Selective Availability (SA) was an intentional degradation of public GPS signals implemented for national security reasons. In May 2000, at the direction of President Bill Clinton, the U.S government discontinued its use of Selective Availability in order to make GPS more responsive to civil and commercial users worldwide. And therefore the GPS open to public usage. Measuring distance from a satellite define by the following:

1. Velocity x time = distance
2. Three perfect measurements can locate a point in 3-dimensional space, means synchorning the satellite and receiver are based on perfect timing (clock). A major element in GPS system.

But security vulnerabilities occurs on the timing machine. Official announcement shown as below:

Siemens Security Advisory by Siemens ProductCERT SSA-197012: Vulnerabilities in SICLOCK central plant clocks: https://cert-portal.siemens.com/productcert/pdf/ssa-197012.pdf