Global implications on the removal of selective availability

In March 1996, President Clinton issued a Presidential Decision Directive (PDD) that directed the Department of Defense (DoD) to discontinue the use of selective availability (SA) by the year 2006, with annual reviews of the DoD´s status in preparing for the event beginning in 2000. Unless the DoD presents compelling evidence to counter the PDD, SA will transition to an “off” state within the next few years. The purpose of this paper is to explore the ramifications of removing SA from the standard positioning service (SPS). The paper summarizes expected GPS orbit and clock error behavior in the absence of SA, and how those errors map into signal-in-space (SIS) user range errors (UREs). This data is then used to assess expected instantaneous position solution errors on a global basis, using the current constellation. This analysis serves to demonstrate the core contribution of the GPS space and control segments to SPS accuracy. The GPS Modernization Program being executed by the GPS Joint Program Office (JPO) includes a new civil signal that will provide SPS users with a two-frequency ionosphere delay correction capability. However, it will be several years before this capability is available, so single-frequency model errors will still be part of the SPS user´s error budget. The paper summarizes an analysis of measured errors in the single-frequency model based upon correlations between the single frequency model and two-frequency measurements taken over an extended period of time from the Federal Aviation Administration´s (FAA) National Satellite Test Bed (NSTB). This error information is then added to our SIS URE dataset, and instantaneous position solutions including the single frequency error are generated. The paper then discusses the implications our findings have for the global positioning/navigation/timing (PNT) infrastructure. The paper provides a summary of how well GPS will support a wide assortment of user communities, and discusses the policy considerations implicit in the decision to discontinue the use of SA