[ntp:questions] DCF/SHM calibration problems
Helmut Wollmersdorfer
helmut at wollmersdorfer.at
Fri Oct 8 10:20:45 UTC 2004
For calibration and test of two DCF77 receivers I had them attached to
my workstation (CPU: AMD Athlon(tm) XP 2500+, bogomips: 3643.80, Kernel
2.4.26-1-386) and got the following values
# fudge to time1 after calibration (peer.awk was -41.933)
fudge 127.127.28.0 time1 0.041933
# fudge to time1 after calibration (peer.awk was -46.114)
fudge 127.127.28.1 time1 0.046114
Now I have them connected to the nodes of my cluster, consisting of two
machines with identical hardware
(CPU: VIA Samuel 2, bogomips: 1183.74, Kernel 2.6.8-1-386). NTP now runs
for some days against the same two stratum 1 servers I used on my
workstation before for calibration, but I get very different values:
via1:~# ntpq
ntpq> peers
remote refid st t when poll reach delay offset
jitter
=============================================================================
mail.happy-fish .INIT. 16 u 14h 1024 0 0.000 0.000
4000.00
xntp1.ien.it .IEN. 1 u 573 1024 377 332.787 -94.915
16.721
*212-82-32-15.ip .PPS. 1 u 588 1024 377 84.259 29.927
5.178
+SHM(0) SHM(0) 5 l 247 1024 377 0.000 -19.142
0.460
+nav.metrologie. 71.80.83.0 14 u 16 64 377 14.884 37.766
19.735
LOCAL(0) LOCAL(0) 13 l 45 64 377 0.000 0.000
0.004
via2:~# ntpq
ntpq> peers
remote refid st t when poll reach delay offset
jitter
==============================================================================
+ruou.ifi.unizh. 129.132.2.21 2 u 841 1024 377 46.083 26.967
1.719
xntp1.ien.it .IEN. 1 u 856 1024 373 385.893 -17.204
70.084
*212-82-32-15.ip .PPS. 1 u 897 1024 377 143.339 55.047
83.569
+SHM(0) SHM(0) 5 l 46 64 377 0.000 -4.704
0.397
-nav.metrologie. 71.80.83.0 14 u 52 64 377 14.264 35.692
21.256
LOCAL(0) LOCAL(0) 13 l 26 64 377 0.000 0.000
0.004
First, the difference to the workstation is hardware, kernel and ttyS1
instead of ttyS0. Maybe kernel 2.6 produces shorter offsets as 2.4?
Second, signal lengths of the two receivers differ systematically around
~5-9 ms. This maybe explains a part of the differences.
See here for the same moments:
via2:~# radioclkd -t ttyS1
[...]
DCD: 51 1 195246
DCD: 52 0 95524
DCD: 53 0 98436
DCD: 54 1 195326
via1:~# radioclkd -t ttyS1
[...]
DCD: 51 1 189162
DCD: 52 0 90625
DCD: 53 0 90749
DCD: 54 1 187530
Can somebody give me examples, how large typical offsets and signal
lengths should be for radio clocks? The part of the radio signal delay
for my distance of 600 km should theoretically be 1.8 ms.
TIA
Helmut Wollmersdorfer
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