Four Stories from the Russian Arctic (newyorker.com)

Redundancy

There is a sentence on about every nautical chart I've seen which states, "The Prudent Mariner will not rely solely on any single aid to navigation."

Basically, don't rely on just one thing as your navigation aid.

We have stories almost daily about some aspect of technology failing on a vast scale, to bet one's life solely on a receiver getting picowatts of signal from satellites in orbit which function at the discretion of the military, and the vagaries of orbital mechanics and luck, is foolish, at best.

Always have a backup.

On a recent sar mission, I was navigating and the chart plotter software crashed and had to be restarted, we had no electronic charts for 5 minutes at 20kts, we were losing primary GPS intermittently, meanwhile it was hard to read the radar picture reflected from the coastline as the sea filter had to be turned right up due the heavy and confused seas. The depth sounder was intermittent as we were getting some air every time we went over a wave. You can be damn sure I had the paper charts out and I was regularly logging our position on paper and keeping track of all the navigation marks we could see out the window when it wasn’t covered in spray.

Simplicity.

No batteries required on the receiver, orbital mechanics and general relativity are not a concern, nor is antenna alignment. (Of course, navigation using maps and known landmarks has its own set of issues.)

Same reason it's still prudent to learn to triangulate your position with a compass and map when heading into the outdoors (especially in the Southern Hemisphere, where there's lower GPS satellite coverage).