Definition
A real-time measurement of how accurately an aircraft's navigation system is currently determining the aircraft's position. ANP is expressed as a radius, in nautical miles, around the computed position within which the aircraft is statistically guaranteed to be located, typically to a 95% confidence level. ANP is continuously compared against the Required Navigation Performance (RNP) value for the phase of flight; if ANP exceeds RNP, the navigation system is no longer meeting the accuracy required for that operation.
Plain English
How accurately the aircraft's navigation system is actually doing its job right now, shown as the size of the area the aircraft is certain to be inside of.
Context Anchor
Seen in area navigation and performance-based navigation discussions, especially when comparing the airplane’s current navigation accuracy with the accuracy required for a route or approach.
Derivation
From Latin actualis meaning 'in fact, real,' contrasted with what is required or expected. The name reflects the idea: this is the navigation accuracy actually being achieved at this moment, as opposed to the accuracy that is merely required.
Why Pilots Care
It determines whether the aircraft can legally and safely continue on a precision RNAV route or approach; if actual performance falls short, the pilot must discontinue the procedure or revert to less accurate navigation.
Analogy
It is like a phone map showing not only your location dot, but also the size of the circle around it that says, “You are probably somewhere inside this area.” A smaller circle means better confidence in the position.
Intuition Check
“Actual” does not mean the airplane is definitely on the correct path. It means the system is estimating how accurate its present position information is.
Example Sentence 1
On the RNP approach, the crew confirmed ANP was 0.1 nautical miles, well within the required RNP value of 0.3.
Example Sentence 2
When GPS signals weakened, actual navigation performance degraded to 0.8 NM and the crew elected to fly the conventional VOR approach instead.