Characterization of GPS Clock and Ephemeris Errors to Support ARAIM

GPS is widely used in aviation for lateral navigation via receiver autonomous integrity monitoring (RAIM). New methodologies are currently being investigated to create an advanced form of RAIM, called ARAIM that would also be capable of supporting vertical navigation. The vertical operations being targeted have tighter integrity requirements than those supported by RAIM. Consequently, more stringent evaluations of GNSS performance are required to demonstrate the safety of ARAIM. ARAIM considers the possibility of two classes of satellite fault: those that affect each satellite independently and those that can affect multiple satellites simultaneously. These faults can lead to different safety comparisons in the aircraft. The likelihood of each fault type occurring can have a significant impact on the resulting ARAIM performance. Therefore, it is important to distinguish between such fault types and to determine appropriate models of their behavior and likelihood. This paper examines the last seven years of GPS clock and ephemeris errors to determine appropriate estimates for the probability of independent satellite failures, Psat, and the probability of simultaneous satellite failures Pconst. Even more importantly, it evaluates performance when there are no failures present. Nominal signal accuracy is characterized by a conservative one-sigma parameter called user range accuracy (URA). This paper examines how well the true error distribution for each satellite is individually described by the broadcast URA value. It further examines how the errors across all satellites are correlated and could combine to create user-positioning errors.