Definition
Instrument approach procedures originally designed around a ground-based navigation aid (such as VOR, NDB, or TACAN) that have been republished to allow GPS to be used as the primary means of navigation along the same approach path. The procedure title carries the suffix 'or GPS' (legacy) or is published as a standalone GPS approach that replaces the original. The lateral path, fixes, and minimums are essentially the same as the underlying ground-based procedure.
Plain English
These are older approaches that were originally flown using ground-based radio aids, but have been re-approved so a pilot can fly the exact same path using GPS instead.
Context Anchor
Seen when using an approved GPS or other area-navigation system to fly certain existing instrument approach procedures listed in the aircraft’s navigation database.
Derivation
Overlay' means to lay one thing on top of another. Here, a GPS path is laid directly over the existing ground-based approach path — same route, different navigation source.
Why Pilots Care
It lets pilots use GPS on many older approaches, increasing the number of usable procedures without requiring ground-based equipment.
Analogy
Think of placing a clear plastic sheet over a paper map and tracing the same route on the sheet. The route is not new; it is the same route shown through a different navigation layer.
Intuition Check
Do not read “overlay” as “replacement” or “shortcut.” Here it means a computer-navigation version is placed over an existing published approach.
Example Sentence 1
Because the VOR/DME approach into the airport was approved as an overlay, the pilot was able to fly it using the GPS as the primary navigation source.
Example Sentence 2
Many airports publish overlay approaches so GPS-equipped aircraft can fly procedures that were once limited to VOR receivers.