Re: [time-nuts] Result of Earth Quake speeds up earth?
Couldn't you rig up a MLBI (medium, not very) setup between you and someone
else in your area..
Could one detect pulses (or a signal) from some quasar (or "infinite
distance" stellar source) with a reasonable small antenna.
Suppose you have two antennas X seconds (at speed-of-light) apart with clocks
accurate to Y seconds. For simplicity, let's assume both antennas are on the
equator and you are looking at something on the plane of the equator and that
it is directly overhead.
Your pointing accuracy is Y/X, or something close to that.
I think I'm on the right track, but I may have overlooked something important.
--
These are my opinions, not necessarily my employer's. I hate spam.
On Wed, Mar 16, 2011 at 1:14 AM, Hal Murray hmurray@megapathdsl.net wrote:
Your pointing accuracy is Y/X, or something close to that.
That describes perfectly when radio can beat optics. The angular
resolution of the system is the aperture size over the wavelength.
So you can see that a radio telescope must be on other 1000 times
wider then an optical telescope if both are to have the same
resolution.
On an amateur budget optics wins because while one can afford a 12"
diameter optical telescope a 1000 foot baseline antenna array will not
fit in the typical back yard.
optical scopes have a limited maximum size. Currently this as roughly
about 10 meters in diameter but technology exists to build a radio
antenna array that is one Earth diameter wide. So radio wins if you
have a government or university sized budget
But optics is catching up and there are now wide optical arrays but
the technology to combine light from multiple scopes in phase is
expensive and depends on precision mechanical devices, long tunnels
and so on. The Charra Array here in LA on Mt. Wilson was designed with
the goal of imaging flares and spots on stars.
There have been proposals to fly a wide baseline optical system in
space, such a system could in theory see continents and oceans on
planets around other stars but realistically the technology is not yet
there. so that will have to wait for the 2nd half of this century.
--
Chris Albertson
Redondo Beach, California
[snip] but technology exists to build a radio
antenna array that is one Earth diameter wide. So radio wins if you
have a government or university sized budget [snip]
Yes. Been done. It's called VLBI.
-John
=================
On 3/16/11 11:31 AM, Chris Albertson wrote:
On Wed, Mar 16, 2011 at 1:14 AM, Hal Murrayhmurray@megapathdsl.net wrote:
Your pointing accuracy is Y/X, or something close to that.
That describes perfectly when radio can beat optics. The angular
resolution of the system is the aperture size over the wavelength.
So you can see that a radio telescope must be on other 1000 times
wider then an optical telescope if both are to have the same
resolution.
On an amateur budget optics wins because while one can afford a 12"
diameter optical telescope a 1000 foot baseline antenna array will not
fit in the typical back yard.
Ahhh.. but what about me and a buddy who lives about a mile away with
line of sight between us? Or me and someone 50km away (albeit without
line of sight between)...
optical scopes have a limited maximum size. Currently this as roughly
about 10 meters in diameter but technology exists to build a radio
antenna array that is one Earth diameter wide. So radio wins if you
have a government or university sized budget
I'm thinking that you could do pretty darn well on a weekend hacker
budget..
Could you make the measurement in, say, 48 hours.. A portable setup
might be reasonable with a 10-20km baseline.
On Wed, Mar 16, 2011 at 11:00 PM, jimlux jimlux@earthlink.net wrote:
Could you make the measurement in, say, 48 hours.. A portable setup might be
reasonable with a 10-20km baseline.
Before you can even think about building a long baseline radio
receiver the first goal is to be able to detect a quasar with just one
receiver from a suburban back yard. Then you can think about
building two receivers. From my reading most amateur targets are
things like Cassiopeia A or the Crab Nebula.
Synchronizing the several receivers that are spread aroud is not
really even required. Many years ago astronomers would mail magnetic
tapes and the data would be combined days after the observations
After all if both receivers are looking at the same distant
transmitters you should be able search for a solution. But of course
having good time tags reduces the search space.
=====
Chris Albertson
Redondo Beach, California
Just for fun I plotted the UT1-UTC data from the IERS Bulletin A.
Here's the raw data:
UT1-UTC
s
-0.18115
-0.18232
-0.18353
-0.1847
-0.18576
-0.18674
-0.18763
-0.18842
-0.18912
-0.1897
-0.1903
-0.19103
-0.192
-0.19324
This is from March 4 to Match 17 inclusive.
I don't know if it's a fluke, but the line takes a definite "step to
the right" before continuing on its merry way.
At a glance x an y graphs seem to show nothing.
Jim Palfreyman
Chris Albertson wrote:
On Wed, Mar 16, 2011 at 11:00 PM, jimluxjimlux@earthlink.net wrote:
Could you make the measurement in, say, 48 hours.. A portable setup might be
reasonable with a 10-20km baseline.
Before you can even think about building a long baseline radio
receiver the first goal is to be able to detect a quasar with just one
receiver from a suburban back yard. Then you can think about
building two receivers. From my reading most amateur targets are
things like Cassiopeia A or the Crab Nebula.
Synchronizing the several receivers that are spread aroud is not
really even required.
You've obviously never tried doing this or you wouldn't make such claims.
Such solution space searches are impractical even today.
Many years ago astronomers would mail magnetic
tapes and the data would be combined days after the observations
After all if both receivers are looking at the same distant
transmitters you should be able search for a solution. But of course
having good time tags reduces the search space.
They still used a hydrogen maser at each VLBI site.
Today they tend to mail hard drives instead.
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).
Bruce
On 3/17/11 12:30 PM, Chris Albertson wrote:
On Wed, Mar 16, 2011 at 11:00 PM, jimluxjimlux@earthlink.net wrote:
Synchronizing the several receivers that are spread aroud is not
really even required. Many years ago astronomers would mail magnetic
tapes and the data would be combined days after the observations
After all if both receivers are looking at the same distant
transmitters you should be able search for a solution. But of course
having good time tags reduces the search space.
Since the goal of this BYMLBI (Back Yard Medium LBI) is to measure the
change in the earth's rotation rate, one would at the least need a time
code recorded on those tapes, since you want to know the instant when
the source crosses some reference plane. It's not quite like a classic
interferometer application where you're looking for high resolution in a
small region, but don't need high absolute position accuracy.
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?
As someone else has pointed out, measuring the earth surface position
relative to spacecraft orbits, e.g. GPS, would be another technique. In
fact, a high resolution measurement of the position of a geosync sat
might do.. If the earth's rotation rate changes you'd have to adjust the
height of the satellites in Clarke orbit to keep them stationary.
Unfortunately, for earth orbiters, there's enough other perturbations
that you probably can't see it. They already have to move satellites
around to compensate for things like solar wind, air drag (for LEO), etc.
But maybe for a spacecraft in deep space, between planets, which is on a
well understood trajectory?
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
As someone else has pointed out, measuring the earth surface position
relative to spacecraft orbits, e.g. GPS, would be another technique.
In fact, a high resolution measurement of the position of a geosync
sat might do.. If the earth's rotation rate changes you'd have to
adjust the height of the satellites in Clarke orbit to keep them
stationary.
Unfortunately, for earth orbiters, there's enough other perturbations
that you probably can't see it. They already have to move satellites
around to compensate for things like solar wind, air drag (for LEO), etc.
But maybe for a spacecraft in deep space, between planets, which is on
a well understood trajectory?
Bruce
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.
As someone else has pointed out, measuring the earth surface position
relative to spacecraft orbits, e.g. GPS, would be another technique.
In fact, a high resolution measurement of the position of a geosync
sat might do.. If the earth's rotation rate changes you'd have to
adjust the height of the satellites in Clarke orbit to keep them
stationary.
Unfortunately, for earth orbiters, there's enough other perturbations
that you probably can't see it. They already have to move satellites
around to compensate for things like solar wind, air drag (for LEO),
etc.
But maybe for a spacecraft in deep space, between planets, which is
on a well understood trajectory?
Bruce
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Le 20/03/2011 05:59, Bruce Griffiths a écrit :
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.
I sincerely doubt that it will be possible to get undisputed
verification of this speed up as the magnitude is swamped by the
irregular diurnal and sub-diurnal rotation rates induced by tidal
effects that are at lease a magnitude greater and for which the error
bars are of the same order or grater than the predicted shift.
There was a similarly predicted rotation shift predicted for the Chile
quake of 28 February 2010 (1,26us). There was IIRC no verification of
that AFAIK. Check the tidal effects at
http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html for the tidal effect
and
http://hpiers.obspm.fr/ for rotational measured speed changes
http://hpiers.obspm.fr/eop-pc/index.php?index=news for the statement
of detectability.
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
As someone else has pointed out, measuring the earth surface position
relative to spacecraft orbits, e.g. GPS, would be another technique.
In fact, a high resolution measurement of the position of a geosync
sat might do.. If the earth's rotation rate changes you'd have to
adjust the height of the satellites in Clarke orbit to keep them
stationary.
Unfortunately, for earth orbiters, there's enough other perturbations
that you probably can't see it. They already have to move satellites
around to compensate for things like solar wind, air drag (for LEO),
etc.
But maybe for a spacecraft in deep space, between planets, which is
on a well understood trajectory?
Bruce
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and follow the instructions there.
No one has commented on my graph. I would have thought that change
would easily be detected.
Jim
On Sunday, March 20, 2011, cook michael michael.cook@sfr.fr wrote:
Le 20/03/2011 05:59, Bruce Griffiths a écrit :
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.
I sincerely doubt that it will be possible to get undisputed verification of this speed up as the magnitude is swamped by the irregular diurnal and sub-diurnal rotation rates induced by tidal effects that are at lease a magnitude greater and for which the error bars are of the same order or grater than the predicted shift.
There was a similarly predicted rotation shift predicted for the Chile quake of 28 February 2010 (1,26us). There was IIRC no verification of that AFAIK. Check the tidal effects at
http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html for the tidal effect
and
http://hpiers.obspm.fr/ for rotational measured speed changes
http://hpiers.obspm.fr/eop-pc/index.php?index=news for the statement of detectability.
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
As someone else has pointed out, measuring the earth surface position relative to spacecraft orbits, e.g. GPS, would be another technique. In fact, a high resolution measurement of the position of a geosync sat might do.. If the earth's rotation rate changes you'd have to adjust the height of the satellites in Clarke orbit to keep them stationary.
Unfortunately, for earth orbiters, there's enough other perturbations that you probably can't see it. They already have to move satellites around to compensate for things like solar wind, air drag (for LEO), etc.
But maybe for a spacecraft in deep space, between planets, which is on a well understood trajectory?
Bruce
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Le 17/03/2011 22:14, Jim Palfreyman a écrit :
Just for fun I plotted the UT1-UTC data from the IERS Bulletin A.
Here's the raw data:
I add the deltas
UT1-UTC
s delta
-0.18115
-0.18232 0,00161
-0.18353 0,00121
-0.1847 0,00117
-0.18576 0,00116
-0.18674 0,00098
-0.18763 0,00109
-0.18842 0,00079<=== 11/3 day of the quake
-0.18912 0,00090
-0.1897 0,00059
-0.1903 0,00060
-0.19103 0,00073
-0.192 0,00079
-0.19324 0,00124
If you check that against the excess day length graph I don't think
that is change is significantly different, and any change would be
expected to be permanent as the mass redistribution is.
This is from March 4 to Match 17 inclusive.
I don't know if it's a fluke, but the line takes a definite "step to
the right" before continuing on its merry way.
At a glance x an y graphs seem to show nothing.
Jim Palfreyman
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Le 20/03/2011 11:02, cook michael a écrit :
Le 17/03/2011 22:14, Jim Palfreyman a écrit :
Just for fun I plotted the UT1-UTC data from the IERS Bulletin A.
Here's the raw data:
Ooops - I correct the deltas ...
UT1-UTC
s delta
-0.18115
-0.18232 0,00117
-0.18353 0,00121
-0.1847 0,00117
-0.18576 0,00106
-0.18674 0,00098
-0.18763 0,00089
-0.18842 0,00079 <=== 11/3 day of the quake
-0.18912 0,00070
-0.1897 0,00058
-0.1903 0,00060
-0.19103 0,00073
-0.192 0,00097
-0.19324 0,00124
If you check that against the excess day length graph I don't think
that is change is significantly different, and any change would be
expected to be permanent as the mass redistribution is.
On 03/20/2011 11:02 AM, cook michael wrote:
Le 17/03/2011 22:14, Jim Palfreyman a écrit :
Just for fun I plotted the UT1-UTC data from the IERS Bulletin A.
Here's the raw data:
I add the deltas
UT1-UTC
s delta
-0.18115
-0.18232 0,00161
-0.18353 0,00121
-0.1847 0,00117
-0.18576 0,00116
-0.18674 0,00098
-0.18763 0,00109
-0.18842 0,00079<=== 11/3 day of the quake
-0.18912 0,00090
-0.1897 0,00059
-0.1903 0,00060
-0.19103 0,00073
-0.192 0,00079
-0.19324 0,00124
I did a different exercise, I tossed the numbers into Gnumeric, removed
the offset of the first sample and linear slope was estimated very
coarsely and all I see is a wobble being sine-like. A shift in rate
would show up in the residues, but I can't see that. One has to have a
longer set of data to see it...
If you check that against the excess day length graph I don't think that
is change is significantly different, and any change would be expected
to be permanent as the mass redistribution is.
I can't see it in this body of data.
Cheers,
Magnus
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.
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
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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and follow the instructions there.
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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