RE: [amsat-bb] AO7 Tumbling!

["Dr Tom Clark" <w3iwi@xxxxxxxx>]

ZS6Y noted

> Hello All.
> Mode A was on at 0615 UTC today..
> This was an over head pass.
> Could not get into the transponder at all for at least 3/4 of
> the pass.
> Sounded if the lady cam on at 0629 UTC then off again at 0634
> with abt 15 deg of pass to go.
> the 28.502 beacon was heards AOS to LOS..
> My loops back faded in and out during the on period.
> Any one else notice this.
> 11 ele Yagi and 50watts were used Inv V for the down link.
> 73's Etienne

This is probably not physical tumbling of the satellite. The 10M antenna on
AO-7 is linear polarization with a tape-measure whip. As a linear polarized
signal passes thru the ionosphere, the plane of polarization rotates due the
Faraday Rotation.

Faraday Rotation happens because the earth's magnetic field gives the
ionosphere two different indices of refraction. The effect is proportional
to the electron content in the ionosphere times the component of the
magnetic field along the line of sight. As the satellite moves across the
sky the geometry of the magnetic field between the observer and the
satellite changes (and the electron density probably varies too). So you
expect to see the satellite fade at different rates and this happens even
when the satellite is perfectly stabilized.

73 de Tom, W3IWI

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