Reference AIM COM 3.14
GPS approaches are generally more efficient because they allow pilots to bypass procedure turns and proceed directly to the FAF. GPS approaches must be retrieved from a current avionics database. Pilot-generated waypoints are not approved for approach procedures. There are two types of GPS approaches: stand-alone and overlay.
Stand-Alone
- approach design is usually based on a 'T' pattern
- charted as "RNAV (GPS) RWY XX"
Overlay
- the underlying navaids do not have to be monitored
- you can use the GPS when the traditional navaid is out of service
There is always the requirement for a RAIM check (for +/- 15 minutes of the ETA) Without it, you have no assurance of the accuracy of the GPS position!
If you want to take credit for a GPS approach at an alternate aerodrome (AIM COM 3.14.12):
- there must be a usable approach at the planned destination which is served by a functioning traditional aid
- the published LNAV minima are the lowest landing limits for which credit may be taken when determining alternate weather minima requirements (not LNAV/VNAV or LPV)
- approach-level RAIM must be available at the ETA for the alternate
- periodically during the flight, and at least once before the mid-point of the flight to the destination, verify that approach-level RAIM is expected to be available at the planned alternate at the ETA
Note: There are GPS and WAAS NOTAM files which can advise of outages / failures
