Session B4: GNSS Integrity and Augmentation

Providing Integrity to a PPP Ground-Based Integrity Monitor System
Yu-Fang Lai, Juan Blanch, Todd Walter, Stanford University
Location: Beacon B

Precise Point Positioning (PPP) is capable of achieving decimeter level of positioning accuracy for receivers in dynamic scenarios, and centimeter level for static receiver. The extremely accurate positioning performance relies on the highly accurate corrections such as precise ephemeris, atmospheric effects, solid tide adjustments, and ocean loading etc. Among all of these corrections, the precise ephemeris is one of the most important external corrections for the PPP users. The accuracy and quality of the ephemeris that are available to the PPP users directly affects the positioning accuracy. As a result, for any PPP users that have safety critical requirements, the integrity of the PPP service, i.e. ephemeris, needs to be monitored and ensured.
In our prior research ([1-3]), we built a Ground-Based Integrity Monitor System that can estimate the ephemeris error, the satellite position error and satellite clock bias error, using code and carrier phase pseudorange measurements. The Integrity Monitor System is driven by a centralized fault-tolerant filter that directly estimate the ephemeris error of the satellites using pseudorange measurements from multiple receivers at different locations. The resulting ephemeris error estimates can be projected to any user locations on Earth to obtain the satellite pseudorange errors that any PPP user may experience at that location, for which the pseudorange errors are induced from ephemeris errors that are not fully corrected by the PPP service. Therefore, we essentially monitor the ephemeris correction send out by the PPP service.
It has been shown in [3] that the Integrity Monitor System can monitor and detect multiple ephemeris faults happening at the same time using single centralized filter. In previous research, we assume that the ephemeris of all the satellites is faulted, and the ephemeris errors are computed based on the receivers correctly receive pseudorange measurements from each of the satellites. However, just like we assume a satellite might be faulted, there is no guarantee that all the receivers of the monitoring network are always correct. We cannot rule out the possibility of receiver faults if we want to use measurements from receivers to monitor the integrity of the ephemeris corrections from PPP service. That is, the integrity of the Integrity Monitor System itself also needs to be ensured. For this reason, we must consider receiver faults when assessing the integrity of the overall system.
To account for the receiver faults in Integrity Monitor System when estimating the ephemeris errors, we apply Solution Separation (SS) for measurements from different receivers to detect the potential receiver fault and place an upper bound on the maximum undetected errors. This is similar to what Advanced Receiver Autonomous Integrity Monitoring (ARAIM) does on detecting satellite faults. We form a number of subset filters with each of it excluding certain combination of receivers to form subset solutions. In this case, the statistics to be compared of is the ephemeris errors from All-In-View (AIV) and subset solutions. By evaluating the differences of ephemeris errors between AIV and subset solutions, we can identify which receivers are faulted and therefore should not be used in the network of Integrity Monitor System. In particular, we focus on investigating relations between single satellite and multiple receivers in the event of receiver faults.
[1] Lai, Y.-F., Blanch, J., Walter, T., Kahr, E., Leahy, E., Silva, P., and Ellum, C. (2023). Prototyping integrity monitors for ppp fault detections. In Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023), pages 2592–2605.
[2] Lai, Y.-F., Blanch, J., and Walter, T. (2024a). Ppp range error integrity using a network of ground monitors. In Proceedings of the 2024 International Technical Meeting of The Institute of Navigation, pages 211–225.
[3] Lai, Y.-F., Blanch, J., and Walter, T. (2024b). Ground-based integrity monitors for ppp correction services. In Proceedings of the ION 2024 Pacific PNT Meeting, pages 514–526.