GPS satellites include ephemeris data in the signals they transmit to GPS receivers.
Ephemeris data is a set of parameters that can be used to accurately calculate the location of a GPS satellite at a particular point in time. It describes the path that the satellite is following as it orbits Earth.
To accurately calculate your location, ephemeris data is only usable for a limited time (a few hours or less). Up-to-date data is needed to minimize error that results from minor variations in a satellite's orbit.
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