Definition
A non-precision instrument approach procedure originally designed for ground-based navigation aids (such as VOR, NDB, or VOR/DME) that has been authorized for use with GPS as the primary navigation source. The procedure retains its original name and chart format, but the pilot may fly the lateral course using GPS guidance instead of the underlying ground-based aid, provided the GPS receiver is approach-certified and the procedure is loaded from a current database.
Plain English
An older approach that was built around a ground station like a VOR or NDB, but which the FAA has approved for you to fly using GPS instead. You follow the same chart and the same path, just with GPS doing the navigation work.
Context Anchor
Seen on instrument approach charts, in IFR flight planning, and when selecting an approach in an approved GPS navigator.
Derivation
Overlay' comes from the idea of laying one thing over another. The GPS course is laid over the original ground-based approach — same path, different navigation source.
Why Pilots Care
It lets pilots use GPS to complete approaches at airports that lack full GPS stand-alone procedures, expanding IFR access without new charted routes.
Analogy
It is like placing a clear route line over an existing paper map. The route is still the original route, but GPS gives you a modern way to follow it.
Intuition Check
Do not read overlay as “any GPS path added by the pilot.” A GPS overlay approach is an approved way to use GPS on a specific published approach.
Example Sentence 1
The pilot briefed the VOR/DME RWY 27 approach and elected to fly it as a GPS overlay approach using the onboard GPS.
Example Sentence 2
When the VOR signal was unreliable, the crew continued on the GPS overlay approach to the same minimums.