[ntp:questions] NTP sever on an isolated Network

Unruh unruh-spam at physics.ubc.ca
Sat Jan 10 02:48:17 UTC 2009


leituras at gmail.com (Diego Ramos) writes:

>All right. I'll show those options to my manager, but I think he will prefer
>the USB Option as we don't need to be extremely accurate.

Some consider 1ms extremely accurate. Some consider it extremely
inaccurate.( Mine runs about 500 times better than this).
 Both would consider 1s pretty crude. 
But then again, some consider running a mile in 12 min the height of
accomplishment.




>Once more, I'm really glad for all the help I'm getting here. I'm stuck with
>this problem for a long time and the help I got here saved me a lot of days.


Actually I withdraw my reccomendation of the USB unit. It seems that Garmin
uses a proprietary protocol on this interface, and I am not sure that there
exists a driver for this. It is not NMEA. I think one needs to do a more
thorough research than I have done. 

Thus, either get a 18PC and supply the 12V it needs from some other source
(It comes with a cigarette lighter type input which you can plug into a car
lighter output. However your manager might get upset if you told him you
have to buy a car as well and park it in your office). or think that the
18LVC is probably the best bet still depite the need to wire it up. There
are a number of places on teh web that will teach you how to do that. 

For anyone with a modicum of soldering experience, hooking up an LVC is
fast and simple. 




>Thanks a lot.

>Diego

>On Fri, Jan 9, 2009 at 10:22 PM, Unruh <unruh-spam at physics.ubc.ca> wrote:

>> leituras at gmail.com (Diego Ramos) writes:
>>
>> >> To sync to a second any gps receiver will do. The Garmin 18PC or USB
>> will
>> >> do  and will get you down to about 5ms accuracy with some tuning. The
>> PPS
>> >>  on the 18LVC will do 1000 times better ( again $50-100 range)
>> >>
>> >> On the 18PC you have to supply the, I think it is 12 V from somewhere
>> else.
>> >> On the USB it will supply its own voltage if I read correctly.
>> >>
>> >>
>> >But I'll be able to use the Garmin 18 USB and xntpd? Which refclock I'll
>> >use?
>>
>> nmea
>>
>> The gps nmea signal message includes the time. It takes about 200ms for
>> that nmea sentence to be sent, but that can be roughly  zeroed out by an
>> appropriate
>> fudge to the nmea driver in the ntp.conf file. The usb port  is a bit
>> flakey in its timing etc, but that will onely be a few millisec. Thus if
>> you are happy with 5ms or so, the nmea driver and the GPS usb should be
>> fine. (someone may have done it so could refine that 5 ms figure to be an
>> accurate estimate of the time error you could get.) It is certainly much
>> better than 100ms (1/10)
>>
>> Why use xntpd? That is ancient.  Use the latest ntpd 4.2.4p6
>>
>>
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>>




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