Hi,
On 04/29/2016 11:45 PM, Michael Wouters wrote:
On Sat, Apr 30, 2016 at 6:14 AM, Magnus Danielson
<mag...@rubidium.dyndns.org> wrote:
Well, giving the conditions mentioned, doing ranging codes such as those
used by GPS is very easy and cheap. Doing this in bidirectional isn't too
hard. Doing a suitably high chip-rate should cost very little.
I've done two-way time-transfer over optical fibre using exactly this
technique. The TDEV is about 1 ps for tau>1s. Not so cheap, about 25K
euro per node (20K signal processing - NI FPGA, 2K laser and power
supplies, 1K detector, 1K RF electronics) in my setup, but that cost
could be greatly reduced since a $100 OEM FPGA could do the signal
processing (I've already done work on this but currently looking for
motivation to finish it off) and a simple, intensity-modulated laser
would probably be fine. A 2K euro budget would be a challenge though.
FPGA-wise, you need a very little FPGA resources.
If you consider the RedPitaya (200 USD) for instance, it is way beyond
what is needed.
The two-way time-transfer is relatively easy, but you will need to do some
calibration to get the precision needed.
At first glance, I would think that you should be able to define the
optical RX/TX path to within 10 cm without any trouble and that gives
you 300 ps accuracy. Even on fibre links, I don't think anyone would
claim an accuracy of better than a few hundred ps.
With a bit of calibration you can remove each nodes systematic asymmetry.
For optical fibers many does not even bother to do a pseudorandom
rangning. A repeating pattern suffice, such as that of SDH frames.
Cheers,
Magnus
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