Improving the detection capability of spatial failure modes using downward-looking sensors in terrain database integrity monitors

This article discusses various methods of improving the detection capability of horizontal and vertical failure modes for a terrain database integrity monitor that is purely based on the inputs from downward looking sensors. Terrain database integrity monitors which use radar altimeter and GPS inputs have previously been proposed for Synthetic Vision Systems (SVS). An SVS provides pilots with either a Heads Down Display (HDD) or a Heads Up Display (HUD) containing aircraft state, guidance and navigation information, and a virtual depiction of the terrain as viewed "from the cockpit." The source used to generate the terrain depicted on these displays is a Digital Elevation Model (DEM). Due to the compelling nature of the displays, it is quite possible for the pilots to use the display for functions other than its original intended function. It may be hard to avoid such a scenario, especially if the system is certified as an advisory system. When using SVS display technology for functions other than advisory it may be necessary to include a DEM integrity monitor whose performance is specified by probabilities of Missed Detection, Fault-Free Detection and Time-to-Alarm. Ideally, the DEM represents the height or elevation of the terrain at corresponding coordinates (e.g. latitude and longitude) expressed in a predefined vertical datum. However, the given elevations deviate from the true elevations due to systematic and blunder errors that are present in the DEM primarily due to the way in which they are generated from different sensor technologies such as photogrammetry, remote sensing, etc. and the manual post-processing process. Another source of error of lesser significance is the flat earth approximation over relatively larger areas, while collecting the data. The described integrity monitor method is based on the compa