Multiple Hypothesis RAIM with Real-Time FDE and Forecasted Availability for Combined Galileo-GPS Vertical Guidance

The number of ranging sources for the aviation user is expected to increase with the Galileo system becoming fully operational over the next decade, and the projected launch of a modernized GPS III constellation. The reduction in nominal error bounds by removal of the ionospheric delay term from the dual-frequency measurements, together with the presence of a larger number of satellites is going to increase the robustness against satellite failures and hazardous pseudorange errors. Consequently, a significant improvement in integrity will be available for the use of satellite navigation during precision aircraft approaches and other critical operations. With a better understanding of the threat model and the multiple hypothesis RAIM algorithm previously reported [Ene, 2006], the final contributions to a new dual-constellation RAIM are made here. A method for Failure Detection and Elimination (FDE) is proposed, with the purpose of improving the navigation Protection Level (PL) where possible. Also, a new capability is added to the RAIM algorithm to provide a conservative forecast of PLs anywhere in the world without the need for real-time range measurements. Since the PL is a direct function of the measurement residuals under this approach, a tool will be developed for predicting PL values ahead of time, when a critical navigation operation is about to begin. Vertical errors are critical during aviation precision approaches, and they are also generally greater than horizontal errors for satellite-based positioning. The purpose of this work is to investigate what Vertical Protection Level (VPL) values could be achieved with RAIM under conservative failure assumptions. Once vertical guidance is accomplished, the same algorithm can be applied in two dimensions to provide a Horizontal Protection Level (HPL) as well. Computer simulations of the new techniques for FDE and VPL prediction have been conducted and preliminary results indicate that VPLs in the 10-20m range are achievable. These protection levels would enable LPV200 landings (the equivalent of Cat I ILS) at all runway ends in the world without the need for a satellite-based or ground-based augmentation system (SBAS or GBAS). A conclusion will be presented on the capabilities of dual-constellation RAIM to assist an aviation user in meeting the integrity and continuity requirements for landing aircraft. The approaches to measurement integrity of both GPS and Galileo [Oehler, 2004] will be compared within the framework of the Weighted Integrity Risk Solution Separation (WIR-SS) algorithm, while possible improvements to the GNSS constellation will be discussed, that could increase the benefits to the RAIM user.