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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