Definition
A value, calculated and displayed by an area navigation system such as GPS or FMS, that represents the system's own estimate of how far its reported position could be from the aircraft's true position. It is expressed as a radius, in nautical miles or feet, and reflects the quality of the navigation signals and geometry currently available to the receiver.
Plain English
The navigation system's own guess at how wrong it might be. If it shows your position with an estimated position error of 0.1 nautical miles, the system is saying its reported location is probably within that distance of where you actually are.
Context Anchor
Seen on GPS and other navigation equipment when checking how trustworthy the displayed aircraft position is.
Derivation
Estimated' comes from Latin aestimare, to value or appraise. 'Error' comes from Latin errare, to wander or stray. Together the phrase literally means an appraised amount of wandering -- a calculated guess at how far off the reported position might be.
Why Pilots Care
It directly affects whether the aircraft meets required navigation performance for the airspace or approach being flown.
Grounding Statement
Think of it as a warning number: the aircraft may really be this far away from the position shown on the screen.
Intuition Check
“Error” does not mean the pilot made a mistake here. It means possible difference between the shown position and the aircraft’s real position.
Example Sentence 1
Before starting the approach, the pilot checked that the estimated position error was well within the limit required by the procedure.
Example Sentence 2
When the Estimated Position Error exceeded the required limit, the crew switched to conventional navigation.