[time-nuts] SE880 GPSDO

[Attila Kinali]

On Wed, 27 Apr 2016 08:25:55 +1200
Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

> 1) Relative position of any pair of clocks located up to 2km apart has to be 
> known to within 3cm or so. Post processing is OK, however differential Earth 
> tides between the clock locations may need to be considered.

That's doable. People at ETHZ got sub cm accuracy from LEA-6T modules
with post-processing of the recorded phase data with an integration time
of several hours. Using phase data of multiple timing modules should give
relative positions with better than 1cm accuracy on these short baselines.
I don't know how much post-processing is necessary though. Haven't looked
into the the field of RTK[1] and PPP[2] yet. Probably data from IGS[3] is
needed as well.

> 2) The difference in the time offset between any pair of clocks located up to 
> 2km apart shall not vary by more than 200ps (1ns time stamp quantisation) or 
> 2ns (10ns timestamp quantisation) over an 8 hour period (at night).
> Post processing of data to fit wander etc is not practical as the SNR is too 
> low to support this.

Now this is quite a bit more challenging. While i'd say 1ns should be doable
(using receivers that are calibrated against each other and using common in
view mode during post-processing of the data), i'm not so sure whether 200ps
is possible. What might work is using an LEA-M8F with it's external frequency
input, to record the phase of an stable external reference (e.g. Rb).
Averaging that over a dozen minutes or so should make it possible to
measure the phase of the reference oscillator with 200ps precision, relative
to the other stations.

Another way would be to use L1/L2 receivers with calibrated antennas.
I know that BIPM has a GPS station that can deliver time transfer
accuracy <2ns over a distance of several 100km. It could be possible
to use such receivers with the <3km distances to deliver 10 times better,
if they are frequently calibrated (eg. every couple of days).
But of course, this makes things much more expensive.

But all this is a wild guess. I haven't seen anything like this done.
If you want a more precise answer i would need to think about the design
of the system for some time.


I guess using some cable/fibre between the telescopes is out of question?


			Attila Kinali

[1] http://www.navipedia.net/index.php/Real_Time_Kinematics
[2] http://www.navipedia.net/index.php/Precise_Point_Positioning
[3] http://www.igs.org/

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