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[jamsat-news:1386] AMSAT Special Bulletin

SB SAT @ AMSAT $ANS-348.01

BID: $ANS-348.01

AO-40 command stations have been quite busy lately as general
housekeeping tasks, system testing and orbital changes have been
underway with the complex circuitry onboard AMSAT OSCAR-40.

AMSAT-DL President (and P3D Project leader) Dr. Karl Meinzer,
DJ4ZC, and AMSAT-NA President Robin Haighton, VE3FRH,
informed ANS of the current status:

AMSAT OSCAR-40 status report / December 13, 2000

Nearly four weeks have now passed since the launch of Phase 3D - and
there are both good things and bad things to report.

Immediately after separation, we could not receive the 70-cm transmitter.
Consequently, we programmed the IHU-2 to act as repeater and use the
V-band middle beacon as our main downlink. In addition, the two S-band
transmitters have been operated occasionally, but the geometry within the
orbit must be right to have the antennas point in the general direction of
Earth. This limits the use we can make of S-band.

The main activity so far for the command stations was to learn to fly the
spacecraft and to prepare it for the first burn with the 400 N motor. This
required a reorientation, as the satellite must point opposite to the
orbital speed vector - and a spin-up to about 9 rpm was needed. In view
of this ongoing work, it was decided to postpone the analysis of the
problem with the 70cm transmitter to sometime after the first burn. The
reorientation of the spacecraft turned out to be more time-consuming
than initially assumed. We had two problems to solve:

1. 	As we neared our firing attitude, the geometry was very poor for
producing direction changes of the spin-vector. The magnetic field of the
Earth is nearly perpendicular to the spin axis meaning that we can
change spin easily, but not direction. It turned out that rounding errors in
the software of the IHU were in the same order of magnitude as the
desired spin-change. Thus, reliable control was not possible. Careful
tweaking of the parameters used in the IHU finally enabled us to solve
this problem.

2.   The geometry was also poor in terms of attitude determination by our
sensors as the Earth and Sun are viewed almost from the same direction.
In fact, for a certain time the Earth sensor was blinded by the Sun and
delivered no useful data at all. The YACE camera (in conjunction with the
IHU-2) came in very handy here, taking pictures of the Earth at strategic
times. We actually used YACE as another sensor looking to the top of the
spacecraft. It gave us badly needed additional data. 

The IHU-2 also played a vital role in processing and storing these pictures.
Unfortunately, it turned out that it was not as reliable as the IHU-1; every
one or two days it crashed. This results in the beacon transmitting only a
weak carrier (no modulation) until the IHU-2 is manually reset from the
ground. So far this has posed no real problem, but it is a nuisance. We
now have some data of how the memory is corrupted by radiation.

Eventually, we hope to install additional software in the IHU-2 which
allows us to avoid (or at least automatically recover from) these crashes.
But this has to wait until things get a bit quieter. Until that time we will
have to live with IHU-2 crashes, which seem to be caused by hard
radiation and typically occur when the satellite comes out of perigee
and again enters the radiation belts.

Around Sunday, December 10th we had finally reached the proper
orientation and spin for the first 400 N motor burn (to take us to
50,000km apogee altitude). After reviewing the various constraints it
was decided to execute this burn on Monday, December 11th in the
perigee of orbit 50/51. The necessary program was uploaded - this is
an automatic sequence since the spacecraft at burn time is at the
equator with only 600 km altitude - and no command stations would
be in contact with the satellite. 

Perigee on orbit 50/51 came and went - but the burn did not take
place. The stored telemetry was analyzed and we learned that the 
sequencer went through all the necessary steps but there was no
helium pressure and consequently no engine burn (helium pressure
is needed to open the fuel-valves).

On Monday, December 11th the situation was investigated and we
found the pressurization command was not resulting in any action.
Initially the investigation concentrated on a software discrepancy,
which may have existed between the test software used in Kourou
and the command version used for flight. We determined that this
probably was not the cause - the valve(s) simply refused to open.
We experienced a similar problem during the test phase of P3D,
and one of the helium valves was repaired as a consequence. All
the indications are that we were again having this type of problem
- which may be caused by the age of the components. Our command
stations then executed multiple open and close commands, which
eventually got the helium flowing, but we also noted that the helium
flow was almost an order of magnitude less than required. Still we
did build up pressure in the tank (to nominal) and we then
programmed the spacecraft to perform the burn on Monday evening.

This time the burn started properly, but there were some anomalies in
the telemetry indicating that the burn did not stop at the programmed
time, but lasted almost 3 minutes longer. Thus, we have now achieved
an orbit with about 60,000 km apogee altitude. For our ultimate plans
this is no problem, we would have used the electric propulsion anyway
to further increase the perigee.

One of our command stations, James Miller, G3RUH, has written a data
collection program for IHU-2, and we have a very detailed account of all
events of this burn. A step-by-step analysis is under way, and we have
some indications of the nature of the problem and possible ways to cure
it - or at least to cope with it, provided the 400 N motor was not damaged
by this event.  

During the next days we will investigate this matter in detail. If we have a
real problem with the 400 N propulsion system, we may need to change
our strategy for achieving a useful final orbit. But it may also mean only
that we have to observe a number of boundary conditions during following
burns and that the situation may have no impact on the mission goals.

For the next few days no attitude changes are planned, and we will be
able to start investigating the problem with the UHF transmitter. During
these tests we will find out if  we can operationally plan on using the
70-cm transmitter or not.

We are living exciting times!

Finally, we would like to express our compliments and appreciation to
the command stations for the superb job they are doing. Each day they
are facing new difficulties requiring a very large amount of time and
dedication. They are doing a super job!   

Dr. Karl Meinzer, DJ4ZC  (and)   Robin Haighton, VE3FRH
 President AMSAT-DL                  President AMSAT-NA

Stay tuned to ANS for additional bulletins from AMSAT, the
official source for information on AMSAT OSCAR-40.

[ANS thanks AMSAT-NA and AMSAT-DL for this information]


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