In GNSS, ephemeris data describe satellite positions as a function of which variable?

• A. Time
• B. Altitude
• C. Revolution
• D. Application

Answer: (A)

The satellite’s position changes continuously, so ephemeris data are built to describe where a satellite will be at any given time. The orbital parameters and clock corrections in the ephemeris are evaluated as a function of time, usually using the time since the ephemeris reference epoch. To know where the satellite was (or will be) when a signal is received, you compute its position at the appropriate time t using those time-dependent parameters. Hence, time is the variable that governs the satellite’s position in the ephemeris model. Altitude, revolution count, or application aren’t what the model uses to define the position.