What is an ephemeris?
An ephemeris is a list of where a satellite is over time. It is the precise counterpart to a TLE: answers instead of inputs.
In astronomy an ephemeris is simply a table of where something is in the sky at given times. For satellites it means the same thing: a sequence of positions, and usually velocities, stamped with the time each one is valid for.
Ephemeris vs TLE
A TLE stores the orbit you must propagate; an ephemeris stores the propagated result. With a TLE you run SGP4 yourself to get a position. With an ephemeris the positions are already computed and listed, often at higher accuracy than SGP4 can reach, because the producer had better tracking data.
Operators publish ephemerides in standard formats such as the CCSDS Orbit Ephemeris Message (OEM) for conjunction analysis and precision operations. Where a public TLE might be good to a kilometre, an operator's own ephemeris built from GPS or laser ranging can be metres-level.
When each is used
- TLEs are the universal, free, public input for general tracking and visualisation.
- Ephemerides are used where accuracy matters most: close-approach screening, maneuver planning, and operator handovers.
Vantafort propagates public TLEs on the GPU to build live positions for the whole catalog. See how that works in SGP4/SDP4 on WebGPU, or start with how to track satellites.
Frequently asked questions
- What is an ephemeris?
- An ephemeris is a table of a satellite's precise position, and usually velocity, at a sequence of times. Where a TLE encodes an orbit you must propagate, an ephemeris lists the computed answers directly.
- What is the difference between an ephemeris and a TLE?
- A TLE is mean orbital elements you feed into SGP4 to compute positions. An ephemeris is the positions themselves, sampled over time, often to much higher accuracy. Operators publish ephemerides (for example in CCSDS OEM format) for precision work.
- How accurate is an ephemeris?
- As accurate as its source. An operator's own ephemeris, built from GPS or laser ranging, can be metres-level, far better than SGP4 propagated from a public TLE.