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Selective Availability: Completely Dead

September 19, 2007

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Selective Availability (SA), the contentious issue of degrading the open GPS civil to advantage military signals, is going away for good under the terms of a presidential decision announced September 18.

Acting on a recommendation of the U.S. Department of Defense (DoD), — including that of Robert England, deputy secretary of defense and cochair of the Space-Based PNT Executive Committee, President Bush ordered the elimination of the SA capability from the GPS Block III satellites, the next generation of spacecraft scheduled to begin launching in 2013. Although the United States stopped the intentional degradation of GPS satellite signals in May 2000, this new action will result in the removal of SA capabilities, thereby eliminating a source of uncertainty in GPS performance that has been of concern to civil GPS users worldwide.

With a new military M-code being transmitted on GPS Block IIR-M and newer generations of satellites, along with in-theater capabilities for denying adversaries access to open GPS signals, military commanders felt comfortable in abandoning SA completely. "Selective availability has not been an effective way to achieve denial of availability," said Lt. General Michael Hamel, commander of the U.S. Air Force Space and Missile Systems Center at Los Angeles Air Force Base, California. "It was a relatively indiscriminate way of implementing [denial]."

"The DoD has demonstrated the capability to selectively deny GPS signals on a regional basis as needed in a military area of operations when U.S. national security is threatened," Mike Shaw, director of the National Space-Based PNT Coordination Office told Inside GNSS. "This was not part of some broader, all inclusive review, but simply recognition that the SA global capability was not needed. The U.S. is embarking on a new generation of GPS III satellites. There is no need to build that capability into this generation of satellites."

A statement by the White House press secretary said, “This decision reflects the United States strong commitment to users of GPS that this free global utility can be counted on to support peaceful civil activities around the world.”

Indeed, international perceptions of GPS seemed to underlie the timing of the decision. Hamel alluded to "rhetoric in international fora about whether or not U.S. could invoke SA," as a consideration for doing away with the SA capability itself. And Shaw noted that "We hope this decision by the U.S. is perceived across the international community as a additional demonstration of the U.S. commitment to worldwide users by showing that GPS can be further relied upon in the future for peaceful civil applications throughout the world,"

SA was implemented on March 25, 1990, by “dithering” or introducing a varying amount of error into the GPS system time transmitted on the C/A-code signal by a GPS satellite. This, in turn, affected a GPS receiver’s computation of the pseudorange to the satellite and thus created an error in the user equipment’s positioning accuracy. However, development of techniques such as differential GPS — the broadcasting of real-time corrections — provided simple means for getting around the degradation of accuracy.

The DoD turned soon SA off, however, to aid allied military users during the first Persian Gulf, war who needed to rely on commercial-grade receivers because of a lack of military equipment that could process the encrypted P(Y)-code. It was reinstated after the end of that conflict and remained in place until President Clinton ordered its elimination effective May 1, 2000.

Despite the halt in the dithering, the fact that GPS satellites retained the capability for its implementation continued to be a source of debate and criticism. The recent presidential announcement is designed to put the subject to rest once and for all.

While SA was in effect, the GPS Standard Positioning Service (SPS) specification stated that autonomous horizontal positioning accuracy would be “no worse than” 100 meters 95 percent of the time.

Copyright © 2007 Gibbons Media & Research LLC, all rights reserved.

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