Thanks for the article link Bruce. I haven't investigated how to best compensate for ionospheric delays yet. Some ionosphere model will surely be necessary. While observing at night will help due to lower TEC I hope to have a good enough model to also get useful data during the day.
I have also recently stumbled over AIPY, a Python package for interferometry which should help a lot with the data processing: http://setiathome.berkeley.edu/~aparsons/aipy/aipy.cgi http://setiathome.berkeley.edu/~aparsons/papers/2008-08-10_LFSW_AIPY_Presentation.pdf Markus On 27.02.2009, 13:38 Bruce Griffiths <[email protected]> wrote: > 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.
