Sam Wormley
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 Posted: Fri Oct 10, 2008 3:14 am Post subject: GPS Performance Standard Document Updated |
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GPS Performance Standard Document Updated
http://www.gpsworld.com/gpsworld/content/printContentPopup.jsp?id=556701
Oct 6, 2008
GPS World
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.
The full SPS document can be found on the National Executive Committee for Space-Based
Positioning, Navigation, and Timing website. |
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