Re: [chrony-users] Setting up Chrony with PPS

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

> pi@gps ~/SystemFiles $ chronyc sources
> 210 Number of sources = 6
> MS Name/IP address  Stratum Poll Reach LastRx Last sample
> =======================================================================
> #* PPS                   0   4   377    13    -19ms[-1103ms] +/- 7321us
> #? NMEA                  0   4   377    14   -140ms[ -140ms] +/-  138ms
> ^- kahuna.ruselabs.com   2   6   377    30   +951ms[ -661ms] +/-  114ms
> ^- zulu.frizzen.net      3   6   377    35  +1433ms[ -178ms] +/-   85ms
> ^- four10.gac.edu        2   6   377    34  +1341ms[ -270ms] +/-   84ms
> ^- gopher.fart.website   3   6   377    33  +1249ms[ -362ms] +/-   98ms

I am really worried about your scatter for the PPS, as I was for the scatter
in your assert line. You should NOT be getting a 7ms uncertainty in PPS. It
should be more like 7us-- ie 1000 times better. There is something wrong.
Either your gpio do not raise interrupts and are being read by very bad
polling, or there is something very wrong with your GPS. 
I also cannot understand your HORRIBLE offsets from external sources ( 1.3
seconds out) That is absurd.  It looks to me like your NMEA is way out as
well. YOur GPS is a piece of junk it looks like.

> pi@gps ~/SystemFiles $ chronyc sourcestats
> 210 Number of sources = 6
> Name/IP Address      NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
> ========================================================================
> PPS                   4   3    45 -54883.652  43980.402  -1149ms    44ms
> NMEA                  4   3    47 -54612.520  50890.305  -1359ms    69ms
> kahuna.ruselabs.com   6   4   321  -5743.294  28636.217  +1711ms   666ms
> zulu.frizzen.net      6   4   321  -7004.703  23417.387  +1429ms   676ms
> four10.gac.edu        6   4   320  -7574.643  21346.463  +1318ms   652ms
> gopher.fart.website   7   4   383  -7677.354  15156.406  +1300ms   691ms
>
> How do these look?
>
> How should I configure chrony.conf so that it locks onto PPS as soon as the 
> signal is available (either after boot up or -- this is a mobile system -- it 
> is moved outside and acquires GPS)?  I don't want a gradual adjustment of 
> time, it should snap right to the PPS time.

And destroy your filesystem in the process? The system does not like jumps in
time. You can do it at startup of chrony (the -r option) but that should not
happen after the system has been running, or things could get upset.

> > When chronyd has some NTP sources, you may want to tweak the offset
> > value of the SHM refclock so the measured offset as reported in
> > "chronyc sourcestats" is close to zero and the PPS signal can be
> > locked to it properly.
>
> Using the above numbers, what is the calculation for offset and delay?

Hard to say, but it looks like it is more than 1 sec.

> Thanks,
>
> Deven
>
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