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White House Agrees to Remove Selective Availability


In a statement late Tuesday, the White House said that President Bush has agreed with the U.S. Department of Defense recommendation to permanently do away with Selective Availability, the intentional degradation of the civil GPS signal.
Specifically, the statement said that the U.S. would no longer require that the ability to introduce timing errors in the GPS signal reserved for civilian use be built into future generations of GPS satellites; it specifically cited Block III spacecraft.

The White House acknowledged that this was following on the decision in 2000 to turn Selective Availability off. "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," the statement said.

In simple terms, GPS satellites currently issue two different sets of signals used for determining location: one for the U.S. military and its allies, dubbed the Precise Positioning Service, or PPS, and one for civilian use, dubbed Standard Positioning Service, or SPS. PPS actually comprises two signals and is encrypted, whereas SPS only uses one and is unencrypted; it was designed from the start to be less accurate than PPS. When GPS came about, the military — the GPS satellite fleet is maintained through the U.S. Air Force — didn't want its own technology being used against it in a conflict, so it intended to make the civilian signal less accurate.

Early on, however, SPS proved more accurate than was comfortable for the military, so it introduced Selective Availability (SA). SA degraded the accuracy of the civilian signal on a global basis by introducing intentional timing errors into the civilian signal.

Back in 2000, the U.S. government decided to turn off SA indefinitely, which is one of the factors in the growing adoption of GPS technology in consumer electronics today. This latest pronouncement from Washington D.C. effectively makes the policy change with regard to SA a permanent one.

The U.S. military says SA is no longer necessary, as it has a range of capabilities and technology to implement regional denial of service of civilian GPS signals when needed in the area of conflict — which is why it originally recommended doing away with SA back in 2000. Furthermore, since the advent of GPS, a range of technologies including supplemental satellite and ground-based navigation systems (such as DGPS, WAAS and EGNOS) have grown up to improve the accuracy of civilian GPS, essentially rendering SA moot.

Furthermore, the U.S. Air Force has acknowledged that a recent upgrade to the GPS ground control system will in the future provide a new "security architecture" for supporting troops in combat.


Hentet fra: http://mg.gpsworld.com/gpsmg/article/ar ... ?id=458483


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