Terrain Referenced Integrity Monitor for an Unmanned Aircraft Systems Precision Approach

This paper discusses the use of an on-board radar altimeter and Digital Elevation Model (DEM) for terrain referenced integrity monitoring during an Unmanned Aircraft Systems precision approach and landing. For operation of commercial UAS in the National Airspace System (NAS), a Performance Based Navigation (PBN) system applicable to all phases of flight is needed for unrestricted airspace and airport access. Methods, such as Radar Altimeter (RALT) Aiding, are currently being researched to create a Global Navigation Satellite System (GNSS) based navigation system with sufficient positional accuracy for a CAT-IIIc like precision approach and landing. While these systems have successfully addressed the vertical accuracy, horizontal accuracy, and vertical integrity requirements, further development for horizontal integrity is needed. This paper builds on previous work by the authors on radar altimeter augmentation to GNSS and discusses additional measures to improve the horizontal integrity of the navigation system. Performance of the proposed system is assessed in nominal and off-nominal conditions using data collected with a DC-3 aircraft at the Ohio University airport.