[time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
jimlux at earthlink.net
Fri Apr 4 18:33:53 UTC 2014
On 4/4/14 9:58 AM, Alex Pummer wrote:
> 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
I don't know many spacecraft that carry a gravity sensor these days.
Apollo 17 carried a gravimeter based on a vibrating string accelerometer.
There's a proposed experiment for 2017 or something to do the
There's also a proposal to put a spring gravimeter (ISA) on an ESA lunar
lander, and to include it in the proposed MAGIA mission, which uses the
two satellite GRAIL/GRACE approach as the primary measurement. I think
Bepi-Colombo is carrying the Italian Spring Accelerometer to Mercury.
GRAIL and GRACE measure the distance between two spacecraft very
precisely (using carrier phase measurements on board) to infer the
gravitational field of the body they are orbiting around.
Gravity science by measuring range and Doppler is popular because it's
cheap. You already have to have a radio on the spacecraft, so the
incremental cost for the science is the labor of the toilers on the
ground, who can be inexpensive graduate students working on their
dissertation in non-real time. There *is* a cost to specifying and
measuring the performance of the spacecraft radio to support this kind
of precision, but that again, is just a cost penalty and it's small. It
doesn't add risk, mass, or power.
Adding an instrument adds mass, power, data bandwidth and programmatic
risk (what if the instrument isn't delivered on time, or interferes with
some other instrument or the spacecraft). Radio science adds no mass,
no power is required and the data is entirely gathered on the ground,
where it's cheap. There is a slight operational cost: doing an 8 hour
gravity science pass with the data rate turned down to minimum so you
can have maximum power in the carrier, as opposed to in the data sidebands.
And, this is changing. As we move towards faster telecom radios,
keeping good radio science performance is challenging. When we sent
bits at 10 bits/second, you needed really good close in phase noise, and
so, the radio science performance came for free. But when you send 10
Mbps, the phase noise inside 10 Hz isn't as big a deal, and people are
starting to ask "why are we spending extra time, money, mass, risk to
achieve insanely high radio science performance"...
That's why Bill Folkner led a development effort to develop a purpose
specific low mass, low power radio science instrument (RSTI). A 1kg,
few Watt, 1 liter box that you could put on anything to do precision
radio science (and nothing else). The prototype is (hopefully) going to
fly as part of LMRST, which is a 2 or 3U cubesat.
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