Definition
An authorization to fly an existing ground-based nonprecision instrument approach (such as a VOR, NDB, or VOR/DME approach) using GPS as the primary navigation source instead of, or in addition to, the original ground-based navigation aid. The approach procedure, course, fixes, and minimums remain those of the underlying nonprecision approach; only the means of navigating along it changes. The approach chart will indicate GPS overlay eligibility, typically with 'or GPS' in the title.
Plain English
It means the pilot is allowed to fly an old-style ground-based approach by following GPS instead of (or alongside) the original radio navigation aid. The approach path and minimum altitudes stay the same, but the cockpit guidance comes from GPS.
Context Anchor
Seen when briefing or flying an instrument approach procedure that allows GPS to be used in place of the original ground-based navigation source.
Derivation
Overlay' literally means one thing laid on top of another. Here, GPS guidance is laid on top of an existing nonprecision approach — the underlying procedure stays put, and GPS sits over it as an alternate way to navigate the same path.
Why Pilots Care
It allows the approach to continue when the original navigation aid is out of service or unreliable.
Analogy
It is like putting a GPS route line on top of an existing road map. The line helps you follow the route, but it does not change the road itself.
Intuition Check
Overlay does not mean the GPS creates a new approach. It means GPS is being used to fly an existing approach path, while the original approach rules still apply.
Example Sentence 1
Because the chart was titled 'VOR or GPS RWY 14,' the pilot briefed it as a GPS overlay of the nonprecision approach and selected the procedure from the GPS database.
Example Sentence 2
GPS overlays let crews use legacy nonprecision procedures with satellite guidance instead of ground equipment.