Ephemeris and clock errors

11Jun07

The navigation message from a satellite is sent out only every 12.5 minutes. In reality, the data contained in these messages tend to be “out of date” by an even larger amount. Consider the case when a GPS satellite is boosted back into a proper orbit; for some time following the maneuver, the receiver’s calculation of the satellite’s position will be incorrect until it receives another ephemeris update. The onboard clocks are extremely accurate, but they do suffer from some clock drift. This problem tends to be very small, but may add up to 2 meters (6 ft) of inaccuracy.

This class of error is more “stable” than ionospheric problems and tends to change over days or weeks rather than minutes. This makes correction fairly simple by sending out a more accurate almanac on a separate channel.

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