The clock and ephemeris error is one GPS issue which users might have to contend with. Correcting these errors is a significant challenge to improving GPS position accuracy.
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 In reality, the data contained in these messages tend to be out of date by an even larger amount.

When a GPS satellite is boosted back into a proper orbit, for some time following this movement, 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 six feet 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.

According to the theory of relativity, due to their constant movement and height relative to the Earth-centered inertial reference of frame, the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly because they are in a weaker gravitational field than the atomic clocks on the Earth’s surface. On the other hand, special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks.

When combined, the discrepancy is 38 microseconds per day. To account for this, the frequency of the clock on board each satellite is given a rate offset prior to launch so that it will run slightly slower than the desired frequency on Earth.

GPS observation processing must also compensate for another relativistic effect called the Sagnac effect. The GPS time scale is defined in an inertial system, but observations are processed in Earth centered and Earth fixed system which is co-rotating and simultaneity is not uniquely defined.

The Lorentz transformation between the two systems modifies the signal run time – a correction having opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds – or tens of meters in position.

The atomic clocks on board the GPS satellites are precisely tuned. This makes the system a practical engineering application of the scientific theory of relativity in a real-world system.

To know more on the solution on problems with GPS system, please visit GPSAutoTracker for more tips on how to maximize the use of your GPS system.

By: Audrey Ly