That is to say, without human input (uploaded data, time base corrections, satellite position control commands, etc) how long will the GPS continue to function after the fall of civilization? If you have a functioning GPS receiver, for how long will it be accurate? Will the satellites cease to transmit after a while (how long?)? Or will the system continue to function, but the accuracy will degrade?
Not a chance. They have to be constantly monitored and given course corrections (IIRC, the Air Force is in charge of this). I don’t know how long it would continue to run without any input from people, but my WAG would be less than a year before it degraded to the point it was useless. I know that it’s already becoming somewhat degraded as new satellites are needed for the network.
I didn’t know Zombies could use GPS…
As for how long they can transmit, the design life of the next generation of GPS satellites, slated to be launched sometime in the future, is 15 years. The most recently launched ones have a design life of 12 years. They might actually exceed their design life, but I’d guess that a couple of decades after apocalypse they would have mostly failed. Of course, long before then, the lack of corrections would probably have rendered them grossly inaccurate, as stated above.
The ephemeris (flight trajectory) of each satellite is updated every two hours. While its position over the next two hours can be calculated to within a few millimeters, the error probably doubles several times a day. The trajectory must be constantly recomputed and updated. Without updates, the position error would probably be 100m after 24 hours. After two days, the error would be off by 1600m (one mile). Within a week, you couldn’t tell what country you were in
Sure they do. Their favorite destination is Braintree, Massachusetts. 
–Mark
Are humans doing this recomputation and updating? I’d imagine its all automated, yes? Not a room full of nerds with abacuseseseseses doing computations. At what point is human intervention required to keep everything in check?
A room full of abacuses (abaci?) makes me immediately think “hmm, that’s a good job for some interns” and so maybe isn’t quite as unrealistic as you might think. 
More seriously, I doubt that any of the computation is done by hand, but I can picture it requiring someone to key in a command into a computer somewhere to initiate the data upload. Just a guess, though. I don’t have any practical experience with these systems.
This wikipedia quote makes me think that it’s nowhere near fully automated:
Not manually (whether by abaci, calculi, or fingers-n-toes).
The folks who manage the GPS constellation use a mission management commercial software product (integrated with the rest of the GPS Control Segment) to compute ephemerides and plan mission events. I don’t feel like advertising, so I won’t name the product, but there are several out there. For a seemingly niche business, it’s a surprisingly large field of commercial software endeavor.
ETA: To bring it back to the OP question, can zombies be given the clearances and physical access to Control Segment systems, and the expertise to use it to manage the GPS fleet? If so, then we’re ok for the service life of the constellation, but I imagine it’d put a dent in the development and deployment schedule for the next-generation GPS Control Segment, or the launch and deployment of the Block IIIA GPS satellites that go with the OCX ground system.
So it does seem that, in the event of a fall of civilization, GPS becomes useless pretty fast. (At least, a lot faster than I would have figured.) I guessed there were inputs required, but if bizerta’s post is correct, it takes a surprisingly short time to get seriously out of calibration.
So, essentially, while the satellites power themselves and will fly for many years, there’s some office building somewhere with some computers in it. The building probably has a backup generator, but people have to drag themselves into the office about every day and they actually click their mice and type a few keys every day to perform these updates. In the typical zombie-outbreak story, the zombies, despite being barely able to walk, kill about 99.99% of the population - enough to leave just a few “named characters” that the audience can remember.
So, the GPS crew would probably become zombies, and they’d lose power to the building where the update gear is installed. It probably has backup generators but only a few days worth of diesel for them, so once it goes down, that’s it. The software’s probably something you could run on a laptop, and there are probably other uplink antenna elsewhere in the world, but those places are also zombie infested, so…
This is the government we’re talking about. They use sliderules.
Or, another scenario is that once the zombie apocalypse starts the people in charge of moving around satellites and other orbital structures and facilities become zombies as well. That means no one is monitoring for space debris and moving those orbital structures out of the way of space junk. One large satellite is struck by a large piece of debris, shattering it and causing more debris. This then causes a cascading effect where other satellites are high, causing more and more debris (sort of the scenario that happened in the movie Gravity…but without the zombies :p). All it would take is for a few of the key GPS satellites to be hammered in this fashion at this time when we’ve drawn down so much on the system and the replacement system hasn’t been put up yet to seriously degrade the thing. That’s really what the folks monitoring and maintaining it are doing…they track the paths of the satellites, check their positions, and also track known debris from all the junk we’ve left up there floating around like high speed bullets, and shift the satellites around when needed.
I almost said sliderules, but I figured I’d go even more low tech to emphasize my point.
In a few previous posts, it seems that posters are disparaging slide rules. If I’ve misunderstood, please forgive my own comment: I submit that anyone who would belittle a slide rule speaks from ignorance. Learn how to use one properly, and you’d realize how ridiculous your statements are.
Well, I wouldn’t trust Siri to get you to your destination.
Actually, in the event of a fall of civilization, GPS becomes useless immediately. Oh, the satellites are still there and everything still works for a while – but, as soon as civilization falls, there is immediately no remaining use for it, and nobody left who had any purpose in using it. So, immediately useless.
Now, it’s possible by then that the whales and dolphins will understand how to use GPS for their needs, for as long as the system continues to function. Maybe even the zombies will find it useful, as long as they have a steady supply of fresh brains available to them.
I am just a few years too young to be of the slide rule generation, so I never learned to use one beyond some really simple “Beginning Slide Rule Lesson #1” examples (like computing 3 x 4). But I understood the basic theory of them (for the main scales anyway). And I had (and still have) an old College Algebra textbook, published in 1948 (that is, older than I am) that has two full chapters on doing complicated numeric computations with logarithms.
So I won’t be one of the posters disparaging slide rules. The government may still use abacuses, or notches carved into bones.
I found a genuine standard-size slide rule in a thrift shop – years after they were long obsolete – and bought it instantly, just for a collectors item if nothing else. I still have that too. I used to take it with me to my college Calculus, Diff Eq, and Statistics exams and set it out on the desk in front of me while I did the exams. Just for fun.
Road GPS would perhaps be useless after the first couple weeks, after the useful stuff in the cities and suburbs have been taken, but a GPS combined with a topo map (either analog or integrated) would be useful for determining where you are in the wilderness.
The Global Positioning System (GPS) is operated and maintained by the Global Positioning Systems Directorate of the Air Force Space and Missile Center, which operates under the Air Force Space Command out of Los Angeles Air Force Base. SMC also oversees the EELV (Delta IV) launches that deploy the satellites, while the Joint Space Operations Center (JSpOC), part of the Joint Functional Command Component for Space of the US Strategic Command, is responsible for tracking space objects, debris, and other hazards.
Control and recalibration of GPS satellites by ground segment is fairly automated and doesn’t really require a lot of direct intervention on a daily basis; it is the maintenance of the ground segment itself that requires human labor and oversight. Like any technological facility, it does require power, maintenance support, and security to continue functioning for any significant period of time (more than a few days), as well as human intervention and decision on any out of specification conditions that aren’t resolved by the normal ground control segment.
The GPS satellites are in MEO at ~20,000 km altitude in size orbital planes. That’s a pretty clean orbit so they shouldn’t have to perform regular debris avoidance maneuvers. In fact, I don’t know that they’ve ever had to perform a maneuver for debris avoidance; all records I can find indicate repositioning in order to optimize constellation coverage, unlike platforms like the ISS that are in the comparative junkyard of the lower altitude of Low Earth Orbit. Most of the debris that would threaten GPS satellites (loose fasteners and other lost material from launch vehicle upper stages) would be difficult to track at that altitude. The system requires a minimum constellation of 24 or 25 satellites to maintain full capability, and is currently robust enough to survive the loss of several satellites.
I don’t understand what you mean by this. As long as an operator has a receiver it is still useful for both land and sea navigation (and air, for what that is worth, though air travel without the support of civilization is likely to be the first mode to be abandoned due to the extreme energy and maintenance requirements). Anyone who has sailed in blue ocean can attest to the benefit of GPS tracking, and the vast majority of blue water cruising sailboats have autonomous power generation via solar panels to recharge batteries for onboard electrical devices including GPS receiver, so at a minimum it will still be valuable to mariners, who are likely to remain in operation the longest in some hypothetical civilization apocalypse. While it is entirely possible to navigate by sextant and dead reckoning between landmarks, it is nowhere near as easy or accurate, and that skill has been largely lost as few yachtsmen bother to exercise the ability.
Stranger