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

[Deven Hickingbotham <devenh@xxxxxxxxxxx>]

With help from the mailing list, I've been able to get PPS working!

chrony.conf:

server 0.us.pool.ntp.org
server 1.us.pool.ntp.org
server 2.us.pool.ntp.org
server 3.us.pool.ntp.org

refclock PPS /dev/pps0 lock NMEA refid PPS
refclock SHM 0 offset 0.5 delay 0.2 refid NMEA noselect

sources and sourcestats:

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

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.

> 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?

Thanks,

Deven

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