Global Positioning System: Encyclopedia II - Global Positioning System - Selective availability

Global Positioning System - Selective availability

When it was first deployed, GPS included a feature called Selective Availability (or SA) that introduced intentional errors of up to a hundred meters into the publicly available navigation signals, making it difficult to use for guiding long range missiles to precise targets. Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, its allies and a few others, mostly government users.

SA typically added signal errors of up to about 10 m horizontally and 30 m vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern US area might read 30 m off, but 30 m off everywhere and in the same direction. In order to improve the usefulness of GPS for civilian navigation, Differential GPS was used by many civilian GPS receivers to greatly improve accuracy.

During the Gulf War, the shortage of military GPS units and the wide availability of civilian ones among personnel resulted in disabling the Selective Availability. In the 1990s the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own, less accurate, radio navigation systems. The military resisted for most of the 1990s, but SA was eventually turned off in 2000 following an announcement by then US President Bill Clinton, allowing all users to enjoy nearly the same level of access. [7]

The US military maintains the ability to use a more advanced version of Selective Availability, called "Selective Deniability", to reduce the accuracty of civilian GPS units in a specific area without affecting the rest of the world. The original SA system could only limit the accuracy of GPS signals world-wide, or not at all. Authorized military units are still able to decrypt the corrected signals using a tamper-resistant hardware module called an SAASM, Selective Availability / Anti-Spoofing Module.

Military (and selected civilian) users still enjoy some technical advantages which can give quicker satellite lock and increased accuracy. The increased accuracy comes mostly from being able to use both the L1 and L2 frequencies and thus better compensate for the varying signal delay in the ionosphere (see above).

• noaa.gov Selective Availability Factsheet (pdf) or [8]

Adapted from the Wikipedia article "Selective availability", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki