What is Selective Availability (or SA)?
When GPS was first introduced the civilian signals were intentionally degraded to reduce their usefulness to users other than the US military. This was called Selective Availability or SA. SA meant that the public could only receive GPS signals with an accuracy of about 100 metres. On 1st May 2000, SA was turned off. GPS users could then receive signals with an accuracy of about 10 metres. The US is committed to leaving SA turned off, but now has the ability to turn GPS signals off on a regional basis when its national security is threatened.
