gravitation measurement, particularly gravitation measurement in space
is based on the Eotvos -effect see here:
http://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_effect and here:
http://en.wikipedia.org/wiki/Lor%C3%A1nd_E%C3%B6tv%C3%B6s and from
the begin of the space exploration many space crafts using accelerometer
based on that Eotvos pendulum, invented by Eotvos in the
eighteen-hundreds [the richest oilfields in the United States were
discovered by Eötvös' Pendulum. The Eötvös pendulum was used to prove
the equivalence of the inertial mass
<http://en.wikipedia.org/wiki/Inertial_mass> and the gravitational mass
<http://en.wikipedia.org/wiki/Gravitational_mass> accurately] so no
speed no time measurement is necessary...
~ a former co-worker of space projects.
A. Pummer
On 4/4/2014 9:01 AM, Jim Lux wrote:
On 4/4/14 5:01 AM, Dr. David Kirkby wrote:
On 4 Apr 2014 08:55, "Tom Knox" <act...@hotmail.com> wrote:
90 microns is approx a freq res of about 1 x 3.66 -12
Thomas Knox
Since the Doppler shift is prortional to the frequency, I can't see how
one can determine the absolute frequency.
But given light travels at 3e8 m/s and they can resolve 9e-5 m/s, I
would
have thought that the frequency resolution needed was 9e-5/3e8=3e-13. We
are differing by more than a factor of 10.
It's actually even more tricky, if you think about it, because what
you are really doing is making the measurement over some time period,
and the path length of signal is continuously varying during that time.
Not only is Cassini doing it's flyby of Enceladus (and you're looking
for small deviations in trajectory from those due to an idealized
point source masses), but you've also got your ground stations on
Earth moving due to planetary motion, daily rotation, as well as
things like solid earth tides moving the DSN station up and down by
tens of cm during the measurements.
Gravity science in deep space is a very time-nutty activity.. it's
basically finding all the various sources of change, modeling them,
and driving the uncertainties as low as possible.
They use a collocated radiometer to compensate for the extra delay of
the atmosphere of earth. JPL has all those folks computing earth
rotation models, and that figures in (hey, you need to know the
rotational velocity of earth pretty accurately, to take that out of
the equation).
The folks who do this spend a lot of time looking at "residuals" plots
and trying to make them look like a flat line of zero width.
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