Comparison of Positions With and Without Selective Availability
12 Hour Data Sets

[graph]

With Selective Availability
April 21, 1997

 Click either image for full size view [graph]

Without Selective Availability
April 20, 1997
The images compare the accuracy of GPS with and without selective availability (SA). Each plot shows the scatter of 12 hours of data (0700 to 1900 UTC) taken at one of the Continuously Operating Reference Stations (CORS) operated by NOAA at Boulder, Colorado. On April 20, 1997, SA was reduced to assist in the search for wreckage of a missing A-10 Warthog. The plots show that SA causes 95% of the points to fall within a radius of 60.7 yards. Without SA, 95% of the points fall within a radius of 7.9 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 Table Mountain National CORS station, National Geodetic Survey, NOAA. Data without SA taken from 0700 to 1900 UTC on April 20, 1997. Data with SA taken from 0700 to 1900 UTC on April 21, 1997. Both data sets were taken at 30 second intervals. Instrumentation was a Turbo-Rogue SNR-8000 GPS receiver with a Dorne Margolin T antenna. 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.