5

GLOBAL NAVIGATION SATELLITESYSTEM DATA ERRORS

5.1 SELECTIVE AVAILABILITY ERRORS

Prior to May 1, 2000, Selective Availability (SA) was a mechanism adopted by the Department of Defense (DOD) to control the achievable navigation accuracy by nonmilitary GPS receivers. In the GPS SPS mode, the SA errors were specified to degrade navigation solution accuracy to 100 m (2D RMS) horizontally and 156 m (RMS) vertically.

In a press release on May 1, 2000, the President of the United States announced the decision to discontinue this intentional degradation of GPS signals available to the public. The decision to discontinue SA was coupled with continuing efforts to upgrade the military utility of systems using GPS and supported by threat assessments that concluded that setting SA to zero would have minimal impact on United States national security. The decision was part of an ongoing effort to make GPS more responsive to civil and commercial users worldwide.

The transition as seen from Colorado Springs, Colorado (USA) at the GPS Support Center is shown in Fig. 5.1. The figure shows the horizontal and vertical errors with SA, and after SA was suspended, midnight GMT (8 p.m. EDT), May 1, 2000. Figure 5.2 shows mean errors with and without SA, with satellite PRN numbers.

Aviation applications will probably be the most visible user group to benefit from the discontinuance of SA. However, precision approach will still require some form of augmentation to ensure that integrity requirements are ...

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