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AO40: Spring feelings!




Dear All, 

Lot's of good news about AO-40! 

The SEU (sensor electronic unit) is triggering the sun again after the
end of eclipse on orbit #147. 

The telemetry channel #100 (analogue spin rate) is now showing a value
of #49. This is the same value that it was showing on orbit #89 when we
lost Sun Sensor triggering. 

Within a few orbits we should see that the Sun Sensor SS2 also starts
triggering and this would allow us to obtain attitude information and
re-start magnetorquing to spin-down the spacecraft. 

A week ago the 3-Axis sun sensors have been turned on and they seem to
work nominally. It was planned to use the information from the 3-axis
omni directional sun sensors to obtain the necessary reference
information for a new algorithm to spin-down the spacecraft without the
SEU. The new software code, which will be patched into the 20ms
interrupt service, is almost ready for upload, but due to the premature
return of the SEU data, this might no more be necessary. 

Analysis of the IHU temperature by DB2OS recently revealed, that the
maximum sun angle, with the lowest temperatures, was much earlier than
anticipated. The temperature of the IHU reached it's minimum exactly on
2001 February 01 with a temperature of about -8.4 °C. This is almost one
month earlier than originally thought. There is no explanation at the
moment, why this is premature. 

The spin rate of around 17.5 RPM did not changed in the last couple of
weeks and whatever it caused to increase, it stopped. As soon as the SEU
will deliver good sun sensor data, the magnetorquers will be used under
the control of the IHU (integrated housekeeping unit) to reduce the
spin, which will than make attitude changes easier. This will also
improve the telemetry reception on S-band. 

The next milestone is to bring the spacecraft into a good orientation to
fire the Arcjet with no electrical power and gaseous ammonia only. The
test firing will be used to check the Arcjet control electronics and
valves. Even without the Arcjet "burning", the thrust from the test
firing will be enough to raise the perigee by about 100km. 

Several independent analysis (including the french space agency CNES)
confirmed, that the current orbit will be stable for many years, i.e.
more than the anticipated lifetime of the spacecraft. 

The test firing should give us additional safety margin and a better
feeling. However, it is later planned to optimize the current orbit with
a full running Arcjet. This is planned after testing of the 3-axis
momentum wheels, which will be used to re-orient the the spacecraft
during these maneuvers. 

While the spacecraft was in hibernation, the command stations were
restless with preparations for the next activities, watching the 
spacecraft telemetry and healthy.

AMSAT-DL expresses his thanks to Paul Willmott, VP9MU for his outstanding 
service in gathering and archiving the AO-40 telemetry from the different 
parts of the world. This is vital additional information for the command
stations in their continuing effort to turn P3-D back into operation. 
Everyone should be encouraged to send captured telemetry (with P3T) to
the AO40 telemetry archive at  'ao40-archive@amsat.org'

After all, the spacecraft seems to be in very good shape, with no further 
damages or other problems detected. The power budget is very good and even
during the maximum sun angle it was not necessary to reduce power
consumption by turning off the S-band transmitter for parts of the
orbit. 

This is indeed a good sign for the next activities. 

Stay tuned.. 

Additional information can be found at the AO-40 web page at: 

http://www.amsat-dl.org/

73s Peter, DB2OS
    AMSAT-DL


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