[ntp:questions] Shall I use a fudge on PPS?

David Taylor david-taylor at blueyonder.co.uk.invalid
Wed Jul 17 06:49:01 UTC 2019


On 16/07/2019 21:19, Andreas Mattheiss wrote:
> Hello,
> 
> I have a cheap GPS hooked onto my PC via a MAX232 level converter - the
> PPS goes directly into the DCD pin of a native serial port.
> 
> Everything is fine; after some time ntpd locks onto the PPS signal, and
> any other servers from the net show a small offset in ntpq -p.
> 
> What is the correct way of dealing with this? We are talking about 3 ms
> here. Is it appropriate to use a fudge on the PPS to bring it in line with
> the mean of a handfull of other servers from the net, or is is more
> appropriate to use the PPS's pings face value, not using any fudge?
> 
> Thanks, regards
> Andreas

Andreas,

I would suspect asymmetric network delays as Bill commented.  I see this 
all the time, and would trust my PPS far more than some network, 
especially any consumer-grade wide-area network.  Here's what I see one PC:


C:\Windows\System32>ntpq -pn kiruna
      remote           refid      st t when poll reach   delay   offset 
jitter
==============================================================================
o127.127.22.1    .uPPS.           0 l    -   16  377    0.000    0.026 
0.013
x127.127.20.1    .NMEA.           0 l    1   16  377    0.000  -53.013 
16.730
*192.168.0.20    .GPS.            1 u   12   32  377    0.273    0.007 
0.023
+192.168.0.3     .kPPS.           1 u   25   32  377    0.171    0.008 
0.039
+192.168.0.71    .kPPS.           1 u   26   32  377    0.524    0.010 
0.012
  uk.pool.ntp.org .POOL.          16 p    -   64    0    0.000    0.000 
  0.000
-83.170.75.28    194.80.204.184   2 u   41   64  377   17.459    2.596 
1.149
-37.220.20.12    85.199.214.98    2 u   51   64  377   20.021    2.315 
1.113
-89.238.136.135  131.188.3.223    2 u   22   64  377   12.805    2.520 
0.857
-195.171.43.10   .PPS.            1 u   12   64  153   27.454    2.916 
0.718

This PC has a PS reference.  Within the LAN other PPS servers show 
offsets up to 26 microseconds.  The servers from the WAN have offsets of 
+2.3 to +2.9 milliseconds.  The WAN is supposedly 200 Mbps down, and 
12Mbps up - at the best of times!

-- 
Cheers,
David
Web: http://www.satsignal.eu



More information about the questions mailing list