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== Ephemeris and clock errors ==
While the ephemeris data is transmitted every 30 seconds, the information itself may be up to two hours old. Variability in solar radiation pressure<ref>[http://ipnpr.jpl.nasa.gov/progress_report/42-159/159I.pdf ''IPN Progress Report'' 42-159 (2004)]</ref> has an indirect effect on GPS accuracy due to its effect on ephemeris errors. If a fast [[time to first fix]] (TTFF) is needed, it is possible to upload a valid ephemeris to a receiver, and in addition to setting the time, a position fix can be obtained in under ten seconds. It is feasible to put such ephemeris data on the web so it can be loaded into mobile GPS devices.<ref>{{cite web|author=SNT080408 |url=http://www.tdc.co.uk/index.php?key=ephemeris |title=Ephemeris Server Example |publisher=Tdc.co.uk |date= |accessdate=2009-10-13}}</ref> See also [[Assisted GPS]].
The satellite's atomic clocks experience noise and [[clock drift]] errors. The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock. However, they are based on observations and may not indicate the clock's current state.
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