Selective Availability OFF forever

I was a recent arrival to the GNSS field in the year 2000 when president Clinton turned off by surprise, to us at ESA at least, the SA degradation to the GPS satellite clocks. At that time at the IGS we were only estimating orbits and clocks and for our work this SA-off had no immediate effect. It did have a longer term effect in the IGS as we started to plan for a clock prediction product latter materialized in the ultra-rapid product updated 4 times a day with 24 hours of predicted orbits and clocks. This product can now substitute the broadcast navigation message for real-time users to provide improved real-time positioning results.

Now president Bush has cleared the air and declared SA dead, by not adding the capabilty in the design of the new GPS satellites, as had been recommended by the DoD. I believe that this reflects the fact that a re-introduction of SA would harm the economic fabric that has been built of single frequency users which rely on stable satellite clocks for increased positioning performance.

SA was artificially “moving the satellite clock” so that the timing measurement which underlie the navigation system could not be as precised as it should be. The GPS satellites have 4 very precise atomic clocks on-board which are used to generate the timing messages, if you purposefully degrade the atomic clock output superposing a signal the timing messages are affected and the position cannot be calculated as precisely. The degradation was based on a function which only authorised military users could effectively remove from the timing message to recover the inherent accuracy of the system. Since SA has been off for almost 8 years and with the improvements in the quality of the GPS navigation message the GPS currently delivers accuracies much higher than originally specified for the civilian single frequency users.

The clock bias values calculated for that day for PRN10 easily show the jump from SA to no-SA on May 4, 2000 at 04:00 UTC indicating the recovery of the stable satellite clock bias :

Clock Bias PRN10, May 2, 2000, SA turned OFF!!

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