[time-nuts] What position is measured?

Hal Murray hmurray at megapathdsl.net
Tue Sep 7 21:33:19 UTC 2010

> 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 output,
> 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 its
> 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:
Use the same type of coax on the PPS signal as you used for the antenna feed. 
 (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 tell 
if the delay comes from the antenna cable or the PPS cable.  You could move 
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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