[time-nuts] Timing Distribution in Mountainous Terrain

[jimlux]

Don Latham wrote:
> jees, Bob, it's called a TDR
> ----- Original Message ----- From: "Bob Camp" <lists at rtty.us>
> To: "Discussion of precise time and frequency measurement" 
> <time-nuts at febo.com>
> 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.