[time-nuts] Fast than light neutrino
Hal Murray
hmurray at megapathdsl.net
Sun Sep 25 22:26:50 UTC 2011
javier.serrano.pareja at gmail.com said:
>> A fiber-based time-transfer would be nice complementary as it would provide
>> an independent timing path.
> Any ideas on how to proceed? This is unknown territory for me.
You can get a lot of good ideas from the radio astronomers. It's been
discussed here in the past, but I don't know what terms to use when searching
the archives. I think it was mostly pointers to their papers. They were
interested is much shorter distances. I think it was 10-20 km.
The idea is to send a signal in both directions over the same fiber. If it's
the same fiber, the transit times are likely to be the same in both
directions. If you send a pulse out and back, you can assume the time the
pulse arrived at the far end was half the round trip time after it left the
start.
Whatever you do, it will require a lot of cooperation from the people who own
the fibers.
Telecommunications fibers can go roughly 100 km between repeaters. Fibers
don't go in straight lines so you will probably need 10-20 repeaters. Maybe
more.
Normal repeaters don't work in both directions. You will have to use pairs
of fibers and swap them to measure the difference in length.
You may have to automate that step. Logically, what you want is a box
containing:
4 fiber connectors
2 repeaters (one in each direction)
several switches that can swap pairs of fibers.
an ethernet port and CPU to flip the switches
Each switch would have 2 fibers "in" and 2 "out". They would either go
straight through or cross over. (You can build that from 4 single-pole
double-throw switches.) I think you need 4 of those switches: two for the
fibers going in each direction, and two to swap the repeaters.
Maybe the delays through the repeaters are consistent enough that you don't
have to swap them.
In the way old days of FDDI, before they invented hubs, they were working on
mechanical switches to bypass a workstation when it was powered off. I
wonder if that technology is still around.
One thing to watch out for... The telephone companies are used to swapping
fibers without telling customers.
If you had a setup like that, would you want to use it for all your tests, or
just once to verify your GPS based timing?
For a one-shot run, it might be possible to use people to swap the fibers.
Assume the repeaters don't need to be measured. Assume it takes 1 minute to
measure the round trip time and 1 minute to swap fibers. With 20 repeaters,
that's less than an hour.
--
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