On 4/3/14 8:17 PM, Chris Albertson wrote:
I just read about a discovery of a liquid water ocean on Saturn's moon
Enceladus. The method used was to measure the velocity of a
spacecraft as it makes a close fly-by. Gravitational anomalies will
cause the spacecraft to speed up or slow down as it flies over massive
objects like mountains. With three pass they now have a 3 dimensional
map of density distribution. It must be very sensitive if they can
tell liquid water from ice by its gravitational field. (or even rock
from ice)
They say they can measure the spacecraft's velocity to 90 microns per
second. They do this by measuring the Doppler sift of the
transmitter. I've been trying to figure out what 90 microns/sec
means in terms of frequency. But I think(?) I need to know the
orbital velocity of Enceladus.
Ranging is done by looking at the round trip time from Earth to
spacecraft back to Earth. The signal on the ground is generated by a
hydrogen maser.
The radio on the spacecraft adds Allan Deviation on the order of 1E-15
at tau of 100-1000 seconds. The uncertainties in things like the
antenna and cables on the spacecraft add similar uncertainty.
The ground station antenna also flexes and moves. I'd have to go look
up what the magnitude of that is, but I think it's in the same order of
magnitude.
For Cassini (which is what they'd be doing for Enceladus), the signals
are in the deep space X-band. Transmitted from earth at 7.15 GHz,
returned from Cassini at 8.4 GHz (roughly). The ratio between
transmitted and received signal is 880/749 (exactly). This is called
the "coherent turnaround ratio" and we spend a fair amount of time
making sure that the turnaround is phase coherent. That is, a phase
shift of 1 radian on the input signal will result in a phase shift of
880/749 radians on the output signal.
The actual time delay through the telecom system is measured on the
ground before launch in a temperature chamber, so any temperature
variation during the measurement can be accounted for.
Radio science and navigation measurements are quite impressive in their
accuracy and attention to detail. measuring range to cm (out of a
billion km, i.e 1 part in 1E14) and velocity to mm/s is sort of standard.
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