The G5 primary mode attitude solution uses GPS aiding to correct for attitude sensor drift. The G5 has a strong preference for using GPS aiding for the primary mode of operation to be immune from an iced/blocked pitot/static affecting the attitude solution. Some say they can detect a blocked pitot/static affecting the attitude solution, but in reality, this is very difficult, if not impossible, to detect.
In the degraded mode failure scenario, GPS is lost (reliably recognized failure) and aiding is done with air data. DO-334 degraded mode required accuracy is attained using only air data drift correction. When we fly the 2+ hours of degraded mode flight tests, we disable the internal G5 GPS receiver, and shut down any external MapMX data being provided by GTX/GNS/GTN. The DO-334 maneuvers are designed to challenge an ADAHRS while it is using an alternate means of aiding (in this case air data).
The G5 does support operation without either air data or GPS data, but we don't spec performance below the dual failure of GPS (primary aiding) and air data (secondary aiding).
Increased availability of primary mode GPS aiding is one of the reasons we built a high performance WAAS GPS into the G5. The most failure resistant installation uses an external antenna for the internal GPS receiver connected to the G5 (which of course is also backed up by the 4 hr backup battery) AND has the RS-232 input wired to receive GPS data from a GNS/GTN.
The G5 will first prefer using the internal GPS, next the externally supplied GPS data from the GNS/GTN, then finally the air data. It is important to recognize that aiding of any kind is only needed occasionally to correct sensor drift.
The GNS/GTN supplied data isn't really required, but some customers prefer this over another GPS antenna and don't mind losing the independence of the WAAS GPS receiver built-into the G5.
As stated above, flight into an area with no GPS service is readily detected by the G5 and air data aiding is used.
We have many thousands of G5 units in the field, and it is very rare for an installation to be completed per the installation manual where the G5 ends up not having a solid source of GPS data. To the best of my knowledge, this was never present in this aircraft until the installation deficiency was addressed.
One additional relevant piece of information. The STC installation manual does not support using a non-WAAS GNS 4XX/5XX as a source of GPS data for the G5. A non-WAAS GNS 4XX/5XX interface uses the Aviation format instead of the MapMX format, and doesn't contain the information needed for the G5 to use it for GPS aiding of the attitude sensors. In an installation with a non-WAAS navigator, the G5 internal GPS receiver must always be used.
Let me also state that we are always looking for ways to improve our products, and we won't hesitate to implement improvements where needed.
This is only a high level description of a very complicated system with lots of safeguards, so please don't try to pick apart the design based on an internet posting.
