Definition
An FAA program that authorizes pilots to use an approved GPS receiver in place of the ground-based navigation equipment normally required to fly certain existing non-precision instrument approaches, such as VOR, NDB, TACAN, or RNAV approaches. The original approach procedure is unchanged; the GPS simply substitutes for the underlying navaid, provided the procedure is titled to permit GPS use (for example, 'VOR or GPS RWY 27').
Plain English
A rule that lets you fly an older instrument approach using GPS instead of the ground station the approach was originally built around, as long as the approach chart shows it's allowed.
Context Anchor
Seen in instrument approach and IFR navigation discussions, especially when older procedures could be flown using approved GPS equipment.
Derivation
Overlay' here means laying one thing on top of another. The GPS guidance is laid over the existing approach, following the same path the original navaid would have defined.
Why Pilots Care
It increases the number of usable instrument approaches at airports without requiring the aircraft to have the original ground-based navigation receivers tuned and identified.
Intuition Check
Overlay does not mean a visual layer on a moving map here. It means GPS approval added onto an existing instrument approach procedure.
Example Sentence 1
Because the approach was titled 'VOR or GPS RWY 18,' the pilot used the overlay program to fly it with GPS while the VOR was out of service.
Example Sentence 2
Because the airport had no ILS, the crew elected to fly the localizer approach using the GPS Approach Overlay Program.