[LEAPSECS] preprint about timekeeping for neutrino experiment
fmeyer at obs-besancon.fr
Fri Sep 30 12:34:13 EDT 2011
On Fri, 30 Sep 2011, Ian Batten wrote:
> On 30 Sep 2011, at 1532, Peter Vince wrote:
>> If they were using stand-alone caesium clocks, then yes - gravity and
>> altitude would make big difference. But they locked their clocks to a
>> single common-view GPS satellite - surely, then, they were both
>> ticking at the same rate, and in sync?
> If you don't have access to the encrypted L2 frequency, what is the lower bound on clock precision for two separated stations observing some common-view satellites? I would have thought that propagation in the ionosphere would introduce enough uncertainty to make 60ns precision unlikely. It's difficult for the non-specialist to know which errors were reduced by the removal of SA and which errors are inherent to the technology, but one paper I found  says that use of the ionospheric model that is transmitted on L1 isn't anything like perfect:
>> Using the broadcast model under normal conditions removes about half of the error (Fees and Stephens 1987) leaving a residual error of around 60-90 nanoseconds during the day and 10 to 20 nanoseconds at night (Knight and Rhoades 1987).
> I presume that some of these errors can be corrected if you know your location accurately, but what is the real state of the art?
Simultaneous common view comparisons precisely aim at reducing the influence of
such one way biases to the differential effect between the two paths.
On such a short baseline, the ionospheric differential effect is below one ns.
This is for one single CV. In their setup there are 2 GPS receivers running continuously,
they probably have something like a few hundreds CV measurement per day available.
And the clocks are not locked to a receiver, they are free but the offset is
continuously monitored through those CV measurement.
The main concern remains the calibration of those 2 receivers and this is what
has been led independantly by the METAS and PTB.
François Meyer Tel : (+33) 3 81 66 69 27 Fax : 3 81 66 69 44
Observatoire de Besancon - BP1615 - 25010 Besancon cedex - FRANCE
Institut UTINAM * Universite de Franche-Comte * CNRS UMR 6213 ***
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