[time-nuts] Precise Time transfer and relative position over a short baseline

[Michael Wouters]

Dear Bruce

I think it will be very difficult to achieve synchronization at the level
you want using GNSS signals. The BIPM would only claim 2 ns accuracy for
calibration of the delays in a GNSS receiver plus antenna. When you process
carrier phase data, you typically see jumps at the day boundary of the
order of 1 ns, reflecting uncertainties in the model. There's  some
reduction in ionospheric noise and systematics because you would be
operating almost zero-baseline eg two identical receivers in our lab track
each other at the level of 100-200 ps, but again, calibration of absolute
delays below 1 ns is hard.

If you really can't use cables then you might consider free space signals
like a laser beam or microwaves. There is plenty of literature about free
space optical links. One example of a microwave timing link is
http://www.locata.com/wp-content/uploads/2015/09/USNO-Wide-Area-Wireless-Network-Synchronization-Using-Locata-ION-Publication-9-20-2015.pdf

On Mon, Apr 11, 2016 at 3:00 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz
> wrote:

> There is a proposal to use multiple light bucket style optical telescopes
> to do Intensity stellar Interferometry over short baselines (up to perhaps
> 1km  or so) by using independent clocks to time tag photon  arrivals. store
> the time tags and process the data off line. Depending on the time tag
> resolution there is a need to measure the time differences between the
> independent clocks to an accuracy in the 1ns to 100ps range. Is there a
> better way of doing this other than using geodetic grade GPS receivers
> capable of GPS carrier phase measurements?Since the local clock flywheel
> oscillators will need to not deviate by more than 100ps or so over the
> several minutes required to perform the carrier phase averaging what sort
> of clock will be suitable apart from a good rubidium standard with a
> cleanup oscillator?
> NB Running fibres or coax between the telescopes isnt an option.
>
> The relative positions of the telescopes has to be known to within a cm or
> so for this to work.
> Bruce
>
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