Abstract: Apparatus for estimating a first height (1) of a vehicle (2) above a first reference surface (3), including a system (4) for determining position, velocity and attitude incorporating at least one sensing means (4b) operable to provide an output signal (5) indicative of a vertical specific force of the vehicle, error-estimating means (6) for receiving as input signals a horizontal reference velocity and position (4a) of the vehicle (2), and a radar altimeter measurement (7) of a second height (1b) of the vehicle (2) above a second reference surface (8) and for providing as an output signal (9) estimates of errors associated with the sensing means output signal (5), and integrating means (10) for receiving said sensing means and error-estimating means output signals (5, 9), and for subtracting the estimates of errors from the signal (5) indicative of vertical specific force while performing a double integration of the results of the subtraction, to provide an output indicative of the required estimated first he