Point of Beginning

Selective Availability Turned Off

June 1, 2000
Each plot shows the positional scatter of 6.5 hours of data taken at one of the Continuously Operating Reference Stations (CORS) operated by the U.S. Coast Guard at Hartsville, Tenn. The plots show that S/A causes 95 percent of the points to fall within a radius of 48.3 yards. Without S/A, 95 percent of the points fall within a radius of 4.5 yards. GPS data were dual-frequency pseudorange (both L1 and L2) incorporating ionospheric correction. Data were processed using the broadcast orbit parameters in the World Geodetic System WGS 84 (G873) reference system.
More than 4 million GPS users worldwide now have access to a military-quality signal as of May 1, 2000, when President Clinton called for the end of Selective Availability (S/A) to the public. Many view the decision to be the beginning of GPS modernization. Steven W. Berglund, president and CEO of Trimble Navigation Limited (Sunnyvale, Calif), said, “The decision to turn off S/A is a milestone in GPS history and it underscores the importance of the technology as a global information utility.”

The decision has caused concern among some surveyors, however. A visitor to the rpls.com bulletin board wrote, “There will be a greatly increased occurrence of abuse by lay persons trying to survey and map with their recreational GPS equipment. It will enhance the ability to use inexpensive GPS for survey recon, corner monument search and recovery.”

Other surveyors voiced concern that there will no longer be a need for differential GPS (DGPS). These fears are unfounded according to the U.S. Department of Commerce, which says that “Even with S/A turned off, GPS alone will not meet all users needs. For users with higher accuracy, availability and integrity requirements—such as surveyors—GPS will still need to be augmented locally with high-fidelity error correction systems based on differential GPS (DGPS) technology.”

Experts from Magellan Corporation (Santa Clara, Calif.) say that “precision-minded DGPS users who rely on transmissions of correction data, such as those broadcast by the U.S. Coast Guard’s reference stations, would normally need correction signals every three to five seconds to obtain a high level of accuracy. Now, the same users will need significantly fewer transfers of data to maintain the same level of accuracy. Any GPS user requiring better than 10 m accuracy should continue to use DGPS corrections to remove the longer term errors in the system such as orbit and satellite clock errors, atmospherics, multipath and receiver noise.”

The change does not require users to purchase new GPS receivers, but manufacturers of GPS devices will benefit from the new signal. John Huyett, president and CEO of Magellan Corporation, said, “This change immediately makes GPS more accurate and reliable, and thus more valuable to our GPS customers.” Jonathan Ladd, Magellan’s senior vice president of worldwide commercial technology, said the decision to end selective availability adds fidelity to the GPS system without costing more. “Prior to this decision, taxpayers were paying for decreased accuracy. Now, they’re getting more accuracy at no additional cost,” Ladd said.

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Selective Availability has been used by the Defense Department to maintain a military advantage by preventing enemies from using GPS against the United States. U.S. industry has long urged the government to provide a military-quality signal to civilian users, but the Pentagon opposed the move on national-security grounds. The Presidential Decision Directive (PDD) fact sheet issued on March 29, 1996, stipulated that the use of S/A would be discontinued no later than 2006. The decision was made sooner, however, after recommendations from the Secretary of Defense, the Departments of State, Transportation and Commerce, the Director of Central Intelligence, and other departments and agencies. In recent years, civil and commercial use of GPS has boomed, leading to a demand for more accurate signals, and the Commerce Department estimates the commercial GPS market will reach $16 billion in U.S. sales this year, possibly doubling in the next three years.

In a White House statement, the President said, “The decision to discontinue S/A is the latest measure in an ongoing effort to make GPS more responsive to civil and commercial users worldwide This increase in accuracy will allow new GPS applications to emerge and continue to enhance the lives of people around the world.”

Specifically, the new availability will improve the accuracy of the GPS signal tenfold, from 100 meters to about 10 meters. This will improve the accuracy of car navigation, enhanced 911 emergency systems and location devices for hiking, camping and other recreational use. Satellite navigation can be used to track a missing person to an area about the size of a tennis court vs. the previous football field-sized area. The new measure will probably mean lower costs for higher levels of accuracy in the near future. The improved signal, which provides time as well as location coordinates, should also allow faster and cheaper data transmission.

Larry Hothem of the U.S. Geodetic Survey said the shut off will make things work better for civilians, but have little effect on military applications that already bypass S/A. “The removal of S/A will improve the unclassified positioning and time precision of GPS by an order of magnitude and provide civilians with improved access to our master clock. The improved positioning stemming from the improved timing will be important to commercial navigation and the Departments of Transportation and Commerce.”

The government hopes to provide more accuracy for GPS users with the addition of another two civilian signals in 2003 and 2005.