[ntp:questions] Adjusting PPS offset

unruh unruh at wormhole.physics.ubc.ca
Sun Sep 4 05:27:39 UTC 2011


On 2011-09-04, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Sat, Sep 3, 2011 at 4:34 PM, A C <agcarver+ntp at acarver.net> wrote:
>
>> What I meant was what method is the best way to determine the value of
>> time1.
>
>
> That is what I meant above when I said, setting it is easy.  Knowing
> what to set it the is the hard part.  The simplest way is to connect
> with a few pool servers then "Fudge" your GPS' time for best match to
> the Internet servers.

Since the pool servers are a few orders of magnitude worse than PPS,
that would not be a vary useful way. 

>
> The get better then that means you need a trusted source of UTC time.
> For most of us that would mean having a "few" GPS receivers of
> different brands and times and hardware (Like a Universal Counter)
> that can measure the phase between PPS signals.  When you get to this
> level you are going to have to measure cable lengths.

The main problem is actually getting the signal into the computer
(interrupt service routines for example). When I tested that time scale
was something like 1-2 us. (I times it by timestamping when I sent a
signal out on a parallel port pin which was connected to the interrupt
pin on the parallel port. the two timestamps tended to be 1-3usec
apart).





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