Curious -- how feasible is it for an aircraft to simultaneously support multiple systems for redundancy (e.g. GPS/Galileo/Glonass/BeiDou)?
Perhaps the most compelling option for multi-system redundancy has been the reintroduction of a complete terrestrial navigation system. There have been perennial proposals to fund the construction of an "enhanced LORAN" system to cover the United States that have gotten as far as early design, basically ever since the decommissioning of LORAN-C, but it's never quite happened. Somewhat contrary to what a lot of people might expect, the technology does exist to build a completely earth-based radiolocation system with substantial (but not complete) land coverage, it's just a pretty expensive infrastructure project and has a hard time getting any traction at a time when the FAA is slashing the budget for existing navaid technology.
The infrastructure that was already "half way there," e.g. FAA VOR/DME amd the Coast Guard DGPS infrastructure built on the back of the cancelled GWEN project, is largely all decommissioned or in a slow process of decommissioning.
ASAT warfare has lead to some renewed interest, but not enough to actually get the money on the table.
True redundancy requires inertial dead reckoning, celestial navigation, or land based navigation aids. Sadly, none of these are being widely deployed.
Is your response "it's not feasible because when one fails, they all do"? Or is your response "it's not feasible because when one fails, they all could"?
There is an important distinction between those two.
Making the entire country dependent on GPS limits the options of the military organizations that established it in the first place, and compromises their mission to provide navigation to troops in the field. (in my opinion)
The vulnerability of satellites to deliberate attack should not place the entire nation in a position of vulnerability as well.
A ground based system in friendly territory, with a large number of separate sites, provides a redundancy in numbers that you just can't get from a few tens of satellites.
There should also be ongoing research into improving celestial and inertial navigation systems as yet another backup.
A long range system similar to the Omega system should also be deployed.
But (IIRC) those other systems are not "officially recognized" by either the FCC (frequency protection) or the FAA. So an airplane operator cannot use them for navigational purposes without opening themselves up to litigation because they're not following FAA directives.
FCC recognition of Galileo is limited in scope:
* https://www.gpsworld.com/fcc-approves-use-of-galileo-for-non...
High altitude, away from any precision navigation areas, not a huge risk.
Flying a GPS approach to an airport - definite risk.
Bear in mind that a GPS satellite has a power budget of ~4kw, and it's >12,000 miles up.
A jammer may only have a watt or two of transmit power, but it can put all of that into a single channel, and it's only going to be a few miles away from your receiver.
https://apps.fcc.gov/els/GetAtt.html?id=110032&x=.
Transmitter power is only 25W. EIRP is ~450W.
In practice, the aircraft will do a go around then proceed with ATC vectoring or inertial navigation.
PPDs are omnidirectional, but quite hard to precisely geolocate when intermittent and moving, as was the case in the well known Newark events (which shut down a major airport).
Airports with difficult approach (like the old Kai Tak in HK) could easily result in CFIT with only small errors, let alone other aircraft.