Selective Availability

Turned off on 2 May 2000


According to a decision of the US President the SA degradation of the GPS system (SA=Selective Availability) was turned off at 00 UTC on 2 May 2000.

The horizontal position accuracy and the vertical accuracy before, during and after the removal of the SA degradation is shown below.
This is valid for accuracy at absolute positioning.

For all pictures below applies:

  • All data come from SWEPOS stations Onsala and Mårtsbo.
  • The data is from dual-frequency receivers, Ashtech Z-12.
  • All diagrams show the position each 15 seconds, during 24 hours.
  • The scale is metres.

    Read the statement of Bill Clinton here (text file).

    Press release from Tribmle.
    Press release from Mowic, (Trimble Sweden).
    Press release from Garmin.

    For more information: www.igeb.gov


    		• GPS day 122, 1 May 2000

    		Horizontal deviation with SA (Onsala day 122)
    Deviation +/- 47 metres at 95% confidence interval.

    		Vertical deviation with SA (Onsala day 122)
    Deviation +/- 95 metres at 95% confidence interval..


    		GPS day 123, 2 May 2000

    		Deviation, northing, during day 123, Onsala
    Horizontal deviation +/- 29 metres at 95% confidence interval.
    Time is shown along the X axis. The picture shows the whole 24-hour period. In the following three images the effect of SA removal, around 05:00 UTC on 2 May, can clearly be seen.

    		Deviation, easting, during day 123, Onsala.
    Time is shown along the X axis. The picture shows the whole 24-hour period.

    		Deviation, up, during day 123, Onsala
    Deviation +/- 47 metres at 95% confidence interval.
    Time is shown along the X axis. The picture shows the whole 24-hour period.




    		Deviation, northting, during day 123, Mårtsbo
    Horizontal deviation +/- 28 metres at 95% confidence interval.
    Time is shown along the X axis. The picture shows the whole 24-hour period. Even in these three pictures the effect of SA removal can clearly be seen.

    		Deviation, easting, during day 123, Mårtsbo
    Time is shown along the X axis. The picture shows the whole 24-hour period.

    		Deviation, up, during day 123, Mårtsbo
    Deviation +/- 48 metres at 95% confidence interval.
    Time is shown along the X axis. The picture shows the whole 24-hour period.


    		GPS day 124, 3 May 2000

    		Horizontal deviation during day 124, Onsala
    Deviation +/- 4.6 metres at 95% confidence interval.
    Longitude is shown along the X axis. Latitude is shown along the Y axis. Tha lack of SA effect is clearly seen.
    Compare to the uppermost picture of Onsala, day 122.

    		Deviation, up, during day 124, Onsala
    Deviation +/- 24 metres at 95% confidence interval.

    		Horizontal deviation during day 124, Mårtsbo
    Deviation +/- 4.4 metres at 95% confidence interval.
    Longitude is shown along the X axis. Latitude is shown along the Y axis.

    		Deviation, up, during day 124, Mårtsbo
    Deviation +/- 24 metres at 95% confidence interval.