[time-nuts] synchronization for telescopes

[Attila Kinali]

Good evening Michael,

On Wed, 4 May 2016 20:11:25 +1000
Michael Wouters <michaeljwouters at gmail.com> wrote:

> There are  differences between solving for position and solving for  time.
> 
> In the case of position, it's constant so you can average over long
> times. In the case of time, you can't assume this and long-term
> averaging is not reasonable. So position uncertainty does not
> translate directly to time uncertainty (although it probably tells you
> about the precision of the individual measurements).
>
> The other key difference (and difficulty) is in your statement "if the
> phase data could be related to a stable reference oscillator in
> post-processing". In the case of position, the solution is for a point
> in space. In the case of time, you have to relate it to the output of
> some physical clock.

Yes and no. Yes, position is simpler as you know your receiver is not
moving (at least in first order, for longer measurements one has to
take earth tides into account). But, if you have a perfect frequency
reference (ie accurately known frequency and zero drift) you can do
the same averaging over time. Of course, there is no such reference,
but using real references you can calculate how accurate/stable it has
to be to achieve the required time precision.

The issue becomes considerably simpler, if all stations have the same
frequency reference, even if it's a little bit more unstable, as then
some of the errors will cancel out in the differences between the stations.

 
> In the case of a single frequency receiver, the measurements are made
> with respect to the internal TCXO, which is operated much like the
> software clock in eg the Linux kernel clock. You then have to know the
> (continuously varying) offset between this clock and the receiver's
> reference timescale, the offset between the nominal output 1 pps and
> the reference time scale in some cases, and the sawtooth correction,
> to finally relate the raw measurements to your external clock.

Yes.

> Several people have mentioned that there are some low-cost receivers
> which apparently allow for an external oscillator. This may result in
> improved time-transfer operation but the key question is the
> relationship between the output 1 pps and the 1 pps derived from the
> external oscillator - it is not obvious that this will be constant
> between eg power cycles of the receiver. This is something you have to
> test for.

Yes.


> > 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.
> 
> I think we're talking about the same thing here. By 'geodetic receiver' I meant:
> L1/L2 + carrier phase measurements + externally supplied 10 MHz and 1 pps.
> This is the typical kind of receiver installed at an IGS station, with
> the external clock a Cs or H-maser. They cost around $10-$15K.

No. I am talking about L1-only timing receivers. Timing receivers contain
hardware and code to get the relation between the internal oscillator and
refer it to an external oscillator. Either by using the PPS or by measuring
an external pulse. Yes, not all do that, but at least some do. And these
L1-only timing receivers are considerably cheaper than L1/L2 receivers.
(you can get them from 100usd up, ~30usd in volumes of 50-100)

> > 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.
> 
> Coincidentally, I am currently writing software so that I can test the
> LEA-8MT for GPSCV time-transfer. This is code-based, in the usual way.
> I will be doing a comparison of a number of different single-frequency
> receivers for time-transfer - this may be of interest to the time-nuts
> community because the testing platform is all open source
> (www.openttp.org) .

This is very interesting indeed. Thanks a lot!


			Attila Kinali 



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