On 29 Dec, 2011, at 23:26 , Terje Mathisen wrote: > Danny Mayer wrote: >> No, they use synchronized Cesium atomic clocks for time accuracy. GPS is >> only used to get a fix on the location and I'm not sure that 10's of >> centimeters is good enough for what they are trying to prove. I'd have >> to look closely at the methods used and the data to even have a clue as >> to what is needed and I have touched that stuff in years. > > Danny, how do you think they keep those atomic clocks synchronized? > > How do they _verify_ that they actually stay in sync (to a single-digit ns > level) over the entire length of the experiment(many months)? > > Even Hydrogen Masers won't give you that performance over a year or so, you > have to have some way to sync them either to each other or to UTC.
Yes, they use GPS to compare the clocks to each other. One of the articles I read even identified the GPS receiver they use. I think it was a Septentrio PolaRx3eTR PRO (or maybe the older model which that one replaced). Those receivers take a 10 MHz and 1 PPS reference in from the atomic clock so that they can produce GPS carrier phase measurements with respect to the local clock's time. Making these measurements simultaneously at both locations gives you data you can post-process to determine the time difference between the two clocks, independent of the GPS system time. The GPS signals are used only as markers that can be measured at both locations. BIPM Circular T lists GPSPPP (that's two-frequency, all-in-view carrier phase measurements) as being accurate to 0.3 ns. The bigger error is the equipment calibration, which they estimate as 5 ns. The traveling atomic clock would have been used for the equipment calibration. Dennis Ferguson _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
