[time-nuts] Result of Earth Quake speeds up earth?

[Bob Camp]

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
>>> 
>>> 
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>> 
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> 
> 
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