The White House made the announcement on May 1. It had long been anticipated that SA would go away, at least during peacetime. Thousands of civilian GPS applications have been developed and many would be enhanced without SA. In addition SA can be defeated through differential GPS, having another GPS receiver at a known location transmit its signal and comparing it with the signal from the GPS receiver in question. While a Differential GPS station is impractical for an individual civilian it's well within the capabilities of adversaries to the U.S.

With Selective Availability on GPS had an accuracy of about 100 meters (328 ft), the length of a football field. With SA off its accuracy is about 16 meters (52 ft), a significant improvement.

Comparison of Positions With and Without SA 6 1/2 Hour Data Sets.
Interagency GPS Executive Board

The Navstar satellites have the capability to turn on SA over a particular region of the world while leaving other areas unaffected. Conceivably this could be done during a major regional conflict to reduce the quality of an adversary's location measurements while permitting friendly military forces access to the precise GPS signal.

Artist's rendition of GPS Block IIR satellite in orbit.
Photo credit - Lockheed Martin

Currently there are 27 Navstar satellites in operation. The minimum required for full global coverage is 24 and nominal constellation consists of 24 satellites plus four spares. The first Block II Navstar, launched on February 14, 1989 has been decommissioned, far exceeding its planned 7.5 years of operation. As we go to press a replacement satellite, Navstar Block IIR-4, is scheduled for launch on May 10 and will go in to the Navstar II-1's location, the E1 slot.

One of the dozens of Delta II rockets used to launch the GPS Block II, IIA, and IIR satellites. Photo credit - USAF.

We've come a long way!

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