For graceful failure, I imagine you would want something akin to radar and a topological map so you can match your position, probably combined with lower fidelity means of locating position (angle or location/angle of sun/moon/stars) to reduce the topological map search space if starting from scratch. I would be surprised if the military didn't already have ways to mitigate lack of GPS, considering it's an obvious information attack vector.
Terrain matching has been around for a long time, with the earliest systems dating from the 1950s, so that's definitely a possibility. This gets a lot of use on cruise missiles.
Another fancy way to navigate without GPS is to use automated celestial navigation. The SR-71 had one of these in the 1960s, and it's also good for submarine-launched nuclear missiles. The hardware is able to sight stars even in the middle of the day (and not just the Sun, funny guy).
For commercial aviation, the typical backups are radio beacons such as VOR and NDB, inertial navigation systems, and good old dead reckoning plus pilotage (i.e. looking out the window).
Old 747s had a window on the ceiling of the cockpit for using celestial navigation.[0]
Also with the reliance on GPS and its inherent fragility has caused the US Navy to restart training in celestial navigation. In order to reboot the training regimen, the Navy is relying on Coast Guard instructors, since the USCG never stopped.[1]
Military aircraft mostly use GPS and INS for navigation. Cruise missiles the same, as well as TERCOM [1], as you alluded to.
I recently spoke to old aircrew guys I worked with and apparently they stopped teaching manual celestial nav in the late 90s finally. Automated celestrial nav was, per my understanding, never too common although SR-71 aircrews commonly used its system.
A variation on terrestrial celestial navigation was used to help orient the Apollo spacecraft en route to and from the Moon. To this day, space missions, such as the Mars Exploration Rover use star trackers to determine the attitude of the spacecraft.
I figured it had existed for quite a while, but didn't imagine it traced back to the 1950's.
> Another fancy way to navigate without GPS is to use automated celestial navigation.
I figured as much. I'm just under-informed in this area, and try not to state things as fact that I don't know as such. I did mention the stars, but it didn't occur to me they are visible during the day with the right equipment. :)
> inertial navigation systems
I'm aware these exist (due to some military fiction I've read), but that's the extent of my knowledge. I'm not aware of how accurate they are.
> good old dead reckoning plus pilotage (i.e. looking out the window).
I was thinking of systems that replace pilots, even if for short whiles, not supplement them, so discounted human correction while in flight.
Inertial navigation is interesting because the error starts out at zero and then builds up with time. Let it run for long enough without recalibration and you'll have no clue where you are. Other techniques tend to have steady error bounds. (Aside from dead reckoning, of course, which is basically just inertial navigation done by hand.)
As far as quantifying that growing error, Wikipedia says it's typically less than 0.6 nautical miles per hour. An airliner after a long oceanic flight could know where it was to within a few miles, good enough to reorient and find the destination airport.
Early efforts in autonomous navigation came out of a strong desire to blow up the Soviet Union, so cruise missiles and ICBMs and such are a good place to look if you're interested in early examples.
Inertial navigation systems are used a lot by submarines, since there's no GPS down there. I think it's pretty accurate, particularly if you combine it with other sensor data like gravitational field strength maps.
As part of a larger system yes, but on their own they're only accurate for a while, as they constantly accumulate error. They require input from other sensors (generally GPS) to provide accurate location.
They need recalibration after every 1-2 sorties for best results but they don't require input from GPS or other sensors. GPS synchronization largely eliminates the need for support staff to recalibrate. INS is incredibly accurate on its own, assuming pre-flight calibration.
Source: Worked on said GPS/INS systems in the military.
And what do you use to tell it where exactly it is during pre-flight calibration?
They "require" GPS input in the sense that if I handed you an INS that was initialized/calibrated with some precision (Oministar/RTK/P-code GPS/ whatever) reference but had been running in the trunk of my car (or dangling from the collar of a feral cat) for N hours/days, there's no way you'd fly a plane with it without initializing it with some sort of position referene.
(If you came up with "sit it on some previously surveyed datum, and you don't need GPS" on your own, you get an A)
