A digital terrain system (DTS) uses a stored digital map of terrain elevation data together with measurements of the aircraft's dynamics and height above the ground, to provide navigation and terrain referenced cues. The paper provides an overview of DTS, however, as specific designs will differ, the technical information presented is generally based on the BAe TERPROM system. The DTS can provide the following functions: terrain referenced navigation (TRN); terrain following (TF); predictive ground collision avoidance system (PGCAS); obstruction warning and cueing; and ranging. A TRN system is an integrated navigation system, processing and combining data from a dead reckoning system, usually an INS (inertial navigation system), radalt (radar altimeter, gives height above ground level) and a DTED (digital terrain elevation data) map database. (6 pages)