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.
GPS satellites include ephemeris data in the signals they transmit to GPS receivers.
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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