Salute!
Where have you flown recently, Global?
The inertial platforms since the late 70's have demonstrated a mile per hour drift on high performance fighters that introduce many more gees and turn rates and... than your basic 747 cruising level for 10 hours. So a good position update can really help things.
The Trident system had super inertial sensors, but the big thing was the mid course position update using a star. The clever Draper folks used that for basic position, but also to calibrate the inertial. The system did not need constant updates or signals from a GPS sat.
If the Navy or Draper says a system can find a star in broad daylight, I tend to believe them. I agree that heavy clouds and such are a biggie, so a system they are talking about should not be the most inportant nav system for ultimate mission success. One thing you can do nowadays that was not possible 50 years ago is reset your strapdown or RLG inertial at last known position, then apply corrections to the inertial sensors.
I like the idea of a system that is independent of the sats. I might also add that one platform I flew long ago used a doppler system to "help" the inertial. There may be fairly simple sensors to help a good inertial provide very accurate positional and body rates.
Gums sends...