Ron Frazier (NTP) wrote:
Hi all,
I have my USB GPS working pretty good at this point on both Windows and
Linux. I have some more Linux specific questions that I'll post later.
At the moment, I'm working on Windows. As has been suggested by others,
I know I can get hugely better accuracy using a different GPS and PPS,
and I plan to experiment with that when I get time and money. (I've
been ignoring lots of other household things while pursuing this
project.) For the moment, I just want to get optimum performance out of
the equipment that I have. For my immediate needs, +/- 6-10 ms accuracy
is perfectly adequate for what I'm doing.
Here's a recent ntpq -p printout:
C:\Windows\system32>ntpq -p
remote refid st t when poll reach delay offset
jitter
==============================================================================
*GPS_NMEA(5) .GPS1. 0 l 1 8 377 0.000 -1.181
0.977
+nist1-ny.ustimi .ACTS. 1 u 25 64 377 53.990 3.458
6.160
#216.119.63.113 .ACTS. 1 u 42 64 377 59.998 22.534
5.595
+india.colorado. .ACTS. 1 u 36 64 377 63.986 1.739
6.604
+ping-audit-207- .ACTS. 1 u 50 64 377 83.998 0.715
3.206
Note that my GPS is closely synchronized with 3 of the 4 NIST Stratum 1
servers. I have intentionally set the fudge factor on the GPS so this
is the case.
Now, here are the relevant server lines in ntp.conf:
server 127.127.20.5 prefer minpoll 3 maxpoll 3 mode 72
fudge 127.127.20.5 time2 0.2950 refid GPS1
server nist1-ny.ustiming.org prefer minpoll 6 maxpoll 13
server nist1.columbiacountyga.gov minpoll 6 maxpoll 13
server utcnist.colorado.edu minpoll 6 maxpoll 13
server nist1.aol-ca.truetime.com minpoll 6 maxpoll 13
Now, note that the NIST servers are being polled every 64 seconds. This
thing has been running this way since yesterday evening. However, in
the configuration file, those servers are allowed to poll up to every 2
hours. My question is, why are the internet servers being polled so
often continuously even though I have a stable GPS source available
which is the currently selected active clock? If I comment out the GPS
line, the internet servers will eventually go back to polling at longer
intervals. Note that I don't want to get banned from using any of these
public servers for hitting them too often.
You are best using servers from the nearest pool.ntp.org group
rather than using distant stratum1 servers.
My own internet connection is 2 Mbit/s adsl. I chose the nearest
from the list of public ntp servers at ntp.org. By nearest I mean
with the lowest delay. I then removed any that were using the
same source. The tools for doing this should be with your ntpd
package.
Note that just now the only stratum-1 source is my own ntp0.
The reason for selection for lowest delay is that internet
connections can be assymetric with route to a server being
different from the route back from the server. I convinced myself
of this by ssh into a remote on a different network and doing
traceroute in both directions. So having selected for lowest
delay that gives the maximum for offset error from that source
as 1/2 of the delay. For my ISP's timeserver the delay is
17.572 msec so maximum possible error is +/- 8.786 msec.
ntpq -c rv -p ntp1:
version="ntpd 4.2.6p5@1.2349-o Tue Dec 27 11:31:58 UTC 2011 (1)",
processor="i386", stratum=2, precision=-18, rootdelay=0.426,
rootdisp=36.828, refid=81.187.61.74, Wed, Feb 15 2012 9:14:14.232,
offset=0.237, frequency=0.157, sys_jitter=0.562, clk_jitter=0.101,
clk_wander=0.006, tai=34
remote refid st t poll reach delay offset jitter
==========================================================================
*ntp0.lordynet.o .MSFa. 1 u 256 377 0.426 -0.059 0.058
-xxxxxxxxxxxxxxx 195.66.241.10 2 u 256 377 17.911 2.872 0.640
+xxxxxxxxxxxxxxx 195.66.241.2 2 u 256 377 17.572 0.532 0.355
-xxxxxxxxxxxxxxx 140.203.16.5 2 u 256 377 30.898 2.129 0.332
-xxxxxxxxxxxxxxx 194.66.31.14 2 u 256 377 26.400 7.166 0.423
+xxxxxxxxxxxxxxx 193.62.22.74 2 u 256 377 32.417 0.811 0.315
ntpq -c rv -p ntp0:
version="ntpd 4.2.6p5@1.2349-o Tue Dec 27 09:47:55 UTC 2011 (1)",
processor="i386", stratum=1, precision=-18, rootdelay=0.000,
rootdisp=11.479, refid=MSFa, Wed, Feb 15 2012 9:14:35.778,
offset=0.400, frequency=-48.742, sys_jitter=0.341, clk_jitter=0.480,
clk_wander=0.000, tai=34
remote refid st t poll reach delay offset jitter
==========================================================================
*SHM(0) .MSFa. 0 l 512 377 0.000 0.242 0.404
-xxxxxxxxxxxxxxx 81.187.61.74 2 u 256 377 0.425 0.220 0.059
+xxxxxxxxxxxxxxx 195.66.241.10 2 u 512 377 17.630 0.941 0.495
-xxxxxxxxxxxxxxx 131.188.3.222 2 u 512 377 32.280 1.153 0.347
-xxxxxxxxxxxxxxx 193.67.79.202 2 u 512 377 20.314 1.401 1.097
-xxxxxxxxxxxxxxx 194.66.31.14 2 u 512 377 26.781 9.677 0.273
+xxxxxxxxxxxxxxx 195.66.241.2 2 u 512 377 18.267 0.978 0.391
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