What is required to certify an IFR APPROACH approved GPS into my aircraft?
For IFR certification, the unit must be installed and interfaced with an appropriate annunciator panel to display required advisory/warning information. It also must be interfaced with a CDI for displaying steering (left-right), To- From, and valid (flag) information. The FAA recently eliminated the requirement to fly over five known waypoints to gather accuracy data. However, a flight test is still required to check autopilot interfacing. The approach feature is to be certified by the installing agency by flying the aircraft on at least two approaches to ensure it is operating in an acceptable manner. Of course, the Repair Station performing the flight test must be certified to perform these procedures and have an FAA approved flight test procedure. Please note that the above information is a very general answer to a complex question. Complete information is available in FAA Advisory Circular AC20-138.