I wonder what the Navy now uses as a backup system? It has to be something the does not depend on satellites. Even in days of sail and before accurate clocks, captains used dead-reckoning involving the log and estimating the wind as a backup for celestial fixes and noon sun lines.
funny thing, the sextant is being taught once again.
The backup is another GPS unit. The military considered GPS so “reliable” that they have backup GPS units as the backup.
Even for aircraft nav GPS is becoming the primary navaid, with another GPS unit as backup. There are still VOR, NDB and other navaids routinely used, but GPS is becoming dominant.
P.S. I know reading my above post probably made you dizzy to even think they do that, but they do.