**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.