I'm astonished that we are investing in another satellite system. I would have thought that pouring every penny into INS research would have been the way forward.
Three systems was the minimum number to throw one into suspect mode, but now we could have multiple sensors and primary read-outs in a thing the size of a small radio. Make this a sensible size to accommodate military spec electronics, and then multiply by a dozen or so, and it could still fit in the skipper's flight bag.
The summing might have to be done on as many autonomous devices as there were sensors, but the only large cost would be the design and initial fabrication. Repeat manufacture would be a minute fraction of hurling very expensive devices into space.
This system would be very difficult to jam, and could only be done locally, and with vast amounts of energy.
I for one am concerned that GPS is almost too good to be true. Certainly, the satellite life-span has passed expectations, but as mentioned above, they are vulnerable to solar radiation. We've been lucky so far, but who knows what's round the solar corner?
GPS signal is pretty small when it's transmitted, but it diminishes - like all electromagnetic radiation - according to the inverse square law. What we receive is mind-bogglingly feint, and consequently vulnerable to simple swamping, with a relatively broad spectrum signal. i.e. Hard to deceive, but easy to kill.