| Re: [chrony-users] Re: Using gpsd |
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There are three problems with NMEA for timing. One is the length of time that an
NMEA phrase takes to communicate. (hundres of milliseconds). Second is that the
length of the NMEA sentence varies, and thus that time is not constant and
will fluctuate by many milliseconds. Third, the gps module places a low
priority on the NMEA sentences and sends them only when all other calculations
it does on the signals have finished, as I understand it. Again, many milliseconds, esp if you are
tracking a variable number of sattelites. IF you want accurate timing(sub
millisecond), use
PPS (good to microseconds or hundred nanoseconds with a bit of work).
If say a tenth of a second accuracy is fine, then use NMEA.
William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
Physics&Astronomy _|___ Advanced Research _|____ Fax: +1(604)822-5324
UBC, Vancouver,BC _|_ Program in Cosmology |____ unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 ____|____ and Gravity ______|_ theory.physics.ubc.ca/
On Wed, 20 Mar 2024, infection.many550@xxxxxxxxxxx wrote:
[CAUTION: Non-UBC Email]
On Tue, Mar 19, 2024 at 11:48:24PM -0700, Bill Unruh wrote:
FWIW, I'm testing a daemon that reads an *i2c* gps device and
feeds to chrony, and I need to bias it by about 0.250s -- I think
it is just that slow to read the bytes from i2c. But I'm still
looking for a foolish logic error like you suggest.
That would surprize me very much, unless you are doing a very slow
polling of the gps pps. .125 sec sound like the original IBMPC I
bought in 1980. Maybe if you told us the hardware you are using and
the OS you are using someone could help more. Now if it is not PPS
but is rather reading the NMEA, then a slow baud rate could give you
that, since the sentences can be about that long at 600Bd. Also, you
really only need the NMEA for the first few seconds until the system
knows the seconds.
I can't (easily) use the PPS line because it's a separate pin, not
available over i2c. So yes, collecting NMEA lines.
Hardware: Raspberry Pi 3B+ with this gadget:
https://learn.adafruit.com/adafruit-mini-gps-pa1010d-module
running legacy (32 bit) raspbian, and my hack here:
https://gist.github.com/nobrowser/285d63410b9c2aec77a53587020cf2b6
I still want to try reading i2c with a super-slim C program instead,
but I'm not too optimistic about it.
--
Ian
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