The Department of Defense turned off Selectively Availability on May 2, 2000 which reduced GPS uncorrected positional error from approximately 100 meters to 10 meters.

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.

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