[ntp:questions] GPS Stratum 1 and PPS

Miguel Gonçalves mail at miguelgoncalves.com
Thu Jul 28 08:59:38 UTC 2011


Hi there!

I have been quite a time nut for a while but recently I bought a GPS 18 LVC
from Garmin and wired it up according to one of the many tutorials around
the Internet. I have a static IP and if this works out OK I am thinking
about opening my NTP port to the world. :-)

I am using FreeBSD 7.4 with a PPS_SYNC kernel and the GPS is connected to
COM1.

I am getting this

tick# ntpq -p
     remote           refid      st t when poll reach   delay   offset
 jitter
==============================================================================
*GPS_NMEA(1)     .GPS.            0 l   11   16  377    0.000    0.007
0.004
+rustime01.rus.u .PPS.            1 u   38   64  377   59.228    1.422
1.749
-ptbtime2.ptb.de .PTB.            1 u   27   64  377   61.864   -2.420
0.922
+canon.inria.fr  .GPSi.           1 u   37   64  377   44.454    1.346
0.728

and I think it's quite good but the problem is that my NTP daemon is
hammering the other servers on the configuration file. As I don't want to
get banned from using these servers I would like some opinions regarding my
config file:

tick# cat /etc/ntp.conf
server 127.127.20.1 mode 2 prefer minpoll 4
fudge 127.127.20.1 flag1 1 flag3 1 refid GPS

server rustime01.rus.uni-stuttgart.de
server ptbtime2.ptb.de

server canon.inria.fr

statsdir        /var/log/ntp/
filegen clockstats      file clockstats type day        enable
filegen peerstats       file peerstats  type day        enable
filegen loopstats       file loopstats  type day        enable
statistics clockstats peerstats loopstats
logfile /var/log/ntp/messages

driftfile /etc/ntp.drift

Any ideas?

I have another machine that is physically close to this one I and got the
PPS signal from the GPS connected to the DCD line of the COM port also. I
intend to discipline the local clock with the PPS. I am numbering the
seconds using stratum 1 servers. Here's the configuration:

tock# cat /etc/ntp.conf
server 127.127.22.1 minpoll 4
fudge 127.127.22.1 flag3 1 stratum 0

server rustime01.rus.uni-stuttgart.de prefer
server ptbtime2.ptb.de prefer

server canon.inria.fr prefer

statsdir        /var/log/ntp/
filegen clockstats      file clockstats type day        enable
filegen peerstats       file peerstats  type day        enable
filegen loopstats       file loopstats  type day        enable
statistics clockstats peerstats loopstats
logfile /var/log/ntp/messages

driftfile /etc/ntp.drift

The idea behind this is that if the server with the GPS is turned off for
maintenance I will still have the other machine getting the PPS and the
seconds from public NTP servers. Is this configuration correct?

By the way, on this second machine I get

tock# ntpq -p
     remote           refid      st t when poll reach   delay   offset
 jitter
==============================================================================
oPPS(1)          .PPS.            0 l   14   16  377    0.000   -0.004
0.002
+rustime01.rus.u .PPS.            1 u  175 1024  377   59.726    1.577
0.143
+ptbtime2.ptb.de .PTB.            1 u  170 1024  377   62.099   -1.806
0.165
+canon.inria.fr  .GPSi.           1 u  493 1024   37   44.637    1.385
0.794

This one is polling the Internet servers every 1024 seconds. Why doesn't the
other?

One final question... Is 1ms difference from stratum 1 close servers (36ms)
normal? For me it's quite good. Can it be better?

stratum 1, precision -18, leap 00, trust 000
refid [PPS], delay 0.03641, dispersion 0.00078
transmitted 4, in filter 4
reference time:    d1dba30e.893d3d9b  Thu, Jul 28 2011  9:57:50.536
originate timestamp: d1dba34e.8d92e2b2  Thu, Jul 28 2011  9:58:54.553
transmit timestamp:  d1dba34e.8a966c79  Thu, Jul 28 2011  9:58:54.541
filter delay:  0.03818  0.03641  0.03738  0.04140
         0.00000  0.00000  0.00000  0.00000
filter offset: 0.002177 0.001189 0.001666 0.003551
         0.000000 0.000000 0.000000 0.000000
delay 0.03641, dispersion 0.00078
offset 0.001189

28 Jul 09:58:54 ntpdate[4779]: adjust time server 193.136.250.246 offset
0.001189 sec

Sorry for the large amount of questions but any help would be appreciated.

Thanks!

Cheers,
Miguel



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