GPS will generally continue to operate as normal, assuming no shift in the overall magnetic field of the earth that would make unstable geosynchronous orbits or drastic magnetic distortion on GPS signals that might create multipath or spurious emissions that upset timing accuracy. (Yeah, the commo guy is claiming ionospheric disruptions are to blame...go figure
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Declination would be trashed..... so if anyone were to actually use map and compass the translation to grid would be problematic. A bit of improvisation and terrain association would fix that up in a jiffy.
