What is the difference between TLEs and ephemeris data?

What is the difference between TLEs and ephemeris data?

• A. TLEs are compact orbital element sets; ephemeris data provide detailed trajectory over time.
• B. TLEs describe spacecraft propulsion; ephemeris data describe attitude control.
• C. TLEs are compact orbital element sets; ephemeris data are used for ground station logs only.
• D. TLEs are ephemeris data; ephemeris data are compact orbital element sets.

Answer: (A)

TLEs encode the satellite’s orbit in a compact set of orbital elements and related parameters, which are then propagated with a standard model to estimate where the object will be at a given time. Ephemeris data, on the other hand, provide a time-tagged sequence of state vectors—positions and velocities—giving a detailed trajectory over time. In short, TLEs are a compact element-based representation for quick propagation, while ephemeris data are a detailed trajectory record you can follow directly. The other choices mix up what TLEs describe or what ephemeris data are used for, so they don’t fit.