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0 pointswillis9362y ago0 comments

It probably won't be 100 ns accurate in motion, but still quite accurate. Practically speaking you won't get better than ~1 s unless you connect the PPS pin to an interrupt or PLL.

With a bare minimum implementation you should be able to get the GPS time from the NMEA strings. You just need to guarantee you'll have GPS lock at least intermittently, which is probably true for a fleet of monitored cars in nearly all situations.

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labcomputer2y ago

Urban canyons aside, the GNSS receiver itself should almost always have better than 100 ns accuracy, even when moving. After all, 100 ns at the speed of light is ~30 meters. At 200 ns you're on the wrong city block.

As for bringing that into a computer, I'm having a hard time imagining a hardware setup where a software PLL won't give you millisecond-level accuracy (assuming you have an internal clock with at least that much precision). 1 second accuracy implies just using the timestamp from the last NEMA string with no further processing...

willis936OP2y ago

Yes I am referring to NMEA string usage because that's as good as you'll get if the PPS output is not connected. You don't strictly need a hardware disciplined oscillator solution, which is why I mentioned connecting to an interrupt pin. This has worked pretty well for me when making raspberry pis into stratum 0 NTP servers.

labcomputer2y ago

> Yes I am referring to NMEA string usage because that's as good as you'll get if the PPS output is not connected.

1. That's only true with a super, super naive approach. You can easily estimate the average latency as part of your prototyping, and subtract that (while "live") from when you receive the full NEMA string. That will knock down the (in)accuracy by at least an order of magnitude, probably two, depending on latency variance. NEMA messages are (almost[1]) always sent synchronously with the PPS.

2. Again I'm having trouble imagining this naive hardware setup where you don't have a better timestamp than 1 second. NEMA usually comes over a UART, so just timestamping the first byte of the NEMA string will give you ~millisecond accuracy. Even a cheap USB 1.1 UART dongle that only polls at 125Hz should give you ~10 millisecond accuracy. As above, you can estimate average latency during development to improve accuracy during usage.

You'll need to do a little bit of software filtering to remove outliers, of course, but that requires no extra hardware.

The point is: The technical bar to getting 10ms accuracy with no special hardware and no PPS is basically zero.

[1] I'm hedging with almost, but I've never seen a counterexample.

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labcomputer2y ago

willis936OP2y ago

labcomputer2y ago

> Yes I am referring to NMEA string usage because that's as good as you'll get if the PPS output is not connected.

You'll need to do a little bit of software filtering to remove outliers, of course, but that requires no extra hardware.

The point is: The technical bar to getting 10ms accuracy with no special hardware and no PPS is basically zero.

[1] I'm hedging with almost, but I've never seen a counterexample.

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