Definition
Precise orbital position and timing data broadcast by each GPS satellite, describing its own location in space at any given moment. The GPS receiver in the aircraft uses this data, combined with the signals from multiple satellites, to calculate the aircraft's position.
Plain English
Information sent down from each GPS satellite that tells your receiver exactly where that satellite is in the sky right now. Without it, the receiver cannot work out where you are.
Context Anchor
Seen in satellite navigation and GPS discussions, especially when explaining how an aircraft receiver calculates position.
Derivation
Ephemeris comes from the Greek ephemeros, meaning 'daily' or 'for the day.' Astronomers historically published ephemerides — daily tables showing where stars and planets would be on a given date. GPS satellites do the same job in real time, broadcasting their own position so receivers always know where each satellite is.
Why Pilots Care
Without current ephemeris data, GPS position accuracy degrades, affecting navigation reliability during instrument flight.
Grounding Statement
A navigation receiver cannot use a satellite signal accurately unless it also knows where that satellite was when the signal left it.
Intuition Check
Do not read “ephemeris” as just general satellite information. It means precise position-and-time information for a specific satellite.
Example Sentence 1
After being unpowered for several weeks, the GPS took longer than usual to acquire a position while it downloaded fresh satellite ephemeris data.
Example Sentence 2
Outdated satellite ephemeris data can cause position errors during an RNAV approach.