Theoretically, as long as the satellites stay in orbit, you could still use GPS if you had a few decent electrical engineers and some side-band communication channel.
You can’t use an off-the-shelf GPS chip to do it, but the satellites are still transmitting, and you can still receive the signal. Use a receiver at a known location to figure out what the current orbits are and anyone that base station can communicate with can input necessary parameters into a mobile unit.
Satellite phones should also be possible to keep running, so you could have satellite-based comms and location post-civilization. I’d bring a sextant, though, just in case.
Ok, now people who know much more about the GPS system can tell me why I’m wrong.
Well, never mind…I guess Stranger is saying that what I had earlier written was incorrect. I had always heard that the Air Force did regular corrections and maintenance on the system to keep it going, but perhaps that’s not the case. I do know that the Russia system had a major failure in 2014 due to a software glitch that took down the entire system, so I still think that any of these systems left completely un-maintained by humans would have a pretty short shelf life to stay viable.
GPS satellites transmit ephemeris and almanac data as well as date and time. (The newer generation of satellites transmit additional state and error correction data.) The satellites require corrections from fixed ground stations to remain current, and would rapidly become unsynchronized without updates. Although you could obtain rough position fixes for a long time even after they became desynchronized, commercial GPS units are reliant upon the ephemeris data to calculate a position.
Most telephone satellites are in Low Earth Orbit and often communicate through ground stations even when going from sat phone to sat phone, so the loss of ground stations would result in effectively disabling the system. I believe Inmarsat performs sat-to-sat communications (Globalstar plans to or does too, if I recall correctly) but ORBCOMM and Iridium probably wouldn’t be functional for long even though the satellites will remain in orbit for a few centuries at least.
Oh, the Air Force definitely monitors and performs maintenance. When we talk about the “ground segment”, it isn’t a separate system from the “space segment”; they’re part and parcel of one very expensive and interconnected system. A loss of the ground segment would likely render the constellation nonfunctional in relatively short order, a fact appreciated almost exclusively by the people who have to maintain it or are strategically dependent upon it. My only point was that they don’t need to reposition satellites frequently or avoid debris at those orbits.
I guess the question is: how much expertise would be required to make a functional system? Presumably there will be plenty of fpgas and solder and antennas around after a zombie apocalypse. If one member of a band of survivors was a reasonably capable electrical engineer, is that enough?
Even accuracy within a few miles would be useful for things like sailing across oceans, since that will get you close enough to see the island you’re aiming for.
I honestly can’t answer that question beyond pointing out that it isn’t like taking a sighting off of the satellite itself and calculating the range based on signal strength and timing as with LORAN. Once the satellite ephemerides become unsynchronized I think the system is pretty much unusable.
However, the sun and stars are still there, and don’t require any user supplied power. Thr lost art of celestial navigation (used by Polynsian islanders for centuries before the sextant was even invented) could become once again a valued skill.
I imagine it’s actually pretty easy to make a very accurate digital sextant, and no one has bothered because we have better solutions. The hard parts of celestial navigation are accurate timekeeping, the calculations, and an accurate model of the earth in relation to the stars. Computers are pretty good at all that.
Stellar updating of inertial navigation is a standard function in systems that predate or cannot rely on the availability of GPS. Litton (now Northrop Grumman) actually produced a stellar/inertial/GPS blended system that uses all three modes to provide a high fidelity solution, as do the upper stage navigation systems of many ICBM/SLBMs.