WAAS measurement processing; current design and potential improvements

Previous papers on the wide area augmentation system (WAAS) provided details on integrity design for such items as clock and ephemeris, signal quality, and ionospheric monitoring. This paper will provide details on GPS measurement processing performed in WAAS that supports these monitoring functions. Measurement processing conducted in the integrity (or safety) processor has evolved during the localizer performance with vertical guidance phase of the WAAS Program. Currently, this processing integrates measurements from all three independent receiving threads at a reference station and performs various cross thread checks to mitigate select multipath and cycle slip events. Underlying this cross thread processing is the assumption that common mode errors caused by environmental conditions are rare and random. The introduction of WAAS reference stations in extreme northern latitudes operating in significant phase scintillation environments, and the design decision to have the receiver operate in very low signal to noise conditions, exposed erroneous measurement conditions that challenged this rare and random assumption. This paper will address the WAAS measurement processing architecture and focus on such erroneous conditions along with the monitoring algorithms WAAS incorporated to mitigate them. The paper also examines potential improvements in WAAS measurement processing during communication outages. Communication outages between reference and master stations that occur primarily from remote reference stations have the potential of causing measurement processing algorithms to reinitialize thus impacting WAAS availability and continuity performance. This paper will investigate algorithms for improving operation through data outage periods without reducing measurement quality.