[time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?

[Jim Lux]

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.

>