Sensitivity Analysis of RADAR Altimeter-aided GPS for UAS Precision Approach

RADAR Altimeter (RALT) Aiding is an augmentation to GPS using a downward facing RADAR altimeter for precision approach and landing of Remotely Piloted Aircraft (RPA). The paper builds on previous work by the authors regarding RALT Aiding and addresses the sensitivity of the RALT-aided GPS position to terrain undulations and RPA attitude. The RALTaided GPS architecture is intended to increase the accuracy, integrity, and continuity of GPS to meet the stringent navigational requirements of a CAT IIIb landing without the use of ground-based navigation aids. To achieve this, the RADAR altimeter height measurement is included in the GPS measurement equation as an additional range. The proposed method involves the combination of three subsystems to obtain the position estimate required to support navigation during autoland. The first subsystem is the selected Global Navigational Satellite System (GNSS); either dual frequency GPS or GPS in conjunction with a Satellite Based Augmentation System (SBAS) (i.e. WAAS). The second subsystem, the RADAR altimeter unit, is a commercially available unit that provides a measurement of the aircraft's height above ground level (AGL) for use in the position computation. The final subsystem is the terrain database used to determine the aircraft's height above WGS 84 ellipsoid given its height AGL. Each of the three subsystems plays an important role in determining the total system accuracy. To ensure the system will meet the performance requirements, various position performance sensitivities must be understood. The sensitivities discussed in this paper are (i) the sensitivity to terrain variations between terrain elevation data points, (ii) the sensitivity to lateral position errors, (iii) and the sensitivity to aircraft attitude.