Spot, have had one for years. The annual plan can be pricey, but if you have no other means of comms and get in trouble, the plan could be priceless.
The "one button" 9-11 call for rescue services can be a life saver.
We got them through a marketing plan between TowBoatUS and Spot for our towboats, good for when we went out of VHF range on the boats.
I used it on any solo bike trip I took to "out of cell range" locations, of which the Western North Carolina area was when the CVO group started going there.
I kept it on top of my stuff in the tour pack and I had the bread crumb feature on so my wife could see where I was and my progress.
Works good. But, you were asking about a back up GPS, what about your cell phone with a good mapping app? Co Pilot is a highly rated app