Don Latham wrote:
jees, Bob, it's called a TDR
----- Original Message ----- From: "Bob Camp" <[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Sent: Friday, September 10, 2010 9:27 PM
Subject: Re: [time-nuts] Timing Distribution in Mountainous Terrain
Hi
The assumption is that you can "bounce" a pulse off the far end of a
single fiber or coax to read it's delay.
Bob
Actually, they don't use a TDR for this kind of thing.. you want to
measure continuously in real time, so you propagate signals in both
directions (at different frequencies) and look at phase differences,
etc. With multiple fibers in the same jacket, you can assume that the
fibers are the same, which makes life easier.
On antenna ranges, with coax, the approach is similar, but you rely on a
known mismatch at the probe.
It's a fascinating metrology problem, especially if you want to actually
"compensate" in real time, rather than just getting knowledge of the
delay and using that in post processing.
http://ipnpr.jpl.nasa.gov/progress_report/42-167/167C.pdf
is a paper by Bob Tjoelker and others about how they do it for DSN. it
has to be hotswappable, etc.
there's some company in Colorado who makes the equipment, as I recall.
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