Europe is working on Galileo and Russia has GLONASS. I imagine that future nav devices will be able to use all three. However, it's hard to imagine that any combination of those systems would be especially difficult to jam or otherwise disable for a modern military.
Jamming is super easy if you can physically locate a powerful radio transmitter where you need to jam. Just pump out lots of radio noise at the right frequency and no one can make out the relatively faint GPS signals above the clutter...
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