The world after SA: benefits to GPS integrity

The Presidential Decision Directive (PDD) on the Global Positioning System (GPS) recommends that selective availability (SA) be removed by 2006. The question remains: if SA were to be turned off, how significant are the benefits to the GPS community? This paper examines the benefits of removing SA as it relates to airborne GPS integrity and demonstrates that there are substantial improvements in the availability of GPS integrity. The study evaluates the current TSO C-129 receiver autonomous integrity monitoring (RAIM) algorithm, which provides fault detection for supplemental use of GPS. Fault detection and exclusion (FDE) functionality for primary means navigation will be contained in the wide area augmentation system (WAAS) equipment (TSO C145/C146), but is designed to be used independent of WAAS. The availability of GPS integrity is examined globally and the results are compared to RAIM and FDE availability with SA on the system. Other augmentations to improve availability, such as the use of geostationary satellites for ranging and additional GPS satellites in the constellation, also are considered, as well as variation of the mask angle. Speculations about improvements from a combined GPS/Galileo system also are provided. The results of this analysis demonstrate a significant improvement in the availability of the fault detection and exclusion functions if SA is turned off and the other augmentations are incorporated. This capability may provide major benefits to countries who either do not plan to implement a WAAS-type system, or may not have one until well into the next century. Although these results hinge on the removal of SA, it is important nonetheless to quantify the magnitude of improvement in the availability of GPS integrity