[time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
alex at pcscons.com
Fri Apr 4 12:58:25 EDT 2014
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.
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" <actast at 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
>> 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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