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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