Eye on the Ionosphere: GPS after SA

Selective Availability (SA), the intentional degradation of the accuracy of the single-frequency GPS position by the DoD, was ended on May 2, 2000. This major policy decision, promised to occur some time in the next 5 years, had an instantaneous and dramatic impact on the users of single-frequency GPS. Figure 1 shows the “before and after” of the turn-off of SA, and illustrates how significant and error source SA was. SA was turned off at 0400 UTC, the point on the graph at which the horizontal and vertical errors were markedly reduced. The magnitudes of the Standard Positioning Service (SPS) circular error probable (CEP), and sperical error probable (SEP) – the uncertainty in a fix – were reduced to less than 5 meters. ? 2001 John Wiley & Sons, Inc.