19
$\begingroup$

Assuming:

  • there was no power, so there was no web/internet
  • you had a mobile phone (one of the smart ones, not a basic one) powered by solar
  • the phone had maps downloaded on to its memory card (so you were not reliant on the internet for map access),

Could you still use satnav (satellite navigation)?

My rudimentary understanding is that satnav works through orbiting satellites constantly pinging down their location enabling you to exactly pinpoint where you are on the earth's surface. Without the internet, could your phone still access this, or is the internet still needed?

EDIT; I just want to say a big thank you to you all. I can't get over how helpful the replies have been. What a great resource Worldbuilding is!

Improve this question
$\endgroup$
28
$\begingroup$

No, the internet is not needed for a GPS-based device to calculate location.

A standalone device learns about updates to the GPS constellation (broadcast by GPS) as part of it's startup. That's why the take a minute or two to startup - they are listening for updates. Phones often receive these updates via internet, which is why they start working faster than standalone devices.

The orbital data requires regular maintenance and the constellation of satellites requires regular replacement. Since nobody will be doing such work after an apocalypse, location fixes will be come increasingly unreliable after a few weeks. After a few months, GPS coverage will become increasingly spotty. The satellites will begin to fail within a couple years.

There are multiple SATNAV systems in orbit - GPS was the first in common use and is still the most popular. The others have the same requirement for regular maintenance and replacement.

Location means just that - a unique latitude/longitude spot on the Earth. Without a database of paths and other locations, that is of limited use to folks. Standalone GPS devices have the database pre-programmed. Phones download databases of varying sizes and uses from the internet.

There are easy alternatives: You can just follow the highway signs and use paper maps, the way everybody successfully navigated before 2000-or-so.

Improve this answer
$\endgroup$
  • 1
    $\begingroup$ Thank you, that's really helpful. I posed the question because of a specific scene in a novel I'm in the middle of. Post-apocolyptic, a person alone, using maps for navigation, but gets lost from taking a wrong turn. Its sunset, and they need to hole up for the night and quickly. They fire up their mobile phone and prays for the intermitant satnav to work (it often doesn't). It does (hurrah!) and they are able to zoom in on their immediate location and find the closest houses. $\endgroup$ – jane Feb 22 '19 at 18:34
  • $\begingroup$ @jane - Hi and welcome to Worldbuilding! I'll just point out the correct spelling of 'apocalyptic' . Hope that's useful :-) $\endgroup$ – chasly - supports Monica Feb 22 '19 at 18:38
  • 2
    $\begingroup$ GPS was not the first satellite navigation system, Transit or NNSS was deployed in 1964, originally for US Navy use. $\endgroup$ – Ward - Reinstate Monica Feb 23 '19 at 4:14
  • 7
    $\begingroup$ It might degrade quicker than that: the atomic clocks on board the satellites need steering every day to keep them in sync with UTC. Without this the accuracy will drift slowly worse and worse. $\endgroup$ – CharlieB Feb 23 '19 at 9:31
  • 1
    $\begingroup$ One of the things you have to remember of the SatNav GPS system, is that any minor deviations in the satellite positions are ignored, and instead the difference is broadcasted. This means that a GPS system that has turned off for a few weeks has trouble finding any satellites until it receives a GPS packet containing this information. Mobile phones usually bypass this lengthy process by downloading these small files from the internet $\endgroup$ – Ferrybig Feb 24 '19 at 11:59
23
$\begingroup$

It depends on how soon after the apocalypse you're trying to use it.

The GPS system is conceptually divided into three segments: the "space segment", the "control segment", and the "user segment". Other satnav systems function similarly.

The "space segment" is the satellites. You need four satellites to be visible to figure out your location; right now, you can typically see nine of them at any given time. In a post-apocalyptic scenario, nobody's replacing these satellites when they fail, but assuming they all survived whatever happened, it'll be a decade or two before you can no longer count on seeing four at any given time.

The "user segment" is your GPS receiver. You've stated that your character has one that works, so that won't be a problem.

The "control segment" is where the problem lies. The GPS system relies on knowing where the GPS satellites are to extreme precision. To do this, the satellites are tracked by the US military and updated orbits are calculated and transmitted to the satellites, which in turn transmit it to your receiver (this transmission is why a non-internet-connected GPS may take a few minutes to get an initial fix -- they're waiting for the orbit data).

This data is updated every two to six hours, and is only good for a few weeks. After that time, the accuracy of the GPS system will degrade rapidly. If it's been more than a month or two since the last update, your GPS probably can't even tell you what city you're in (or near).

Even if you're in the timeframe where the GPS orbit data is still good, you won't have WAAS data. This is data on ionospheric effects and other unpredictable accuracy reductions. It's updated every five minutes, and without it, your GPS fix will only be good to within 20 meters or so, rather than the 3-meter accuracy you're used to.

Improve this answer
$\endgroup$
9
$\begingroup$

In addition to the maintenance of the satellite constellation there's another problem that's going to degrade your fixes unless you have a military device--the weather. Lightspeed is only constant in a vacuum and the upper fringes of Earth's atmosphere aren't quite a vacuum--enough to matter at the precision that GPS needs.

Normally, part of the information your receiver gets from the satellite is an update on this weather--but nobody's going to be making the weather reports anymore.

There is a second broadcast on another frequency that can be used to calculate and correct for the weather (the effects vary with the frequency, by seeing the difference between the two signals you can figure out how much atmosphere got in the way) but it is encrypted, if you don't have a military device you can't read it. Note that this is also why military devices are a bit more accurate than civilian ones--they get realtime calculations across the actual path the signal took, civilians get the periodically-updated weather reports that are an average over a large area.

Overall, the effect will be the fix degrades in a somewhat random/somewhat predictable way (a satellite that has drifted away from where it should be will make the same error every time it comes around--I think this will cause an error that repeats on a 12 day cycle but I'm not sure) and eventually starts getting periodic holes in the coverage as satellites die and sometimes you don't have 4 satellites above the horizon. Note that there are receivers that are built to look at all the satellites up there, not just the US ones. These will degrade the same but go a bit longer before getting no solution at all. (These units are commonly sold for polar use as the Russian system is optimized for high latitudes.)

Improve this answer
$\endgroup$
  • 2
    $\begingroup$ My Xiaomi phone says it uses GPS, GLONASS, and BeiDou, so perhaps the situation is not so uncommon as I understand from your text? $\endgroup$ – Rui F Ribeiro Feb 22 '19 at 23:34
  • $\begingroup$ @RuiFRibeiro I didn't realize any cell phones had that degree of coverage. It's missing the EU birds, though, and so far the Chinese birds aren't anything like a complete system. The only fully operational constellations are GPS and GLONASS. $\endgroup$ – Loren Pechtel Feb 23 '19 at 4:36
  • $\begingroup$ "but it is encrypted, if you don't have a military device you can't read it" - I would be very surprised if there weren't civilian receivers capable of breaking through the encryption on the military signal. $\endgroup$ – Sean Feb 23 '19 at 5:42
  • 2
    $\begingroup$ @RuiFRibeiro The iPhone X (and newer models) supports “GPS, GLONASS, Galileo, and QZSS”. support.apple.com/kb/SP770?locale=en_GB $\endgroup$ – Mike Scott Feb 23 '19 at 17:23
  • $\begingroup$ @Sean They would have to be custom built receivers--a standard GPS receiver doesn't have the circuitry to receive the encrypted signal. The frequencies are far apart, you can't just tune one radio to do both. $\endgroup$ – Loren Pechtel Feb 24 '19 at 4:34
3
$\begingroup$

First, we must assume that the GPS (or Galileo) satellite constellation is intact, and the satellites have not been shot down as part of the apocalyptic events in your world, and have not been subjected to EMP from nuclear blasts. If all is working up there, then they can continue to transmit their signals, which are nothing more than very accurate clocks (plus an almanac of where every satellite in the constellation is supposed to be).

Down on the ground, you have a device (smart-phone or standalone GPS) that is battery-powered, and a means of charging it. The complicated maths that allows GPS to work operates entirely on board this device. Comparing the time signals from multiple satellites and using the minute (ie pico-seconds) differences (which exist due to the speed of light) to derive a geolocation co-ordinate, is all done on-board your device. Once the co-ordinate has been calculated, it can be compared against the index of a map stored on your device and voila, you know where you are. You can then use your choice of algorithm to calculate the best route to another location in that index, also done entirely on-board the device.

In summary, Yes, SatNav would still work. However, as has been said in comments and other answers, without replacement satellites, and continual updates to both the satellite orbits (and/or the almanacs they transmit) and to the maps on your device, the accuracy and usefulness of the navigation will degrade rather quickly, within a matter of years at best.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.