A Case for Updating the GPS Standard Positioning Service Performance Standard

In 2001 the United States Department of Defense issued a performance standard for operating the Global Positioning System for its open access positioning service, called the Standard Positioning Service (SPS). The SPS Performance Standard was a revision to the SPS Signal Specification issued in 1995, reflecting expected improvements in GPS standard positioning service following the decision in May 2000 to set Selective Availability to zero. At that time the US Government did not know how good the GPS performance would be in the post-Selective-Availability era. The parameters incorporated into the standard, therefore, were quite liberal, providing ample margin to the system operators to ensure the standards would be met. Now with four years of GPS performance to examine, we have found actual performance well exceeds the standards set in 2001. Furthermore with the addition of improved satellite clocks and more efficient satellite operations, GPS has steadily improved in performance throughout this period, both in accuracy and availability. The European Union has made commitments for its own space-based position, navigation, and timing service, called Galileo, and these commitments exceed those made in the US SPS Performance Standard. The time has come, therefore, to revise this document to better reflect operational service levels and US Government desired commitments. This paper looks at what is needed to modify the current commitments. It presents the results of a study of GPS performance data from 2004 showing historical service levels in all the categories of performance, including range and position domain accuracy, geometric and position availability, and system reliability. The paper then presents a case for a revised set of performance standards. The 2001 SPS Performance Standard reflects an effects-based view of GPS performance levels. By this is meant, the service is not defined merely from a constellation centric viewpoint of geometry and user range error commitments. Rather it looks at the performance from the standpoint of the user GPS receiver. This paper explains the advantages to such an effects-based approach in laying out the standards, and builds on this approach, by recommending additional standards for several other classes of users, including aviation and timing users. This paper makes the case for updating the SPS Performance Standard, bringing the document in line with current performance levels and user expectations for service. The result will be a service standard that more closely reflects actual performance, thereby increasing confidence and user satisfaction in the service provided by the Global Positioning System. This paper solely reflects the views of the author, and does not represent the position of the US Government or Overlook Systems.