USCG & USN binned the eLoran system for GPS
Do a bit of digging to learn how our "we don't need no backup, we've got all these shiny new satellites!" geniuses dumped the partially-built eLoran enhancements to the existing Loran system, after spending million$ to upgrade the existing stations. eLoran provided an order of magnitude improvement to Loran, and is essentially proof against jamming.
Yes, ye lubbers ... JAMMING! A disgruntled teen with a pocket jammer off the internet can easily blank the extremely low-power GPS signals for miles around. It's happened several times in US port locations. GPS super-high freqs are also susceptible to physical and environmental degradation.
eLoran is very low freq and very high power, virtually impossible to jam. As for hacking, it's also relatively 'low tech' and defensible.
Backup? We don't need no steenkin' backup. We gots GPS! (Think of all the military and domestic aviation, transport, commerce, and surface transportation systems that are now locked in to total GPS dependence ... and this Navy commander sez, "We'll probably just turn it off!" Oh, my achin' head!)