In GNSS, what does ephemeris refer to?

In the context of GNSS (Global Navigation Satellite Systems), an ephemeris is essentially a detailed set of data that indicates the precise position of a satellite at any given moment in time. This information is crucial for GNSS receivers to calculate their own positions accurately. The position of the satellite continuously changes due to its orbit, and the ephemeris provides a time-dependent record of these variations.

The ephemeris data includes not only the satellite's position but also other relevant parameters that support satellite positioning, such as velocity and corrections for gravitational effects from celestial bodies. This time-related information allows GNSS receivers to synchronize and determine their position relative to the satellites as they move in space.

The other options, which might suggest different relationships (like altitude, revolution, or application) do not accurately capture the essence of what ephemeris data represents. Only the option addressing satellite positions as a function of time encompasses the critical, dynamic aspects necessary for GNSS operations.