Comparison of Positions With and Without Selective Availability
6 1/2 Hour Data Sets

[graph]

With Selective Availability
May 1, 2000

 Click either image for full size view [graph]

Without Selective Availability
May 2, 2000
The images compare the accuracy of GPS with and without selective availability (SA). Each plot shows the positional scatter of 6.5 hours of data (0730 to 1400 UTC) taken at one of the Continuously Operating Reference Stations (CORS) operated by the U.S. Coast Guard at Hartsville, Tennessee. On May 2, 2000, SA was no longer present. The plots show that SA causes 95% of the points to fall within a radius of 48.3 yards. Without SA, 95% of the points fall within a radius of 4.5 yards. 

As illustration, consider a football stadium. With SA activated, you really only know if you are on the field or in the stands at that football stadium; with SA switched off, you know which yard marker you are standing on.

For additional information:

Dr. Richard Snay
National Geodetic Survey, NOAA
Richard Snay
Fine Print: Data taken at the Hartsville National CORS station, National Geodetic Survey, NOAA. Data with SA were taken from 0730 to 1400 UTC on May 1, 2000. Data without SA were taken from 0730 to 1400 UTC on May 2, 2000. Both data sets were taken at 30 second intervals. Instrumentation was an Ashtech Z-12 receiver. GPS data were dual-frequency pseudorange (both L1 and L2) incorporating ionospheric correction. Data were processed in accordance with the GPS Interface Control Document ICD-GPS-200C, using the broadcast orbit parameters in the World Geodetic System WGS 84 (G873) reference system.