Tom,
L1 code and carrier phase GPS will be challenging.
Tracking it as such is relatively straightforward.
For ionspheric and tropospheric shifts there will be a significant
common mode, thus cancels fairly well as the time-difference.
What is a concern is the multipath, which will shift around differently
for the sites as constellation shifts and reflections moves around.
Getting such antennas cheap to keep it within the budget is however
troublesome. With luck you can find them.
I believe more in a direct link to lock the nodes together.
Cheers,
Magnus
On 05/04/2016 07:06 PM, Tom Van Baak wrote:
If every station has its own GPSDO, what is the purpose of the optical transfer?
The purpose of the optical transfer is to keep the LO at each site in sync at
all times to within 500 ps. GPSDO are not good enough for this level of timing.
That's why some sort of optical transfer is being discussed.
A optical transfer would allow them to either:
1) measure the phase difference of each LO and phase lock them to within 500
ps, or
2) measure the phase drift amongst all the LO and then back out the drift
during post-processing.
Another proposal is using L1 code and carrier phase common view GPS techniques
at each site and back out the observed Rb phase drift during post-processing.
The question is if this gets you down to 500 ps.
Any of these methods is going to be a challenge, given their 500 ps requirement
and their $2k budget.
/tvb
----- Original Message -----
From: "Azelio Boriani" <azelio.bori...@gmail.com>
To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com>
Sent: Monday, May 02, 2016 5:20 AM
Subject: Re: [time-nuts] Fw: Optical transfer of time and frequency
If every station has its own GPSDO, what is the purpose of the optical transfer?
On Mon, May 2, 2016 at 11:14 AM, Michael Wouters
<michaeljwout...@gmail.com> wrote:
One other possibility occurs to me that might be doable with surplus
gear and sticks to the budget. Instead of using WR, give up on
getting time of day and just send a 1 kHz pulse stream in each
direction. Each station then measures against its own GPSDO clock
using a standard/homebrew TIC and records the difference. This is
ambiguous modulo 1 ms but this is trivially resolved using GPS. You
also probably know the distance between the stations to much better
than 1 ms = 300 km :-) . You then post-process but this can be done
with very little latency if you're keen.
Cheers
Michael
On Mon, May 2, 2016 at 1:44 AM, Tom Van Baak <t...@leapsecond.com> wrote:
Has anybody experienced with free-space optical gigabit Ethernet
links? I am curious about whether the transceivers have a fixed
latency or at least a latency one can easily quantify online. This is
the trickiest part for adding WR support on top of a given physical
layer.
Hi Javier,
When searching this topic I ran across a commercial laser solution:
http://www.laseroptronics.com/products.cfm/product/27-0-0.htm
http://www.laseroptronics.com/index.cfm/id/57-66.htm
http://www.laseroptronics.com/index.cfm/id/57-69.htm
etc.
But, according to /57-67.htm it "starts" at $15k per node. Plus there's the cost of
all the WR pieces, assuming the two are even compatible. So this is vastly above the ~$2k
budget mentioned by OP. I also assume OP is not ready to embark on a one-off, multi-man-year
R&D project.
This particular issue -- how to synchronize (or, at least phase compare)
multiple oscillators by a two-way laser link over a few km to within 500 ps --
is really quite interesting. It would, for example, allow me to do live
monitoring of 5071A Cs time dilation on my next mountain-valley relativity
experiment.
/tvb
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