So what would break if GPS went down for 4 days?
Sat nav obviously, Amazon package tracking, Grindr? (I assume that falls back to cell tower triangulation???).
I guess theres some more serious things, any examples?
Sat nav obviously, Amazon package tracking, Grindr? (I assume that falls back to cell tower triangulation???).
I guess theres some more serious things, any examples?
Without GPS, these reference oscillators can free-run for a while but they will eventually drift out of tolerance.
Basically, without GPS, most/all cellular/digital radio networks will fail in some manner after a while. Precise failure modes will depend on a lot of things including exactly what is wrong with GPS and the type of emission, how the base station software is written, etc.
I'm not too sure about LTE, but some initial googling suggests that while it does need a precision frequency and time source, there are ways of deriving that from the network, which makes sense if you think about indoor base stations that often don't have a clear view of the sky.
Or do they do something simpler, and just derive a frequency reference from the frequency of the carrier signals and sync the local to some fixed ratio of the carrier frequency?
The former would be more accurate, but would break if something went wrong with the data. The latter would probably not be as accurate, due to Doppler effect (I'm getting +/-0.00041% shift from Doppler effect), but would keep working as long as the carrier is being broadcast. You could probably correct for a good part of the Doppler effect just from the last known orbital parameters of the satellite--even if the data updates have been broken for a long time that should work.
It's the first - the GPS modules used for these purposes decode the time and then generate a digital pulse on an output pin at the top of each second (this is called a 1PPS output). A hardware counter counts the cycles from a local oscillator between pulses. The count between pulses is latched (this is all in hardware so far) and then the software reads the latched value. If your local reference oscillator is supposed to be 10MHz, your counter should report 10,000,000 ticks exactly. If it's high or low, the software can adjust the temperature of a little mini-oven that the oscillator resides in to speed it up or slow it down (some oscillators just use a voltage to speed up or slow down instead of an oven). Usually this is done in a PID loop or similar feedback loop. At this stage you can apply some filtering logic to help ride out certain errors (if you're suddenly off by 50% something else is probably wrong, etc.).
To ensure the GPS timing is as accurate as possible, you can have the GPS module survey its position for a lengthy period of time (since it is presumably mounted on an antenna mast somewhere and not moving) and average it out to establish a more accurate well-known location. The GPS can then use that known location to apply a correction to the GPS signals. Basically, if it's currently-computed position based on GPS signals is 50 meters away from the true well-known location of the receiver, it can figure out how far off the timing computation is and adjust. This is mostly handled by the GPS module itself, and is probably even more automated and accurate now than when I worked on it ~15 years ago.
There's also ways to increase GPS accuracy using supplementary broadcasts that contain localized offset correction information, referred to as Differential GPS (DGPS) run by many groups including the US Coast Guard and a similar technology called WAAS (wide area augmentation system) run by the US FAA. The GPS we were using didn't have the capability to use those sources but modern units probably do.
There is actually an article from the New Yorker answering your question: much less traffic, all planes grounded, no train, etc. [1]
> The U.S. Department of Homeland Security classifies sixteen infrastructure sectors—including dams, agriculture, health care, emergency services, and information technology—as critical, and therefore particularly vulnerable to sabotage. All but three require G.P.S. for essential functions.
[1] https://www.newyorker.com/tech/annals-of-technology/what-wou...
trucking companies rely on it to enforce rest periods. so more truck accidents?
turn by turn direction on phones would go out. wi fi is enough for nav, unless your phone and the wifi are on different ntp networks maybe I had a phone with broken gps and I didn't really notice.
I suppose the Qualcomm unit might eventually be off by a second or three. But I don't know if its clock is updated by GPS, phone or wifi. I'm guessing phone.
We're also required to carry paper log sheets in case of Qualcomm outage. So then it would be paper, watch and honesty. The honesty can be sanity checked by speed an distance.
Base stations should be using GPS (and others) to discipline a local oscilator that should be ok enough for a while in absence of signal. They can probably also get a timing signal from their backhaul.
It works better with precise timing, but I think it's workable with best efforts.
I certainly noticed in the UK on an iPhone 6 where the GPS chip was borked. You would drive for 10 minutes and then you would get an updated location. This was in outer London.
How could I test that?
Galileo hasn't reached full operating capability yet, so that's probably why few people have noticed (as why would you depend on an incomplete system?)
Something similar is used to test German Toll Collect On-Board Units (OBUs) before refurbishment.
"Everyone go to Area 51 -- they can't stop all of us" essentially.
As to if it's just internet cranks, the US military shifting views, or aggressive, foreign AGI-PROP is debatable. My money says it's a distraction, but I don't know what it's a distraction from.
Can you imagine if these people decided to, say, storm immigrant detention centers, or abusive corporations... or anything that productive in a meaningful sense?