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
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