An ephemeris is quite simply a table giving the coordinates of a celestial body at specific times during a given period. The word comes from the same Greek root as “ephemeral”, which strictly means short-lived, but has come to mean inconsequential. However, in terms of GNSS systems, the ephemeris is certainly not inconsequential.
Each GNSS satellite includes ephemeris data in the signal it transmits. This comprises a set of parameters that can be used to accurately calculate the location of a the satellite at a specific moment in time, and hence describes the path the satellite is following as it orbits Earth.
As the name accurately implies, ephemeris data is only valid for a limited time (a few hours or less). Therefore up-to-date ephemeris data is needed to minimize errors that result from minor variations in a satellite's orbit.
When testing GNSS receivers using a GNSS simulator, ephemeris data play a significant role in several tests. For example, when testing a receiver's time to first fix (TTFF) performance there are differing requirements for the cold-, warm- and hot-start tests. For a cold-start test, the receiver needs to receive time, almanac and ephemeris data. For a hot-start test, all the data are already in the receiver. But for the warm-start test, the receiver already has the time and almanac data but needs to receive fresh ephemeris data.