Definition
The small, accumulating inaccuracies in position and velocity output by an Inertial Navigation System (INS), caused by tiny imperfections in its accelerometers and gyros that compound over time. Because an INS calculates position by continuously integrating sensed accelerations from a known starting point, any sensor bias or drift grows steadily into a position error the longer the system runs without an external reference update.
Plain English
An INS works out where you are by adding up movement from where you started. The sensors that measure that movement aren't perfect, so the calculated position slowly drifts away from the real one. The longer the flight, the bigger the drift.
Context Anchor
Seen in instrument navigation, long-range flight, and discussions of how an INS must be aligned, monitored, and sometimes updated by another navigation source.
Derivation
Inertial comes from inertia, the tendency of an object to keep moving as it is unless acted on by a force. An inertial navigation system uses sensed motion to figure out where the aircraft has gone, so any small sensing error can affect the position it computes.
Why Pilots Care
Uncorrected INS errors can produce large position offsets, leading to navigation deviations, airspace violations, or arrival at the wrong fix if the system is not updated with external references.
Analogy
It is like walking in the dark while counting your steps and turns from a known starting point. If your first direction is a little off, or your step count is slightly wrong, your estimated position gets farther from your real position as you keep going.
Grounding Statement
Small sensor imperfections add up over time so the INS position slowly drifts even though nothing external has changed.
Intuition Check
Do not read “error” here as only a pilot mistake or a system failure. In INS use, an error can be a normal difference between the position the system calculates and the aircraft’s true position.
Example Sentence 1
After eight hours over the Pacific, the crew compared the two INS units and noted the expected INS errors had grown to about three nautical miles.
Example Sentence 2
The pilot performed an in-flight position update to reset the INS errors before entering oceanic airspace.