Comment by rcxdude
19 hours ago
>Typically a receiver should be able to get time within seconds from cold start if there's enough sky visibility, and once it does it is as accurate as it gets regardless of position fix status.
It'll be good to far better than seconds when it first gets a signal and will get more accurate (microseconds to nanoseconds) once it has a position fix because it can then compensate for the delays in the signal propagation (you can think of a gps fix as a 4d fix because it also needs to solve for time). You will probably need to use the PPS output to resolve this, though.
I once researched how accurate GPS time is, and the answer is it's within 40 ns. Good luck finding a way to transmit that to your CPU that doesn't add a much larger delay.
What I meant was that it will take seconds to get an accurate time from the sky (as accurate as it gets without specialistic techniques or equipment to consume it, and better than NTP already).
For a cold start, you have to wait 30 seconds for the whole ephemeris data to be broadcast unless you have another data path getting you A-GPS data. (it's literally just broadcast that slowly)
You can still get pretty accurate time (microseconds) with the GPS almanac which is broadcast over ~15 minutes and only updated every few months.
Doing really fast time acquisition isn't a hard problem but it's a hardware problem, you can't really change how an ordinary GPS receiver operates, you have to have a custom receiver.