[ntp:questions] Re: GPS Truetime NTP server and offset/jitter peak

Ralf Gross usenet at ralfgross.de
Tue Oct 18 10:28:11 UTC 2005


Martin Burnicki wrote:
> Ralf Gross wrote:
>> I've a problem with a Truetime NTS-100-GPS device connected by RS232 to a
>> CentOS 4 Linux server (ntp-4.2.0.a.20040617-4).
>> 
>> Every night at 2am (CEST) offset an jitter suddenly shows very high values
>> and as a result ntpd loses sync to the GPS stratum 0 device for about 30
>> seconds.
> 
> 2am CEST is UTC midnight ...

Exactly!

> [...] 
>> 02:00:08
>>       remote       refid   st t when poll reach   delay   offset  jitter
>>
> ==============================================================================
>> xTRUETIME(0)     .TRUE.    0 l    1   16  377    0.000  -999.57   999.525
>> ...                                                     ^^^^^^^
>> ---------------------------------- peak ------------------/
> 
> This is about exactly -1 second. Did you hear about the upcoming leap second
> which is scheduled for UTC midnight after Dec 31, 2005?

Not until now ;) Ok, there will be a extra second in 2005.

> The leap second is currently already announced in the GPS satellite data
> stream, and if the GPS receiver doesn't handle the announcement correctly
> it might introduce the leap second every UTC midnight. 
> 
> We've read here about at least one other type of GPS receiver which was
> going to insert the leap second at the end of September instead of the end
> of December.

I think the problem first occurred at the end of August.
 
29 Aug 05:47:09 ntpd[28396]: synchronized to TRUETIME(0), stratum 0
30 Aug 02:00:02 ntpd[28396]: synchronized to         $IP, stratum 3
30 Aug 02:00:37 ntpd[28396]: synchronized to TRUETIME(0), stratum 0
31 Aug 02:00:05 ntpd[28396]: synchronized to         $IP, stratum 3
31 Aug 02:00:55 ntpd[28396]: synchronized to TRUETIME(0), stratum 0
 1 Sep 02:00:10 ntpd[28396]: synchronized to         $IP, stratum 3
 1 Sep 02:01:03 ntpd[28396]: synchronized to TRUETIME(0), stratum 0
 2 Sep 02:00:12 ntpd[28396]: synchronized to         $IP, stratum 3
 2 Sep 02:00:29 ntpd[28396]: synchronized to TRUETIME(0), stratum 0
 3 Sep 02:00:01 ntpd[28396]: synchronized to         $IP, stratum 3
 3 Sep 02:00:34 ntpd[28396]: synchronized to TRUETIME(0), stratum 0
 4 Sep 02:00:14 ntpd[28396]: synchronized to         $IP, stratum 3
 4 Sep 02:00:31 ntpd[28396]: synchronized to TRUETIME(0), stratum 0
 5 Sep 02:00:04 ntpd[28396]: synchronized to         $IP, stratum 3
 5 Sep 02:00:34 ntpd[28396]: synchronized to TRUETIME(0), stratum 0
 6 Sep 02:00:06 ntpd[28396]: synchronized to         $IP, stratum 2
 6 Sep 02:00:38 ntpd[28396]: synchronized to TRUETIME(0), stratum 0

The GPS device is an old Truetime NTS-100-GPS from 1996.

> Do you have the possibility to log the GPS serial output across UTC midnight
> in order to verify that?

Well, how would I do that without interfering the clients (the server is
our master ntpd server right now). I think it wouldn't be a big problem,
because there are 3 stratum 3 servers too.

Ralf




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