Some very good discussions here - earlier I (hope) I provided some useful info on the incident.
Again - hopefully this will help when looking at defences...The military often use what they call a CRPA (Conformal Radiation (some use "Reception" if you want to look online) Pattern antennas. Principle of these is that the antenna pattern steers nulls in the field in the direction of Radio Frequency interference at GPS frequencies. The antennas are typically 4 or 7 elements (determines number of nulls which is N-1 (where N=number of elements). As GPS signals come in below thermal noise - any signal the CRPA detects is regarded as interference and a null is formed in the direction of the source. They work really well and make it much harder for a spoofer (who now has to worry about setting very precise power levels which is difficult for all but most determined). I've been an advocate of these systems being made available to Commercial aviation( but only from my ground based knowledge of aviation) for a long time but so far restricted for military use only.. following is a very technical paper on how they work but I think explains some of the principals. I'd be interested in hearing your views on whether this kind of solution could work well enough for you on the flight deck....

https://web.stanford.edu/group/scpnt...-Jam_final.pdf

On my aircraft the assumed ADIRU drift if GPS is lost is 8 NM per hour. If you’re already on an RNAV approach when you lose GPS, it’s perfectly feasible to complete the approach before you get NAV UNABLE RNP.

8NM per hour!!!

are you sure?
That rate might be ok for the first 30 minutes of a flight but is too high for anything over an hour.

I believe 8 nm per hour is the typical assumed limit with no updating at all (no GPS, no DME/DME). This is a conservative limit.

Otherwise, Part 121 requires 2 nm per hour IRS system accuracy for flights up to 10 hours.

Hoppy is right IMHO. I'm accustomed to 3+3T and almost invariably the results after even longish sectors came in way under that on the B747 classic to which I was then assigned.

Strange then that so many IFR equipped GA aircraft are carrying out RNAV GPS approaches without any inertial reference whatsoever.

It's very common not to have inertial in the smaller business jets too, and they also fly RNAV and RNP with approvals.

Apples and oranges maybe?

The IRU drift rate isn't constant. The IRU will potentially have high drift rates during the taxi out / takeoff / departure climb and also on the way down during the descent, approach, landing and taxi back to the gate.

Conversely, the IRU should be relatively stable during cruise.

On trans-oceanic flights with long stable cruise times, 3 + 3T is a good rule of thumb for approximating the drift limit. But 3 + 3T isn't a good approximation for very short flights.

And during an active approach, especially with a go-around or missed approach, the IRU can drift at much higher rates than 3 + 3T, generally assumed to be 2 nm per 15 minutes (which is 8 nm per hour) based on empirical testing.

For RNAV GPS approaches, if the GPS is lost then (typically) the system is not certified to fall back to DME/DME updating and will only rely on the IRUs.

So for safety reasons, during procedure design in case of GPS loss the IRU is assumed to be exhibiting high drift (8 nm/hr) for N minutes of activity before stabilizing back to 2 nm/hr (with N being a variable depending on what is being designed.)