Mitigation of short duration satellite outages for Advanced RAIM and other integrity systems based on GNSS

In most systems providing GNSS integrity to aircraft, like Satellite-based Augmentation Systems (SBAS), Groundbased Augmentation Systems (GBAS), Receiver Autonomous Integrity Monitoring (RAIM), or the proposed Advanced RAIM, the user position is computed using only the measurements of the current epoch (albeit carrier-smoothed). This snapshot approach is well suited for integrity, because it does not require a complex characterization of the ranging errors. In particular, except for multipath, no assumptions need to be made on the time correlation of the ranging errors. Also, the snapshot solution has been proven to be sufficient for SBAS and RAIM, and availability simulations suggest that it will be sufficient for Advanced RAIM. However, these availability simulations typically assume that as long as a satellite is above a given mask angle, it will be tracked. That is, they do not take into account that measurement outages above the mask angle can occur. Among other reasons, these outages can be due to aircraft banking, interference (either intentional or not), or ionospheric scintillation -this latter cause is especially relevant as both dual frequency SBAS and Advanced RAIM are intended to provide service in low latitude regions, where GNSS signals are frequently affected by scintillation. The temporary loss of measurements will degrade the user geometry, which could result in the loss of service. In Advanced RAIM, the Protection Level is very dependent on the worse subset geometry (out of the set of subsets that contains a fault free solution with high probability). As a consequence, ARAIM is potentially more sensitive to the loss of measurements. In this paper we describe, develop, and test an algorithm designed to mitigate the effect of short duration outages. The proposed technique exploits the temporal correlation of the pseudorange errors, which is not currently exploited. This correlation is routinely exploited in Real Time Kinematic and Precise Point Positioning techniques to fix the carrier phase ambiguities, and has been proposed to improve Advanced RAIM performance by exploiting the geometry diversity provided by satellite motion. However, these techniques rely on temporal error models that, while realistic, might not be sufficiently conservative for integrity purposes. The algorithm proposed here exploits the temporal correlation of the errors in a simple way and with a low computational load.