Evaluation of RADAR Altimeter-Aided GPS for Precision Approach using Flight Test Data

This paper discusses the use of a downward facing RADAR Altimeter (RALT) as an augmentation to the Global Positioning System (GPS) for precision approach and landing of Remotely Piloted Aircraft (RPA). This architecture, known as RALT Aiding, aims to improve the accuracy, integrity, and continuity of the GPS position solution to meet the stringent navigational requirements of a Category (CAT) IIIb landing. This method improves the position solution by incorporating the additional range measurement provided by the RALT into the measurement equation used to solve the aircraft's position and aircraft's protection levels. This system is of high interest for RPAs as all currently approved and in development coupled autoland systems, such as the Instrument Landing System (ILS) and Ground Based Augmentation System (GBAS), are reliant on navigational aids located on the airport's grounds. Successful integration into the National Airspace System (NAS) requires these aircraft to be capable of fully coupled autoland at many airports. The most feasible way to achieve this is to develop a navigational system independent of ground aids. This paper extends work previously performed on RALT Aiding by the authors and compares the previous simulation results to results based on collected flight data. The flight test data used for the evaluation in this paper comes from various flight tests in which Ohio University has been involved. The flight tests took place at multiple airports, using multiple different commercially available RALT units. The airplane-airport combinations included in this study are Gulfstream V-Reno airport (NV) and DC3-Ohio University airport (OH).