Definition
An authorization that allows a pilot to fly an existing ground-based instrument procedure -- such as a VOR, NDB, or holding pattern -- using GPS for navigation guidance instead of, or in addition to, the original ground-based navaid. The underlying procedure remains the same; only the navigation source used to fly it changes.
Plain English
It means you are flying an old procedure that was designed around a ground station, but you are allowed to use your GPS to fly it. The path is the same -- the equipment guiding you along it is different.
Context Anchor
Seen in instrument flying when GPS is used to fly a published hold or other procedure that was not originally written as a GPS-only procedure.
Derivation
Overlay' here keeps its everyday sense -- one thing laid on top of another. The GPS guidance is laid over the existing ground-based procedure, so the procedure underneath is unchanged but a new way of flying it sits on top.
Why Pilots Care
Allows pilots to use GPS for procedures that were not originally published as GPS procedures, expanding usable navigation options during training or when ground signals are unavailable.
Analogy
It is like placing a clear GPS path on top of a printed route. The highlighted guidance helps you follow the route, but the printed route is still the one you must fly.
Intuition Check
Do not read “overlay” as “replacement.” A GPS overlay helps you fly the existing procedure; it does not turn it into a different procedure.
Example Sentence 1
The hold was originally based on the VOR, but the chart shows it is approved as a GPS overlay, so the crew flew it using the FMS.
Example Sentence 2
Because the approach was approved for GPS overlay, the crew could track the inbound course with the GPS instead of the localizer.