I have ActiveCaptain on a laptop, iOS apps on iPhone and iPad, CP at the helm, and a smaller (older) Garmin MAP GPS 420 that runs on batteries (or house bank) stored inside a metal oven (should work like a Farraday Cage) in case of lightning strike.

IMHO: If the entire GPS constellation is shut off by the DOD or jammed by hostile force; we will all have much bigger concerns than pleasure craft GPS navigation.