Is US military GPS jam proof?
I have jammed (it really was an accident) a GPS satellite - the signal we (accidental) emitted, the bird failed to receive it's time update from the source in CO.
Then again, not everyone has access to a multi-gigawatt (Erp)autotracking jammer. The point is, almost anything can be jammed with brute force jamming.
Ya, it was that high power - we would bounce a signal from the moon from time to time as part of a test routine. I miss my old job, to say the least.
Nothing that uses radio waves is jam-proof.
Some secure comms hop between frequencies, so that jamming is impractical -- it would require jamming way too much spectrum, would shut down way too much RF communication, and would be dead easy to track. I don't know if secure GPS uses something similar, but even that could be denied for a short period of time in a small area.