SA (selective availability)

Selective Availability (SA) was a feature implemented by the United States Department of Defense (DoD) in the Global Positioning System (GPS) to intentionally introduce errors in the positioning data provided to civilian users. The primary purpose of SA was to degrade the accuracy of GPS signals available to civilian users while preserving the accuracy for military and authorized users.

SA was introduced in the early stages of GPS development and was initially implemented to prevent potential adversaries from using GPS for precise navigation and targeting. By introducing intentional errors into the GPS signals, SA reduced the accuracy of civilian GPS receivers, making them less reliable for certain applications that required high precision, such as surveying, mapping, and some scientific research.

The errors introduced by SA were primarily caused by altering the timing signals transmitted by GPS satellites. The timing signals were intentionally manipulated to induce small inaccuracies in the distance calculations made by GPS receivers. These inaccuracies, known as "positioning errors," affected both the horizontal and vertical accuracy of GPS signals.

Before the implementation of SA, civilian GPS receivers could provide position accuracy within a range of tens of meters. However, with SA enabled, the accuracy of civilian GPS signals degraded to several hundred meters or even more, depending on various factors such as the number of satellites in view, signal obstructions, and atmospheric conditions.

The intentional degradation of GPS accuracy through SA was a significant limitation for civilian users, as it restricted the potential applications and reliability of GPS technology in various industries. This limitation led to the development and use of alternative positioning systems and technologies that offered higher accuracy for civilian users, such as differential GPS (DGPS) and other augmentation systems.

In May 2000, the U.S. government announced its decision to discontinue the use of SA, following a series of advancements in GPS technology and a shift in the national security strategy. This change was made in response to the increasing importance of GPS in civilian applications and the availability of more precise positioning systems in other countries.

By discontinuing SA, the accuracy of GPS signals improved significantly for civilian users. Today, GPS receivers can provide positioning accuracy within a few meters or even centimeters, depending on the quality of the receiver and the availability of augmentation systems. This enhanced accuracy has enabled the widespread use of GPS technology in numerous industries and everyday applications, including navigation, transportation, geolocation services, precision agriculture, and more.

In summary, Selective Availability (SA) was a deliberate degradation of GPS signals introduced by the U.S. Department of Defense to limit the accuracy available to civilian users. However, SA was discontinued in 2000, resulting in a significant improvement in GPS accuracy for civilian applications and the widespread adoption of GPS technology in various industries.