S/A RNAV

Selective availability (S/A) is a tool that the USA could use to degrade the accuracy of the GPSGPS —Global Positioning System (USA) fix obtained by anyone that is not US military - this was regarded as the time as a national security measure. It works by dithering the atomic clocks so that the transmission schedule is in error - hence, any users will not know when the pulses are transmitted and cannot get a fix. S/A was turned off many years ago but could, hypothetically, be reinstated.