Ephemeris
Pronunciation: eh-FEM-er-iss
A table or data file giving the precise position and velocity of a satellite as a function of time, used by ground stations for antenna pointing and orbital analysis.
Full Definition
An ephemeris (plural: ephemerides) is a dataset that provides the position and velocity state vectors of a satellite at closely spaced time intervals, enabling highly accurate prediction of the spacecraft's location. Unlike a TLE, which encodes mean orbital elements and degrades in accuracy over time, an ephemeris is derived from precision orbit determination (POD) using ranging data and is accurate to metres or better. Ground-station antenna control systems ingest ephemeris data to compute look-angles (azimuth and elevation) for pointing. Editors should be aware that the plural 'ephemerides' (eh-FEM-er-ih-deez) is the correct Latin-derived form; 'ephemeri' and 'ephemerises' are not accepted. In satellite operations documents, 'ephemeris' is always singular when referring to a single satellite's data file.
Usage
Usage note: Plural is 'ephemerides', not 'ephemeri'. Distinguish from a TLE, which is a compact, lower-accuracy orbital description. An ephemeris is a high-accuracy time-series dataset.
In Context
- "The flight dynamics team uplinked a fresh ephemeris to the ground control system every 12 hours to maintain pointing accuracy." — Operations Procedure
- "Editors should note that the correct plural is 'ephemerides', not 'ephemeri' or 'ephemerides'." — Editorial Style Guide