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Asked by: Dan Chitty
I read about the sensor FAF in the AIM and Instrument Procedures Handbook (IPH). Is there more to the sensor FAF than what is in the AIM and IPH? I think I understand the purpose but if anyone can explain further please share.
John D. Collins
on Apr 04, 2013
The sensor FAF is a fictitious waypoint defined for use by the GPS in the approach database when a FAF is not part of the approach procedure. Early on when GPS navigators with approach capability were being introduced, there were no GPS standalone or RNAV (GPS) approaches. To get the technology into more widespread use, the FAA initiated a GPS overlay approach program where an existing ADF or VOR approach could also be flown using a GPS. These approaches were designated by including “or GPS” in the title of the VOR or ADF procedure. Not all VOR or ADF procedures use a FAF, for example if the VOR or NDB is located on or close to the airport. These procedures typically use the VOR or NDB as the IAF and MAP, with a leg flown to a procedure turn and once having completed the procedure turn and established inbound, the pilot descends to the MDA without any FAF. This works fine for a VOR or ADF as the navigation equipment, but presents a problem for a GPS that is designed to always fly a leg (from a FAF waypoint to a MAP waypoint), Several things change in the GPS as the aircraft approaches the FAF, for example the CDI reduces from +/- 1 NM full scale to +/- .3 NM full scale. The integrity is also checked two miles prior to the FAF to verify RAIM is suitable for the approach and the RAIM alarm conditions are reduced to approach levels. So without a FAF, the logic encapsulated in the GPS doesn’t work. The solution is to invent a FAF called the sensor FAF and put it in a convenient location on the inbound leg.
As time went by, more and more GPS stand alone approaches were introduced. Later the GPS approach standard was updated and is now incorporated in the RNAV (GPS) approach standard. As these new approaches came on line, the majority of them had lower MDAs than the overlays and could also be constructed straight in to the runways. As this happened, the overlays were removed. In the same way, as time has gone by most of the GPS procedures were updated to the RNAV standard. As things stand today, most of the overlay approaches are gone along with most of the GPS stand alone procedures. What is left are the RNAV (GPS) procedures many of them enhanced with WAAS to provide LPV approaches with its near ILS capability.
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Dan Chitty on Apr 04, 2013
Your explanation makes perfect sense. Thank you for the always very informative feedback.
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