Near Term Improvements to WAAS Availability

Since 2003, when it was first declared operational, the Wide Area Augmentation System (WAAS) has been increasing its availability through successive software updates (for example, new monitoring algorithms) and hardware updates (expanded reference receiver network). Today, WAAS provides vertical guidance to more than 3000 runways in the United States. With the current GPS constellation, WAAS has very high availability, and is even robust to some degradation in the constellation. However, even with the current GPS constellation, WAAS availability can be affected by the loss of a single satellite. In addition, there is growing interest in using WAAS for more demanding operations, such as Autoland. For these reasons, it is worthwhile investigating possible improvements in the ground monitors (which would not require avionics updates). In this paper, we propose and evaluate changes in three areas of the monitoring algorithms: signal deformation monitoring, code noise and multipath characterization, and clock and ephemeris monitoring. We show that even without reducing the ionospheric delay error bounds, which are the biggest contributor to the Protection Levels, it is possible to significantly increase WAAS performance.