So, thats precisely the question. What is the process, if any.
I think at that point it might be possible to size the effort. Note, the receiver per se, in otherwords the hardware, is already TSO'ed.
I'm hoping some folks who have worked on non-TSO'ed avionics might have some info.
To save some time, I've already looked at the applicable advisory circular, TSO C149, 161b and 162b, 192 etc.
They don't say anything specific other than "meets DO-178B" which is also pretty non-specific.
Is the only way to do IFR GPS to be garmin?