GPS Approaches

Recall the basic operation of GPS: it triangulates your position by measuring distances from satellites by precise timing of radio signals.  4 satellites are required to obtain a 3D position fix. 

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