GPS is mission critical in that INS13 fuses all the sensors together to run the IMU and INS13 basically can't fly without it. One time I had a bench test running INS13 indoors and sats dropped to 2 or 3 at which point it thought it was moving westward at high speed (as if a strong wind was blowing). If it had been flying, it would have flown eastward at high speed. I've never had this happen outdoors, even with an old (USA satellites only) GPS. But then again, I have never used one of those small GPS's (small antenna has poor performance).
This gets into having different failsafes for different failures. Normally you make the one failsafe do RTB if it runs out of RC range. Do you want to give that up to handle a case where the GPS comes off?
You might consider checking out the new AttiEstAlgos in the latest next. I know there are some new ones that are designed to make GPS flight modes work with less than optimal mag sensors. One of those new algos runs the CF (Basic) in parallel to INS13 IIRC and from there it would be possible to enhance the code to switch to CF and Attitude mode if you lost GPS, but then what? I guess it would be OK if you weren't flying waypoints out of RC range and could fly it back home manually.
My thoughts anyway.
