Abstract: A Terrain Following Apparatus for a vehicle such as an aircraft includes means (1) for effecting autonomous following of terrain by the vehicle, a control device (2), operable by a person operating the vehicle, to engage and disengage the means (1), a Terrain Referenced Navigation system (3) operable to provide navigation data for the vehicle, and validation means (4) for validating the integrity of the navigation data to provide when the integrity of the navigation data is validated for engagement of the autonomous Terrain Following means (1) by the device (2) and when the integrity of the navigation data is invalidated, for preventing operation of the means (1) when the control device (2) is operated.

Abstract: The terrain referenced navigation system uses data from a global positioning system (GPS) to determine a vehicles position, velocity and height above ground. The system uses input data from an altimeter device, a digital terrain elevation data, and a GPS position, velocity and time data. The system applies a GPS error process model in addition to a TRN measurement model, to obtain a state vector which includes estimates of current errors in the GPS data. The PVT data input from a GPS receiver is used to determine a current vehicle position and height in geographic axis. Using specifically constructed Kalman filter states, a geographical position and height of the vehicle is reference to the digital terrain elevation data, and an estimated ground clearance at the vehicle position is determined. The ground clearance is differenced with a radar altimeter output, and the residual is processed by Kalman filter to determine a new state vector, including estimates of current errors in the GPS data.