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- To: chrony-users@xxxxxxxxxxxxxxxxxxxx, Hal Murray <hmurray@xxxxxxxxxxxxxxx>
- Subject: Re: [chrony-users] Multi-PPS+RTC Setup Questions
- From: Matt Corallo <ntp-lists@xxxxxxxxxxxxxxx>
- Date: Tue, 29 Sep 2020 16:33:27 -0400
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Going based on my M9 hooked up to a serial line using the binary protocol, its pretty stable (+/- 5ms with an offset
around 75ms), except for the 1.5-hour cycle on which it jumps 30ms
(https://noc.as397444.net/ntpgraphs/peer-offset-127.127.20.1.png). Docs seem to indicate it varies based on the time to
calculate a fix, so I presume there's some correlation across signal quality/sats used, though haven't tried digging
since I use PPS.
As for the cycles, well, I'm waiting to hear back from u-blox on that, but I'm assuming there's something funky.
Matt
On 9/29/20 4:13 PM, Hal Murray wrote:
So NMEA is one of the worst ways of getting accuracy (usually much worse than
internet ntp.) (PPS, network, NMEA, wristwatch is the order of accuracy.)
Some networks are really crappy - see bufferbloat. I have an old DSL line.
Round trip times are often over a second when I'm downloading something.
Some NMEA is really crappy. SiRF is the one that comes to mine.
http://users.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif
There is nothing fundamental about timing on a serial port being crappy. If
the line is idle and the software gives it some data to send, I'd expect the
data to start within a bit time.
Does anybody have data on any UBlox chips? NMEA may be different than their
binary mode.
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