Improved Ephemeris Monitoring for GNSS

Abstract

GNSS performance monitoring is an important component of aviation safety. RAIM, SBAS, GBAS, and ARAIM all meet their safety and availability analyses by assuming certain levels of performance from the GPS L1 C/A signals. In the future, most of these systems will utilize new signals and new constellations. It will be vitally important to verify the level of performance of these additional resources. Currently, GPS flags its integrity status in a variety of methods including broadcast flags in the navigation data as well as by broadcasting alternative codes and/or alternative data bit sequences. Unfortunately, these latter two methods are not necessarily reflected in archived navigation data sets. When a receiver observes NAV data bits that do not pass parity it typically discards the data and the NAV bits are not saved for further evaluation. Aviation receivers are required to set a GPS satellite unhealthy if parity fails on five successive words (3 seconds). However, nonaviation receivers may elect to continue tracking in this situation, which may lead to the appearance that all is well with the satellite. We propose a new navigation data archive format that saves all of the navigation data bit data regardless of parity checks. This format allows the subsequent detection of non-standard data (NSD) broadcasts. It would also reveal the use of non-standard codes (NSC) that also can be difficult to detect. When an NSC is broadcast, most receivers stop tracking the satellite. This fact is not recorded in navigation data files, but may be seen in the observation files. Further, sometimes receivers cross-correlate the missing satellite’s PRN code with another satellite’s, and report that observation data as though it came from the satellite broadcasting NSC. This discrepancy can be detected by comparing the expected pseudorange to the observed data. However, this process is cumbersome. By recording the raw navigation data bits, an NSC broadcast becomes more obvious. Either there is a lack of data, which is readily apparent, or the recorded bits correspond to the other satellite being mistaken for the absent PRN. In the latter case, identical bits can be seen for two PRNs. A voting method across multiple receivers can be used to determine which is the true data set. Alternatively, some of the new signals will contain the PRN in the navigation data. This inclusion will make the mistaken record obvious.