Explanation of Graph Labels:

SPS- Standard Positioning Service- the normal civilian positioning accuracy obtained by using the single frequency C/A code (Coarse/Acquisition).

CEP- Circular Error Probable (horizontal only, 2D)- a statistical statement that describes a 50% confidence level. In other words, 50% of the positions obtained will be within the stated distance from ground truth.

SEP- Spherical Error Probable (horizontal and vertical, 3D)- a statistical statement that describes a 50% confidence level. In other words, 50% of the positions obtained will be within the stated distance from ground truth.

UTC- Universal Time Coordinated- local solar mean time at the Greenwich Meridian. A uniform atomic time system maintained by the U.S. Naval Observatory.

The transition on this graph shows the reduction of error on May 1, 2000 at Midnight, Eastern Daylight Time, when Selective Availability (S/A) was turned off.