President Makes It Official — No more SA
This just in courtesy of Wired.com.
Full text of Wired Selective Availability Ending Article here:
The White House said today (19 sep 2007) that it is permanently ending a program that had intentionally built error-producing mechanisms into the nation’s fleet of Global Positioning System (GPS) satellites.
The so-called Selective Availability (SA) degradation capacity has been part of the military-run satellites since their launch, although it has been turned off following a presidential order in 2000. The idea was to prevent U.S. rivals or enemies from using the system for unwanted purposes …
The timing of the decision may or may not have anything to do with the development of Europe’s Galileo program, which is intended to provide a rival, but partially complementary network to the today-unique GPS system …
Like Wired I would certainly add a disclaimer about how important Galileo is to the decision. SA has always been a “Red-headed stepchild” to the GPS program overall and whatever the motivation this decision today just helps all concerned plan better for the future.
SA was originally conceived in this way: The satellites would broadcast two main systems of signals, one useable by anyone and the second set only accessible to those using receivers with a government-controlled decryption module. The system still operates in that way today, but the vast majority of both government an commercial users access only the civil, available to all, signals. The actual navigation information available to both sets of users is, essentially, the same when SA is turned of.
Back when SA was turned on what it did was “dither” or randomly vary the timing of the public signal channels. But if a user had access to accurate timing … such as a GPS correction base station … readily available to the public, the effects of the SA “tinkering” could be completely negated … so the actual utility of SA never lived up to the “denial of accuracy” originally envisioned.
During the first Gulf War (1990/91) the military made relatively little use of government-procured GPS receivers, but even troops on the line we buying GPS receivers for their own use … so SA was effectively “turned off” in that the timing error was set to zero for long periods of time.
During the years between 1991 and 2000 internal discussion, studies and down-right bickering argued incessantly for and against turning SA back on and how radically the signal should be degraded. Finally in 2000, President Clinton “toned down” the arguments by signing a statement committing the US to “discontinuing the use of SA by 2006“. Effectively, SA hasn’t been used since and this months acceptance of DoD recommendations by President Bush effectively drive the final nail in the coffin. RIP, Selective Availability.