Terrain following and terrain avoidance algorithms

The purpose of terrain following (TF) for military aircraft is to use the protection from enemy sight afforded by terrain masking. A usual requirement is to remain as close as possible above a given distance, the `set clearance height´, from the ground. Conventionally a forward looking terrain following radar (TFR) is used to provide range and angle information of points on the ground for processing into a TF pitch plane command. A problem with using a TRF for implementing TF is that it is far from covert. The author considers the application of terrain referenced navigation systems (TRN) for guidance. These techniques improve the accuracy of the aircraft primary dead reckoning system, typically an inertial navigation system, by using additional information obtained from radar altimeter returns and a stored digital terrain map