Abstract: Terrain navigation apparatus (1) for a stationary or moving vehicle (2) when the apparatus (1) is mounted thereon, including a system (3) for determining position, velocity, altitude and attitude incorporating at least six sensing means operable to provide signals indicative of a first measured height (5), a first velocity (7) and a first position (9) of the vehicle (2), scanning means (11) operable to scan terrain (13) around the vehicle (2) and measure a range (15) from the vehicle (2) thereto, first combining means (17) for combining signals indicative of the first measured height, the range and the first position (5, 15, 9) of the vehicle (2), together with error estimates (19) associated therewith, and for outputting results of said combining process, error-estimating means (21) for receiving as input signals results of said combining process and for providing said error estimates(19), such that the error-estimating means (21) and first combining means (17) operatively interact to effect terrain navigati