The National Executive Committee for Space-Based Positioning, Navigation, and Timing (PNT) has released an updated civil GPS Standard Positioning Service Performance Standard, committing the United States Department of Defense (DoD) to an improved level of GPS accuracy for civilian signals.
It is the fourth revision of the standard positioning service (SPS) performance standards document, and the first update since October 2001. In addition to specifying GPS minimum performance commitments, the SPS performance standard serves as a technical document designed to complement the GPS Signal in Space (SIS) Interface Specification.
The most significant change in the updated SPS standards is a 33 percent improvement in the minimum level of SIS range accuracy, from 6 meters root mean square (rms) accuracy to 4 meters rms (7.8 meters, 95 percent), according to the document, which is drafted by the DoD and released through the PNT committee.
Other notable changes are the addition of minimum levels of SIS range velocity accuracy and range acceleration accuracy, which were unspecified in the previous version of the SPS performance standard. The updated document also introduces a definition for an “expandable 24-slot” GPS constellation with more than 24 satellites, although the baseline 24-slot GPS constellation definition remains unchanged from the previous version of the SPS performance document.
While the stated dedication to improvement is notable, it has a built-in conservative margin for minimum performance; as the documents authors note in the executive summary: “with current (2007) SIS accuracy, well designed GPS receivers have been achieving horizontal accuracy of 3 meters or better and vertical accuracy of 5 meters or better, 95 percent of the time.”
One notable item missing from the updated document is a commitment to semicodeless GPS access. This isn’t a surprise, as the U.S. Department of Defense (DoD) published a notice in the Federal Register Tuesday, September 23, stating that it will cease to support codeless/semi-codeless GPS access as of December 31, 2020. Prior to that, on May 16, the U.S. Dept. of Commerce’s (DoC) Office of Space Commercialization first issued a Notice for Public Comment on the DoD proposal to discontinue supporting P(Y) codeless/semicodeless on both GPS L1 and L2 frequencies broadcast from modernized satellites (Block IIR-M, Block IIF and Block IIIA/B/C) beginning December 31, 2020.
The SPS document only addresses the L1 GPS signal. Although three new modernized civil signals will be available in the future, L2C, L5, and L1C, the performance specifications in this version of the SPS apply only to the L1 C/A signal, since this is the only civil GPS signal that is currently fully operational, the SPS authors noted. (Sumber GPS World)