Definition
In GPS, ephemeris data is the precise orbital information broadcast by each GPS satellite that tells a receiver exactly where that satellite is at a given moment. The receiver needs this position data, combined with timing signals, to calculate its own location.
Plain English
Information sent down by each GPS satellite saying 'here is exactly where I am right now.' Your GPS receiver uses that, along with similar messages from other satellites, to work out where you are.
Context Anchor
Seen in GPS operation discussions, especially when explaining how a GPS receiver finds satellites and calculates position.
Derivation
From the Greek 'ephemeros,' meaning 'lasting only a day' or 'daily.' Astronomers used 'ephemeris' for tables giving the daily positions of celestial bodies. GPS borrows the term because each satellite is, in effect, a moving celestial object whose position must be known precisely at every moment.
Why Pilots Care
Fresh ephemeris data is required for the GPS receiver to produce accurate navigation solutions; stale data reduces position precision and can affect instrument approach reliability.
Analogy
Like a published timetable that shows exactly where every bus on a route will be at each hour of the day.
Grounding Statement
A GPS receiver cannot use a satellite accurately until it knows where that satellite is and what time correction to apply.
Intuition Check
Do not think of an ephemeris as the aircraft’s position. It is satellite position-and-time data that helps the receiver calculate the aircraft’s position.
Example Sentence 1
After power-up, the GPS receiver downloaded fresh ephemeris data from each satellite before it could provide a reliable position.
Example Sentence 2
Without updated ephemeris information the receiver could not maintain the required navigation accuracy for the arrival procedure.