Satgen 471  Ao10 Enigma Variations  by GM4IHJ (BID SGEN471) 1998-04-04

What is happening to Amsat Oscar 10 ? The spin rate dropped steadily
from 1987 to early 1998, and it appeared to have almost vanished in
February 98 , when the fade pattern indicated a complete Z axis
rotation roughly every 6 minutes. All be it, a rotation which as ZL1MO
suggested, is the product of Z axis nutation causing the satellite
antennas to conically scan the sky . Now as March 98 ends rotation
around the Z axis seems to have a period of about 144 seconds duration
,with every rotation usually , but not always, having one deep fade of
beacon signal and two shallow fades.

Around the 29th March however the beacon began to FM, shifting about
45 Hz just after the deepest fade of the beacon. This shift appeared
regularly in the same place each rotation cycle so it was clearly not
due to over powerful users on the transponder. However before
observers could be sure the FMing and the deep fade were tied
together, they began to drift apart, with by 31st March, the beacon
FMing some 10 to 12 seconds after the recovery from the deep fade.
Suggesting that while the FMing might be due to poor solar panel
illumination at that point in the rotation cycle, the beacon fade
clearly had a different cause. In addition a pattern of minor FMing at
other points in the rotation cycle has also become evident.
 
There is also a puzzle , whereby the signals coming down from the
transponder are very much weaker than the downlink beacon note on
145.81 MHz. Such that with the satellite near apogee, the IHJ receiver
fed by a simple 5 element 2m yagi (negative gasfet pre amp) , produces
a beacon signal 20 dBs over noise, whilst the qsos on the downlink are
on or in the noise and barely readable even at fade peaks. The qsos
can of course be improved by adding a good preamp in the loop but then
the beacon simply rises to an almost unheard of near 26 dBs over
noise. Frankly these are not the conditions this operator has
experienced in the past when 2 by 10 XY yagis and a gasfet were used
to guarantee copy of all users on the transponder , whilst the beacon
came in at a level very little stronger than the qsos. Making one
wonder if, somehow , the satellite can have switched to use the high
gain satellite downlink 2m antenna, whilst the up link is switched
through the low gain 70cms antenna ?

Polarisation possibly offers a further clue, in respect of variation
in the beacon signal between January 98 and March 98, whereby in
January when the beacon signal faded briefly, switching between the
horizontal 5 element yagi and the vertically polarised 5 element yagi
instantly brought up the signal . Indeed watching both polarisations
at the same time showed that they were almost exactly antiphase. In
March 98 this effect was no longer present. So, what is this
marvellous survivor of space radio's pre digital stone age, doing ?

IHJ will continue monitoring Ao10 regularly, and is interested in the
ideas and thoughts of anyone who thinks he has some explanation for
what is happpening, and/or , can suggest what might happen in the
coming months.