[time-nuts] Fw: Optical transfer of time and frequency

Magnus Danielson magnus at rubidium.dyndns.org
Wed May 4 17:51:46 UTC 2016


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.boriani at gmail.com>
> To: "Discussion of precise time and frequency measurement" <time-nuts at 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
>> <michaeljwouters at 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 <tvb at 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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