On 03/20/2011 08:26 PM, Bruce Griffiths wrote: > The beam from the interferometer/phased array can be swept over the sky > by varying the phase shift between the elements during the data > reduction process allowing high resolution imaging. > Compensating for Earth rotation and consequent changes in the > atmospheric delay are necessary. Differential phase shifts of a few tens > of picosec are significant in the imaging process. > The effect of atmospheric refraction has to be accounted for if accurate > positions of the source relative to the Earth's surface are required. I would use cross-correlation between two (or more) antennas. The noise of the individual antenna assembly would cancel out for sufficient correlation length. That would help to boot-strap the direct correlation if not the FFTed correlation will do. Long FFTs is cheap these days. Then again, the real VLBI people would beat me up for such naive approaches. Cheers, Magnus

Hi I have no doubt that VLBI works. I'm also quite confident that with great big dishes and fancy attachments you can do a really good job. My confusion is more as this relates to the back yard "time the earth to 1 ms" (or 1 us) question. Of the files that downloaded for me, the pptx file has some pretty good data in it. In slide 15, they show quite a bit of noise (100's of us) and "jitter" of 10's of us day to day. That's with a massive setup way beyond anything a backyard system is likely to achieve. The slide is justifying a longer baseline, but it's still pretty interesting data. Bob On Mar 20, 2011, at 3:26 PM, Bruce Griffiths wrote: > The beam from the interferometer/phased array can be swept over the sky by varying the phase shift between the elements during the data reduction process allowing high resolution imaging. > Compensating for Earth rotation and consequent changes in the atmospheric delay are necessary. Differential phase shifts of a few tens of picosec are significant in the imaging process. > The effect of atmospheric refraction has to be accounted for if accurate positions of the source relative to the Earth's surface are required. > > It is claimed that VLBI ought to be capable of measuring changes in dUT1 faster than optical methods: > http://ipnpr.jpl.nasa.gov/progress_report2/42-42/42H.PDF > > Some idea of the capabilities of VLBI in determining Earth orientation parameters may be gleaned from: > > http://ipnpr.jpl.nasa.gov/progress_report2/42-42/42H.PDF > > http://astrogeo.org/petrov/papers/ptd_eng.pdf > > http://science.nrao.edu/vlbaworkshop_2010/present/boboltz.pptx > > http://www.scielo.cl/scielo.php?pid=S0717-65382004000300009&script=sci_arttext <http://www.scielo.cl/scielo.php?pid=S0717-65382004000300009&script=sci_arttext> > > http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/35119/1/93-0683.pdf > > http://acc.igs.org/erp/ut1+lod_iers09.pdf > > http://astrogeo.org/petrov/papers/opteop.ps.gz > > http://www.jive.nl/~campbell/geod3.ps.gz <http://www.jive.nl/%7Ecampbell/geod3.ps.gz> > > Bruce > > Bob Camp wrote: >> Hi >> >> Maybe I missed something here. It would hardly be the first time. >> >> If the objective is to come up with a sub 1 ms resolution on observing the object. And we have chosen this all so indeed we get "fast" changes. Isn't a 1,000 second integration going to get in the way? If we need the integration to simply "see" the signal, then determining it's "center" within the integration time to less than 1 ppm seems unlikely. On a hand waving basis that's sort of a 60 db signal to noise. >> >> As I said, I may be missing something. >> >> Bob >> >> >> On Mar 20, 2011, at 2:31 PM, Bruce Griffiths wrote: >> >> >>> jimlux wrote: >>> >>>> On 3/19/11 10:41 PM, Bruce Griffiths wrote: >>>> >>>>> Bruce Griffiths wrote: >>>>> >>>>>> jimlux wrote: >>>>>> >>>>>>> >>>>>>>> A 10-12m diameter dish is probably close to the minimum feasible >>>>>>>> aperture. >>>>>>>> A 4m dish can be made to work in conjunction with a mauch larger dish >>>>>>>> (eg 30m). >>>>>>>> >>>>>>>> >>>>>>> The original speculation was for measuring the small change in earth >>>>>>> rotation rate, for which some sort of interferometric measurement of >>>>>>> a stellar source could be used. >>>>>>> >>>>>>> The source has to be bright (so you can detect it with a practical >>>>>>> antenna.. not everyone has a 30m dish in their back yard) >>>>>>> The source has to be small angle (or at least something that you >>>>>>> could accurately determine the centroid of) >>>>>>> The source has to be "not moving" (which I think leaves out using >>>>>>> something like jupiter) >>>>>>> The frequency of measurement has to be somewhere that the atmosphere >>>>>>> won't dominate the uncertainty (leaving out optical, I think) >>>>>>> >>>>>>> >>>>>>> SO what's the brightest small angular radio source out there? >>>>>>> >>>>>> 3C273 >>>>>> >>>>>> RA 12:29.1 DEC 02:03.1 >>>>>> >>>>> Its flux density is around 30 Jy in the waterhole region. >>>>> ie about 3E-17W per square meter for a 100MHz bandwidth. >>>>> The radio spectrum is relatively flat due to the synchroton nature of >>>>> the blazar source. >>>>> >>>> >>>> Ok, so lets say our ambitious amateur has a 3 meter diameter dish.. that's about 7 square meters. Knock that down to 4 square meters to make up for illumination and feed issues. So we're looking at 12E-17 W >>>> or 1.2E-13 mW or -130dBm, in 100 MHz BW. >>>> >>>> Say we want the "signal" to be comparable to the noise power, what do we need for a noise temperature.. kTB = -130dBm. kT = -174dBm/Hz for 300K, B = 80dBHz. (so at room temp, kTB would be -94dBm.. we need to drop noise power by at least 40 dB, so T needs to be down in the "sub 1 K" area, which is totally impractical. >>>> >>>> Looks like we need a bigger antenna.. >>>> Unless there's some clever correlation scheme. >>>> >>>> >>>> >>>> >>>> >>>> >>> With 2 or more antennas and integration times of 100sec to 1000 sec its routine to image objects well below the thermal noise level. >>> The fluctuations in the source signal correlate whereas the thermal noise in a receiver/dish pair do not. >>> >>> Modeling of the relative drift and frequency (and phase) offset (even if they are hydrogen masers) of the 2 sampling clocks involved is sometimes necessary. >>> >>> Bruce >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.

Hi If the "signal" is actually 80 db below the noise, you will need a lot of cross correlation simply to find it at all. Your digitizers are going to have to be *very* good just to let you get that far in the first place. Bob On Mar 20, 2011, at 6:36 PM, Magnus Danielson wrote: > On 03/20/2011 08:26 PM, Bruce Griffiths wrote: >> The beam from the interferometer/phased array can be swept over the sky >> by varying the phase shift between the elements during the data >> reduction process allowing high resolution imaging. >> Compensating for Earth rotation and consequent changes in the >> atmospheric delay are necessary. Differential phase shifts of a few tens >> of picosec are significant in the imaging process. >> The effect of atmospheric refraction has to be accounted for if accurate >> positions of the source relative to the Earth's surface are required. > > I would use cross-correlation between two (or more) antennas. The noise of the individual antenna assembly would cancel out for sufficient correlation length. That would help to boot-strap the direct correlation if not the FFTed correlation will do. > > Long FFTs is cheap these days. > > Then again, the real VLBI people would beat me up for such naive approaches. > > Cheers, > Magnus > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.