Markus Article discussing modelling effects of the ionosphere at low frequencies: http://ens.ewi.tudelft.nl/pubs/tol07isscs.pdf
I presume you intend to make the VLBI observations at night when the ionosphere has the least effect on the propagation delay at 50MHz. Bruce Markus Kern wrote: > On 24.02.2009, 21:37 Bruce Griffiths <[email protected]> wrote: > > >> Markus Kern wrote: >> >>> On 22.02.2009, 21:12 Bruce Griffiths >>> <[email protected]> wrote: >>> >>> >>> >>>> Markus >>>> >>>> >>> >>> >>>> Even with sawtooth correction the performance of the M12+T was found >>>> inadequate for the LOFAR <http://www.lofar.org/p/systems.htm> array. >>>> They use SRS FS725 rubidium sources disciplined by M12+T GPS timing >>>> receivers. >>>> >>>> >>> I didn't mean using the M12 by itself, obviously a clock stable enough >>> over the time the M12 pps must be integrated has to be used. >>> >>> If we are using the ADEV limits you proposed then at 50 MHz (= 3ns >>> acceptable error) the timing requirement is an ADEV of 3*1E-(8+x) at >>> tau = x seconds. From the measurements at >>> http://www.leapsecond.com/pages/gpsdo/ it seems the Thunderbolt gets >>> pretty close to that. >>> >>> The LOFAR clock system is described at [1]. In section 3.1.3.3 they >>> say: >>> >>> "Some Crystal Oscillators have the advantage that they have a better >>> Allan variance for periods of up to 10s and therefore it can be claimed >>> that they have a better performance than the SRS-FS725 Rb-reference >>> standard. The performance for time periods above 10s, the SRS-FS725 >>> performs better. Therefore choosing an OCXO would require a maximum >>> calibration interval of 10s and it would require a significantly better >>> GPS (or GALILEO) receiver because de Rb-reference is used to average >>> the PPS signal from the GPS receiver thereby making it possible to >>> identify the time difference between stations at receive frequencies >>> above 10MHz." >>> >>> I think this means that they are using pps integration times above 10 >>> seconds. I couldn't find any reference to the actual value though. >>> >>> LOFAR is also working at frequencies up to 240MHz so the timing >>> requirements are definitely higher. They say that a station time >>> offset of 200ps does not affect performance as long as it remains >>> stable over time. >>> >>> >>> >>>> They also state that the ionosphere contribution to ADEV is about 8E-12 >>>> @10s. >>>> >>>> >>> Yes, from which they infer that "the reference clock shall have an >>> Allan variance of 1e-11 or less over 10s." I am not sure if this has >>> to do with the propagation of the GPS signal or if they mean that they >>> need a clock stable enough to later compensate for the different >>> delays of the observed signal through the ionosphere. >>> >>> Markus >>> >>> [1] >>> http://www.lofar.org/operations/lib/exe/fetch.php?id=public%3Adocuments%3Alofar_documents&cache=cache&media=public:documents:19_detailed_description_of_clock_sync.pdf >>> >>> >>> >>> > > >> Markus >> > > >> The ionosphere contribution to the Allan deviation at GPS frequencies is >> much smaller (by a factor of 10-100 or so) than that, as is evident from >> carrier phase measurements. >> At 50MHz the ionospheric phase shift, dispersion and instability will be >> much greater than at GPS frequencies. >> They are merely ensuring that the LO contribution to Allan deviation is >> much smaller than that of the ionosphere. >> > > Yes, that's what I thought. > > >> If you look at the Allan deviation plot on the PRS10 page: >> http://www.thinksrs.com/products/PRS10.htm >> > > >> This indicates that the likely disciplining loop time constant will be >> several thousand seconds. >> > > Ok, so it may indeed be necessary to use a rubidium oscillator which > has the required stability over that time frame. > > I suppose a GPS disciplined Rb-clock will be much more expensive than > a Thunderbolt. However there are relatively cheap rubidium oscillators > like the LPRO 101 out there. Are they suitable and has anyone tried to > slave them to GPS? From reading the mailing list archives it seems > Brooks Shera's circuit won't be suitable for this. > > >> Close isn't good enough: the phase differences between pairs of stations >> is significant, the Allan deviation needs to be at least 30% lower per >> station. >> If the errors at station pairs have significant correlation the >> requirement can be relaxed somewhat. >> > > I realize that the phase difference is important and that things won't > work if it's not low enough :) > > >> Bruce >> > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
