[LEAPSECS] preprint about timekeeping for neutrino experiment

[Ian Batten]

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 [1] 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?

ian

[1] Dana, P. H. Global Positioning System (GPS) time dissemination for real-time applications. Real-Time Systems 12, 9-40 (1997).