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Discrepancy Detected In GPS Time
jones_supa writes that on Tuesday, 26th January, Aalto University's Metsähovi observatory located in Kirkkonummi, Finland, detected a rare anomaly in time reported by the GPS system (Google translation). The automatic monitoring system of a hydrogen maser atomic clock triggered an alarm which reported a deviation of 13.7 microseconds. While this is tiny, it is a sign of a problem somewhere, and does not exclude the possibility of larger timekeeping problems happening. The specific source of the problem is not known, but candidates are a faulty GPS satellite or an atomic clock placed in one. Particle flare-up from sun is unlikely, as the observatory has currently not detected unusually high activity from sun.
http://www.navcen.uscg.gov/?Do...
Actually GPS receivers on earth are in a constant state of being updated. Part of the transmission from the satellite includes a continuous update of the orbital data for the GPS constellation, and other related data. Also, in North America, the WAAS system downlinks atmospheric correction data in real-time so that the GPS receiver can compensate for changes in the ionosphere.
...si hoc legere nimium eruditionis habes...
As someone who works in GPS software, you see these kinds of off-by-miles readings from time to time. If it was one time and not constant, we already have software in place to ignore the anomalous reading.
Peter predicted that you would "deliberately forget" creation 2000 years ago...
Using only the exact time from it and not having it calculate your location would only be useful for something like dead reckoning. They are not looking at a clock and deciding when to turn, they are looking at a GPS device to tell it where they are. The GPS is using the exact timing data to calculate it's location and even tiny errors can cause big errors in your location.
For every nanosecond off you get about 1 foot (0.3 meters) off on your location, 13.7 microseconds is enough for the GPS to think your location is 2.55 miles (4.11 KM) away from where you really are. That is also the error from only one satellite so the error from other ones adds even more error to where the GPS thinks you are.
http://www.montana.edu/gps/und...
https://en.wikipedia.org/wiki/...
The status of each SV is also part of the datastream, and all it takes to "turn off" an errant SV is to set the flag in the data stream that says it is unusable.
WAAS doesn't know about atmospheric corrections. What WAAS does is use a network of fixed ground stations that detect deviations in position and generate data to correct those deviations. The assumption is that the WAAS receiver isn't moving, so any deviations are from propagation errors. This is the same kind of thing that has been used by surveyors and other high accuracy GPS users for decades. At the highest level of accuracy it is called realtime kinematic GPS, and it uses both the correction data and actual carrier phase information to give centimeter level accuracy. There is also "differential", which makes use of the correction data to get multi-cm level accuracy. Both were very big issues when selective availability was on.
In regards to WAAS, I think you're talking about something else. WAAS was developed for the FAA to allow the use of GPS in all stages of flight, including precision landing.
It's based on a network of high precision ground receivers which are used to calculate two sets of correction information. The first is intended for all receivers in the WAAS footprint (basically North America), and consists of estimates of the error in the satellite position, and clock errors. The other breaks the continent up into a grid, and provides local estimates for errors in the ephemeris, clock errors, and ionospheric delay.
...si hoc legere nimium eruditionis habes...