Ephemeris data describe satellite positions as a function of time.

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

Answer: (B)

Ephemeris data provide the satellite’s location at any given moment by tying position to time. Each data set includes a reference epoch and time parameters that let you compute the satellite’s 3D coordinates for that moment, incorporating the orbital motion and perturbations. Without specifying the time, you don’t know where the satellite is, even though altitude is a derived detail and revolutions are just counts of orbits based on time, while application refers to use rather than the position itself. So the variable that describes the position as the satellite moves is time.