radar altimeter compares that measured elevation profile with onboard tercom data set to make necessary corrections.
The GPS system uses the network of GPS satellites and an onboard GPS receiver to detect its position with very high accuracy. But only downside if the environment has jamming probability or in any manner there is an interference between the GPS receiver and the satellite it becomes useless.. Of course with advantage of faster processors, this feature gives a much more accurate methodology of Terrain hugging then TERCOM as GPS data can also have elevation level points which helps a far more accurate sense of missile path.. Thus it helps in any route and no necessity of pre planned route is there.
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. (CM in AShM mode)
See this to understand
View attachment 295623
View attachment 295624
See how the onboard DSMAC flight unit senses the target position and compares it with onboard imaged scans in Binary Frames ..
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