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I made some inquiries about the 'official" beacon frequency recently, and
the command station "standard" frequency was reported as 2401.323 MHz.
However, I see variations of +/- 2 KHz or so from that.

There are lots of factors in that.  One is that if you have your converter
remote mounted, it is going to be, even with temperature stabilization,
sensitive to ambient temperature.  For instance, when my G3RUH dish is
looking into near the sun, I get a real jump in frequency.  Hi Hi

Another is the freshness of the keps.  At mode S frequencies, small velocity
differences can make noticeable differences in the Doppler shift.  I can run
an ephemeris using sequential keps and sometimes, but not always, see a
measurable shift in predicted Doppler values.  Nothing huge, but enough to
be detectible in an audio passband.

Finally, I find consistent variations of the frequency in the course of an
orbit, even with constant terrestrial ambient temperatures.  I use Uni_Trac
to do the tracking and radio tuning, and have cross checked its Doppler
compensation against several tracking programs, including the gold standard
InstantTrack.  Dead on.  That suggests either a change in the actual beacon
frequency along the orbit, or there are some residuals in the tracking
algorithm everybody is using that shows up at these frequencies.  Remember
that you need to make some simplifying assumptions to make this
computationally tractable.

As a practical matter it probably doesn't matter, though I am using hardware
frequency tracking for extended TLM acquisition.  I know at least one person
who is collecting extensive and detailed data on this general matter.  It
should be possible to factor all sorts of things into that analysis.



To get the attention of a large animal,
be it an elephant or a country, it
helps to know what part of it feels
pain. Be very, very sure, though, that
you want its full attention!

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