[time-nuts] What position is measured?

Tom Holmes tholmes at woh.rr.com
Wed Sep 8 03:16:02 UTC 2010

One other delay contributor  would appear to be processing delay in the
receiver, which thus begs the question of how the PPS signal is actually
synchronized to the GPS system.

Tom Holmes, N8ZM
Tipp City, OH

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Hal Murray
> Sent: Tuesday, September 07, 2010 5:33 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] What position is measured?
> > Does this mean that while the antenna feedline cable length does not
> > influence the measured position (at the phase center of the antenna),
and it
> > does not influence the accuracy of a disciplined frequency reference
> > it does introduce an error into the absolute time output (i.e., adding a
> > delay to the PPS output)?
> > In other words, do I correctly assume that I may safely ignore the
length of
> > my TBolt's antenna feedline if I am only interested in its 10 MHz OCXO
> > output, but I may want to compensate for it if I ever find a need to use
> > PPS output as an absolute time marker?
> Yes.  Here is the way I would look at it.
> Consider the PPS case where you have a setup like this:
>   antenna-cable(RF)-receiver-cable(PPS)-testgear.
> Use the same type of coax on the PPS signal as you used for the antenna
>  (Or measure the length of the cables in ns rather than meters.)
> If you measure the offset of the PPS signal at your test gear, you can't
> if the delay comes from the antenna cable or the PPS cable.  You could
> the receiver back and forth or move chunks of cable from one side to the
> other and the PPS signal wouldn't move.
> This also means that you can use the GPS receiver's antenna correction to
> correct for the delay in your PPS distribution.
> --
> These are my opinions, not necessarily my employer's.  I hate spam.
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