[time-nuts] synchronization for telescopes

[Attila Kinali]

Hoi Michael,

On Mon, 2 May 2016 07:52:09 +1000
Michael Wouters <michaeljwouters at gmail.com> wrote:


> I don't think a cheap receiver like a LEAxxx will quite get you there.

I have some numbers of an project of the ETH that did use standard
LEA-6T recording the phase data and got in the post processing
to an uncertainty of <4mm averaging over several hours. Translating
that to timing resolution would mean an uncertainty of less than 13ps.
Of course, this number is completely theoretical, but it shows that
it should be possible to go below 1ns in time resolution, if the
phase data could be related to a stable reference oscillator in
post-processing and if the offsets between the different receiver
and antenna combinations are calibrated out.

> The number I quoted is for high quality geodetic receivers. There are
> crucial differences between these and the cheap receivers in regard to
> time-transfer. The first is how you relate your external clock's 1 pps
> to GPS time.

That's why I'm proposing timing receivers. They are the ones that have
the additional software and hardware bits which allow to relate an
external oscillator to the satellite phases.

> The other important difference is the resolution of the receiver's
> measurements. A cheap receiver reports the code measurements at
> relatively coarse resolution, sometimes a few ns, whereas a geodetic
> receiver reports at much higher resolution. If you had a cheap
> receiver, the code measurement resolution is seldom specified so you
> would have to test candidate receivers.

I don't know what resolution the LEA family offers there, but the
spec of the protocol defines a 1ps resolution in the data. So I would
guess that the phase data resolution is probably in the order of 10-100ps.

> I have many years of raw code measurement data from many identical
> receivers operating on baselines of a few km up to 20 km. I will try
> to have a look later this week to confirm/deny/make ambiguous what I
> said above.

If you have time to dig that data out, that would be very nice.

				Attila Kinali

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