Early on pilots crossed the oceans without VORs, GPS, Lorans and many other navigation aids. ADFs are still available even if you have to tune in to an AM radio station. The Inertial navigation system (INS) is the real backup to GPS.
INS is a series of gyros and sensors that measure all movement and direction change. Combine that with a magnetic compass and you have a fairly accurate navigation system. The best part of the INS is it is totally independent from any ground or space base device.https://en.wikipedia.org/wiki/Inertial_navigation_system
The original INS cost over $10,000 (1950's dollars) and weighed quite a bit. Today all the elements needed for a INS is in the average smartphone.
There's an APP for that:https://play.google.com/store/apps/details?id=com.olympio.inav&hl=en
The next generation of glass panels for our planes will have INS integrated along with a backup battery. If the GPS goes out while on approach you will still have full 3d awareness for over an hour.
Klaus Marx - Piper Pacer Pilot
Juneau, AK & Wenatchee, WA . . . . . . Click on Link Bar to view the 'non-profit' group.