L5 Satellite Based Augmentation Systems Protection Level Equations

The current L1 Space Based Augmentation System (SBAS) protection level equations were agreed upon nearly a decade ago. These equations are provided for L1-only users and are based upon covariance propagation of zero-mean gaussian errors. While this description is reasonably accurate for some nominal error sources, it is not always a good model for actual error characteristics. When departures from the zero-mean gaussian model are significant, the broadcast confidence terms must be inflated in order to provide protection for all user geometries. This leads to a loss of availability even for users who do not observe the satellite with the problematic errors. Since these equations were first adopted, a significant amount of work has gone into accurate characterization of the error sources and the treatment of non-zero and non-gaussian errors. New SBAS signals planned for the L5frequency offer an opportunity to revisit these decisions and make changes to improve integrity and availability.