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From: F/32 Eurydice on 5 Mar 2010 10:15

What do you figure causes the three smaller steps in the audio tone of
the x-band signal of Mars Express passing Phobos, besides the obvious
answer that Phobos' gravits causes the big one?

http://bit.ly/bz9isr
From: Mike Dworetsky on 8 Mar 2010 08:58
F/32 Eurydice wrote:
> What do you figure causes the three smaller steps in the audio tone of
> the x-band signal of Mars Express passing Phobos, besides the obvious
> answer that Phobos' gravits causes the big one?
>
> http://bit.ly/bz9isr

According to the source you cite, the reason for the changes are that NASA
controllers commanded the Mars Express to switch to a frequency locked to
their transmitter, and the other changes (I think the ones you mention) are
due to the amateur eavesdropper switching his receiver frequencies to stay
locked on to a band where the audio would be in the human audibility range.
The audio is, I presume, the difference between signal and local tuner
frequency, or something closely related to it.

The variation of frequency is due to the orbit of Express around Mars,
mainly. They don't claim to have found the variation due to Phobos gravity
yet, though they seem to be looking for it. It wouldn't be a step, but a
slight change in the curvature during the fly-by.

Quote:

"In the graph above, there is a sharp zig-zag at the start of the recording
(the time starts at the bottom and goes to the top) - this is the effect of
Mars Express catching onto the "sweep" of NASA's 70m ground station. This is
a technique where the station on Earth varies its frequency to find one
where Mars Express is listening. Then the Mars Express transmitter 'follows'
this frequency from the ground, which is a much more stable frequency source
than the one on board the spacecraft. This extreme stability is necessary to
find the minute variations in the signal due to Phobos.
"In the audio, the pitch of the Mars Express tone sometimes jumps from low
to high: this is caused by Bertrand shifting the receiver to stay locked on
the Mars Express frequency as it drifts due to the Doppler effect - exactly
like re-tuning a station on your radio at home."


--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

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