[ntp:questions] GPX18x LVC 3.50 firmware - high serial delay problem workround

Chris Albertson albertson.chris at gmail.com
Fri Jan 14 22:12:00 UTC 2011


On Fri, Jan 14, 2011 at 1:30 PM, unruh <unruh at wormhole.physics.ubc.ca> wrote:
> On 2011-01-14, Chris Albertson <albertson.chris at gmail.com> wrote:
>>> When the software (any software) only receives a PPS signal and a
>>> serial message conveying the absolute time, but it does not know how
>>> much the serial message is offset from the true time, how should it
>>> determine the true time?
>>
>>>From a configuration file that describes the relationship between the
>> PPS and text message.
>
> How in the world does whoever set up that config file know that
> difference? The program can use the same algorithm you do to determine
> that.

The only way to know is to compare to another reference assumed to be
correct.  Pool NTP servers would be accurate enough for that.    The
GPSes (Motorola Oncore) I use have a related "problem" in that they
allow the pulse to be adjust so that it happens before the UTC second
or any time during the second.  So I actually have a choice.  But how
to set it and know it is right?

What you'd do is adjust the timing until it was a best match to the
other reference clocks you have or lacking that to a set of pool NTP
servers

I think what this proves is that setting up a Stratum One NTP server
requires that you have access to multiple clocks in your lab.  eBay
makes this very inexpensive now.  Many people have three or more
clocks and test equipmnt so that they can be compared.  Lacking this,
it is just a gues if your server has corect time

Could this be automated?  Maybe, to some degree.  The reference clock
driver would need to have a "survey mode" setting where it would run
for many hours and compare it's own time to others.  NTP does this
already, almost,  what it lacks is a way to capture the measured
offset and fold it back to a config file.





-- 
=====
Chris Albertson
Redondo Beach, California



More information about the questions mailing list