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 -- [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. >> >> > > > > _______________________________________________ > 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.
