In message , greywolf42
writes
Craig Markwardt wrote in message
news

"greywolf42" writes:

Jonathan Silverlight
wrote
in message ...


There's an article in the current issue of Nature that seems relevant
to
the Pioneer anomalous acceleration question ("A test of general
relativity using radio links with the Cassini spacecraft" B Bertotti,
L
Iess and P Tortora, Nature vol 425. No. 6956 p. 374,
doi:10.1038/nature01997)
AFAICS they have accurately modelled emission from the RTGs and they
don't see any unexplained acceleration.

They haven't looked for anomalous acceleration with this experiment. The
referenced experiment measures the time-delay of the signal in a
gravitational field. Nothing more.

Incorrect. The referenced Cassini experiment, as with all radiometric
Doppler tracking experiments, measures the Doppler shifts of the
carrier due to the motion and other intervening effects. The time
delay is not measured (i.e., it is not a ranging experiment). The
Shapiro effect enters into the observable as the *time rate of change*
in the signal travel time (including relatistic effects).

It is also incorrect to say that the Cassini experiment does not
measure an "anomalous" acceleration. By construction, the experiment
requires a modeling of all forces on the spacecraft, "anomalous" or
not. My reading of the article is that no anomalous forces were
required.


And my reading of your above statement is that there is no such conclusion
contained within the paper. The paper is nothing more or less than a
calculation of the PPN parameter, gamma during a single Solar conjunction.

There are no modelling of forces on the spacecraft in the paper -- construct
or otherwise.

Have you actually looked at the paper??
They wouldn't be able to see anything without accurate modelling of the
gravitational and non-gravitational forces on the spacecraft.
It's the latter which are much more interesting here - solar radiation
and the anisotropic thermal emission from the spacecraft, which is the
prime suspect in the Pioneer acceleration.

Quoting
"Deriving this acceleration from a model of the spacecraft is a
difficult task; but its estimation from Doppler measurements, combined
with attitude data, is routinely carried out for spacecraft navigation
with good and consistent accuracies. The largest component (along the
Earth-spacecraft axis, _pushing towards the Earth_
[my emphasis]
) is about 3 x 10^-9 m s^2..... This component has been determined with
a formal error of ~3%".

Bertotti et al. figure 3 shows Doppler residuals of about +- 4 x 10^-4
Hz over the 30 day period of solar conjunction, and their RMS value is
1.2 x 10^-4. This works out to about 5 x 10^-11 Hz/s, much less than
the figure Anderson et al. found (6 x 10^-9 Hz/s)
--
"It is written in mathematical language"
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