[ntp:questions] Peer Review of ntpq -c rv

unruh unruh at invalid.ca
Thu Mar 8 15:42:55 UTC 2012


On 2012-03-08, Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
> On 3/7/2012 7:10 PM, David Lord wrote:
>> Ron Frazier (NTP) wrote:
>>> I'm a little confused.  Are you currently getting offsets in the 10's 
>>> of milliseconds range or in the 10's of microseconds range.  Also, 
>>> how are you measuring offset from UTC, as opposed to offset from the 
>>> GPS board?
>>>
>>> Sincerely,
>>>
>>> Ron
>>>
>>>
>>> On 3/7/2012 7:26 AM, Alby VA wrote:
>>>>   I'm looking to get a little feedback on if the following output of my
>>>> Sure GPS / FreeBSD
>>>> setup looks like its running smoothly and keeping time correctly.
>>>>
>>>>
>>>> assID=0 status=0115 leap_none, sync_atomic, 1 event,
>>>> event_clock_reset,
>>>> version="ntpd 4.2.6p5 at 1.2349-o Mon Feb 20 22:00:33 UTC 2012 (1)",
>>>> processor="amd64", system="FreeBSD/9.0-RELEASE", leap=00, stratum=1,
>>>> precision=-19, rootdelay=0.000, rootdisp=0.269, refid=PPS,
>>>> reftime=d301d22f.dc1393c2  Wed, Mar  7 2012  7:25:19.859,
>>>> clock=d301d230.cc08bad9  Wed, Mar  7 2012  7:25:20.797, peer=41909,
>>>> tc=4, mintc=3, offset=-0.034, frequency=-25.214, sys_jitter=0.002,
>>>> clk_jitter=0.002, clk_wander=0.001
>>
>> From above the offset is -0.034 msec, ie 34 usec
>>
>>
>> David
>>
>
> At first I was confused and thought he was ms off.  However, if he's 
> within 34 us, that's pretty good.  Sounds like his board is working OK, 
> but could possibly be a bit better.  If he were communicating 
> exclusively on the serial port and the PPS were not working, would he be 

PPS on a linux machine will give offsets of around 5us (standard
deviation around 3)  with occasional
larger excursions. 
With a local network connection to a stratum 1 server, you will get
around 20us fluctuations in the offset.

> getting performance this good?
>
> Even though it's not as critical with PPS, I thought I'd pass a long a 
> couple of tips to making the serial NMEA connection perform as good a 
> possible.
>
> My advice to stabilize the NMEA data as much as possible, do the following:
>
> A) maximize the baud rate, as long as it doesn't destabilize the system.
> B) minimize the number of NMEA sentences, to preferably one.
> C) make that NMEA sentence one that doesn't vary much, if at all, in 
> length, like GPZDA or maybe GPZDG.  I don't know much about GPZDG.  
> However, the manual for the Trimble Resolution T recommends GPZDA for 
> timekeeping.
> D) Set ntp.conf to read only the desired sentence.
>
> Here's something else that could be affecting his results, or possibly, 
> I could be all wet.  It occurred to me that BSD (which I know nothing 
> about, but am assuming it's a linux like system) might be automatically 
> loading the virtual com port driver for the USB connection, like Ubuntu 
> did when I plugged in the USB GPS that I have, which is a Globalsat 
> BU-353.  So, my thought was, if it were trying to communicate both by 
> USB and serial, that it might be causing a problem.  Or, it could be 
> totally unrelated.  But, I just thought I'd throw it out there.
>
> Sincerely,
>
> Ron
>
>
>>
>>>>
>>>>
>>>>   Here are my website graphs/stats I've been keeping since day 1.
>>>>
>>>>
>>>> NTP Clock Offset: http://godzilla.empire.org:9999/
>>>> NTP Offset from UTC: 
>>>> http://godzilla.empire.org:9999/godzilla_ntp-b.html
>>>>
>
>
>



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