AO-40 transponder hiatus looms

From the ARRL...



Necessary adjustments to AO-40's attitude to compensate for
unfavorable sun angles over the next several months will silence the
satellite's transponders for a while. Recent reports indicate that
AO-40 continues to operate well, providing coverage between many
parts of the world.




A scheduled attitude shift to compensate for the unfavorable sun
angle will leave AO-40's antennas pointing away from Earth until
next spring and lead to a transponder shutdown period that could
start as soon as late December.



The satellite is currently in a long period during which Earth
eclipses the sun near perigee--its point closest to Earth. AO-40
relies on solar panels for its power.



Command station team member Stacey Mills, W4SM, said that testing
and development continue on AO-40's three-axis control system, to
account for significant changes in the final orbit, the so-called
''mystery effect'' and the loss of some sensors. But he said that
three-axis control would not be ready in time to avoid the
unfavorable solar-angle season, so AO-40 will remain in spin mode,
with attitude controlled by onboard magnetorquers. The onboard
magnetorquing system--which consists of solenoid coils--makes use of
Earth's magnetic field to control the spacecraft's spin and
orientation.



''Within a few weeks, we will have to change ALAT (AO-40's attitude
with respect to Earth) dramatically, probably to about -50 degrees,
to allow the sun to pass us by for about three months,'' he
explained. The resulting high ''squint angle'' will render the S2
transmitter ineffective for transponder use, and the passbands will
be shut off temporarily.



Mills estimated that ground controllers may need to start shifting
the satellite's attitude starting sometime just before Christmas. He
didn't expect a favorable sun angle that would again allow pointing
AO-40 directly toward Earth (ALON/ALAT 0/0) until mid-April. ''It's
possible that we can leave the transponders on during the first part
of the move and turn them back on slightly before April 15 as we
start back toward 0/0,'' Mills said, ''but you can figure that
things will be sub-optimal from about Christmas until April 15.''



During the transponder shutdown period, Mills pointed out, telemetry
also will be harder to come by. He urged AO-40 telemetry gatherers
to be as active as possible during the transponder downtime.



The current AO-40 transponder operating schedule and more
information are available via the AMSAT Web site.