[time-nuts] What position is measured?

[jimlux]

Mark J. Blair wrote:
> On Sep 7, 2010, at 6:30 AM, jimlux wrote:
>> Another analogy is that if you had a machine that recorded all the signals, mounted right at the antenna, and then carried the recording half way around the world, and then ran the recording into a receiver, it would give you the position of the antenna, not the receiver.  The cable is just a time delay.
> 
> 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.. except that the cable's physical and electrical length *do* vary 
with temperature, so if you're looking at the gnat's eyelash sort of 
thing, you need to take that into account.  Maybe 10 ppm/degree, so a 20 
meter run will change a bit less than a millimeter.  That's down in the 
fractional picoseconds time-wise.

It's an issue if you're doing things like interferometry at higher 
frequencies..