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Re: AO-40 K Band 30Jan03 VS 3 AXIS STABILIZATION



On 31 Jan 2003, at 14:27, Jon Ogden wrote:

> 
> What we NEED is to get the 3 axis stabilization routine working.  Then we
> won't have squint problems!
> 
> 73,
> 
> Jon
> NA9D

I know this question was ask many times and i just read back AMSAT-DL-BB bulletins since january 2001 and i 
isolate the  works progress on the 3 axis stabilization question. Hope this can help to answer your questions.




On January 8 2001 Karl Meinzer wrote:

4. Test the momentum wheels to determine if AO-40 can be put into a
    three-axis mode which would greatly reduce the impact of the loss of the
    omni-antennas. 



 On Mon, 15 Jan 2001 Peter Guelzow wrote:

* Attitude Control System * 
The attitude control system is fully functional. Unfortunatly due to the
high spin of about 17 RPM the sun changed faster than we could change
the attitude. Because the sunangle is now about 60 degrees, the sun
sensor electronic stopped working, that's why the indicated Spin in the
telemetry is now almost 0 RPM. Without sun and attitude information no
magnetorquing can be performed, thus no further attitude or spin change 
was done. However, Karl Meinzer DJ4ZC is developing a new de-spinner
software algorithm, which does not use the Sun sensor information. It
will be tested in the next week and will than be used to reduce the spin
and move to better spacecraft attitude. 



On Fri, 16 Mar 2001  Robin Haighton VE3FRH  on
[amsat-bb] AO-40 Special Bulletin wrote

Before the satellite could be regularly used for general Amateur Radio
communications, it was necessary to carry out orbital changes, stabilize
the satellite, open the solar panels, etc.



On Tue, 01 May 2001 11:50:34 -0600 Jim White wrote:

The plan for 5/2/2001 includes additional software loading, some initial 
tests on RUDAK-B, and possibly some initial telemetry.  If time and squint 
angles allow the CAN server will be loaded and we will take a look at the 
16 temperatures available through RUDAK.  It may also be possible to power 
up the CEDEX experiment and obtain some initial radiation data.

Testing and commissioning plans after that are uncertain and will depend 
mainly on squint angles and pass visibility.



On Mon, 24 Sep 2001 Wayne Estes wrote:

We all know that the AO40 momentum wheels were successfully tested in
August, but I haven't heard anything since then. I have a few questions:

1. Is there an online document that describes the planned sequence of tests
that could potentially lead us to 3-axis stabilization of AO40?

2. Will there be a trial 3-axis stabilization operation in the near future?
Are such tests waiting for some other event such as completion of RUDAK
testing or cessation of eclipses?

3. If 3-axis stabilized, could AO40 obtain sufficient solar energy with the
solar panels un-deployed?  (so that we could easily return to spin
stabilization if a reaction wheel fails)

Wayne Estes W9AE
Mundelein, IL, USA



___________________________________________________________________
On Mon, 05 Nov 2001 Stacey E. Mills wrote:

3-Axis
-------
Testing and development continues on the 3-axis control system, to account 
for significant changes in our final orbit, the mystery effect, and the 
loss of some sensors.  Expect further feasibility testing and announcements 
on this shortly.  We are approaching the end of the favorable solar angle 
"season", so within about one month we will have to either initiate 3-axis 
control or offpoint for several months until the solar angle improves.  No 
shift to 3-axis control will be made without adequate testing of a 
mechanism to revert back to spin control.



On Date sent:      	Sat, 01 Dec 2001 Jens Schmidt wrote:

Hi all,
local scuttlebutt says that ARRL is better informed
than AMSAT-BB about the state of AO40.
Read here:
http://www.arrl.org/news/stories/2001/11/29/4/?nc=1
73 Jens    ZL2TJT
RE78nv
AMSAT ZL #218

The Text:

AO-40 Transponder Hiatus Looms
{PRIVATE "TYPE=PICT;ALT=Stacey Mills, W4SM"}AO-40 ground controller Stacey Mills, W4SM.

NEWINGTON, CT, Nov 29, 2001--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. 
Command station team member Stacey Mills, W4SM, says, however, that he's puzzled and disappointed by 
the relatively low numbers of users on AO-40 during the last month or so. "I'm not sure why," he told ARRL 
this week. "This is prime time, and we won't have such good conditions much longer until the three-month 
'bad angle' cycle passes." 
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. These periods, which began in late August, will rapidly increase in length and continue well into next 
June. The satellite relies on solar panels for its power. 
Mills 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. 
"We will stay in spin mode for at least a few more months," Mills said. "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 high "squint angle" (the satellite's antennas' 
aperture with respect to Earth) 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 thetransponder downtime. 

On	Sun, 02 Dec 2001 01:11:Stacey E. Mills wrote:
 
...With AO-40, we have no omnis......  During these times, the passbands will be on if the squint is reasonable, 
but  there is no reason to have them active if the squint is prohibitive.   I had hoped to outline all of this in an 
update for the amsat-bb last week, but my day job and the actual planning of  the events has kept me too busy.  
When we start the move, we'll outline the plan and time line.
The perigee eclipses will continue for many months but don't last very long.  Therefore,  don't really affect the 
power budget, but they do affect magnetorquing because without the sun, the sensors can't  "despin" the 
magnetoquers.  So torquing is not as efficient, particularly when the eclipse is right at perigee.



On Sun, 2 Dec 2001 Wayne Estes wrote 

I think the rosy pre-flight predictions all assumed the best-case situation of zero off-pointing angle and S1 antenna. 
 Off-pointing angle and antenna
gain make a much bigger difference than a 50% higher apogee.  3-axis might partially solve the off-pointing 
problem and make near-perigee operation
very practical for people with marginal 2400 MHz receiving systems.


On	Mon, 03 Dec 2001 Stacey E. Mills wrote:

Magnetorquing is a little like "magic" and although we will do everything 
possible to follow the above scenario, we must juggle the mystery effect, 
efforts at attitude determination, decreased torquer efficiency due to 
perigee eclipses, etc.  The simulators say this will work, but ultimately 
the above must be viewed as our current best estimate of upcoming events.
...stay tuned.


On Sat, 22 Dec 2001 Stacey E. Mills

The mystery effect  appears to have changed (decreased) somewhat as we have 
increased perigee, and the amount of change is consistent with an 
interaction with the earth's magnetic field. Althought drag was initially 
considered when the perigee was rather low, our current 1000+ km perigee 
completely excludes that possibility.  If the solar panels were opened we 
would be able to tell by changes in the solar panel current patterns, 
changes in magnetorquing efficiency, changes in center of gravity as seen 
in the photos, changes in temperatures of the panels and objects shaded by 
them, etc.  There is no evidence for any of these things so it is clear 
that the panels remain closed.   (Even if one were open, any effect from 
this would not be "despun" as discussed above.)

If the mystery effect is due to AO-40 having a net Z-axis magnetic field, 
then aligning that field with the earth's magnetic field, should cause the 
magnetic force and resultant mystery effect to disappear.  This is one 
approach we are considering for counteracting the ME during perigee under 
3-axis control.


OnTue, 19 Feb 2002 Stacey E. Mills  wrote

>Now how does this situation affect the eventual (?) move to 3 axis
>stabilization using the momentum wheels?  If I understand things correctly,
>even under momentum wheel control, the bird will still experience periods of
>non-optimum squint angle in order for the solar panels to catch the sun.

True, though in theory we should be able to tolerate a few hours of bad 
solar angle to give optimal squints, then swing back to recharge the batteries.

>Now will the additional, unfolded panels help to decrease periods of bad sun
>angle since you have more panels out there and so thereby we have a better
>point to earth?

I'm not quite sure if I understand your point, but I think the answer is 
no.  In part we have to try to stay within solar sensor ranges.

>Finally, what is the status and plan of moving to spin up the momentum
>wheels and open the panels?  Is it ever going to happen or are we just too
>unsure of the condition of the bird to risk it?

As soon as we have a final timetable, I'll sure pass it along.  It's not a 
matter of uncertainty about the spacecraft.  It's a matter of testing a lot 
of software, making sure all the contingencies like the mystery effect and 
protracted eclipses are covered, and that we can get back to spin mode if 
we need to. The original plans for 3-axis mode were based on a much higher 
inclination....  Some of the software routines will need to be loaded in 
the spacecraft and tested in bits and pieces.  Others can be tested in 
ground simulations.  This is all under active development, but we do not 
want to rush it or back ourselves into a corner from which we cannot retreat.


On Sun, 18 Aug 2002 
Mike Miller KA5SMA

Does anyone know who will make the decision and if it will ever be made 
...concerning the the deployment of the solar panels?? I know there are 
unknowns,,as in everything,,, damage to spacecraft...and so forth...but these 
bad pointing angles  are discouraging...Do I understand that an advantage of 
3 axis stabilization...with solar panels deployed would mean satellite always 
pointing at earth???   Can someone elaberate on the possible negatives and 
positives of deploying the solar panels...?/

Mike Miller KA5SMA


On Tue, 20 Aug 2002 Don Woodward and Jens Schmidt  ZL2TJT wrote:


Three-axis stabilization of Ao-40.
-----------------------------------
With the transition of spinstabilized mode to three-axis stabilized
mode of Ao-40, longer service life would be obtained with improved
Squintangles.
But after the propulsion incident in December 2000 and the consequences
from it, new more complicated basic conditions result. In the following
contribution, the central problems and the further procedure are described
on the basis of summarized information by the project manager Karl Meinzer,
DJ4ZC, and the command team.

After the incident in the drive system and with the lost fuel, several basic
conditions have changed in such a manner that the subject of three-axis
stabilization was to be reconsidered completely again.
To these changes belong:

1. By the inclination of up to 10deg (fig. 1, A) as opposed to the originally
planned approximately 61deg (fig. 1, B), has the geometry between earth,
sun and satellite become different, which makes another strategy in the
present situation necessary.

2. The so-called " Mystery Effect (ME) " makes the movement of the satellite
uncontrollable, if the spin of AO-40 is so far reduced that the three spin
wheels (fig. 2) can take up the remaining angular momentum, without pushing
to their speed limit.

3. Since the omni antennas failed, each larger change of attitude, (ALAT),
leads to the loss of communication (fig. 1, C).

4. The communication in perigee proximity, with bad Squint, permits
practically no real time control of the three-axis automatic control loop in
the satellite.

For a long time the question was simply, whether there is at all a safe way
to be able to reach a three-axis stabilization without then loosing the control
of the satellite and communication.
In the meantime a strategy was developed, by project manager Karl Meinzer,
which permits a return, with some probability, to the spin mode from each
phase of the transition in time.
Thus it is not sure yet whether Ao-40 can be actually transferred into the
three-axis mode. A high risk remains.
However with the development of the strategy, it turned out that the procedure
is less dependent on the solar angle than original was accepted. That takes
some of the pressure from the further procedure.
This is better for all involved, in order not to risk that Ao-40 points uncontrolled

parallel to the magnetic axis of Earth, with the antennas to the north. That would
most likely be the end of Ao-40.

The strategy plans now to hold the Z-axis of the satellite such in the orbit, as to
have at least for a certain period in each orbit access to Ao-40. In order to
implement an satellite-internal automatic controller, actually a three-dimensional
automatic control loop of second order is necessary.
However a set of simpler differential equations could be found, which make it
possible to control the satellite movements in a predetermineable way.

To a final three-axis regulation one will only be able to approach step by step.
In a first step it is necessary to bring the spin to a value of approximately
0.4 RPM so that the spin wheels can take up the residual torque around the Z-axis.
This spin reduction must be tested, in order to be able to determine the behaviour
of the satellite as caused by the Mystery Effect.
If it is possible to still manoeuvre the satellite at this number of revolutions
the wheels can transfer the residual torque in a next step.
>From then on Ao-40 then slowly 'drifts', and one could return at any time to the
original situation with spin around the Z-axis.
If Ao-40 manoeuvring should be successful so far, the next steps can be planned.

For all these steps the appropriate mathematically physical fundamentals in
program code are to be converted.
This must also be progressed carefully. In addition trials of the code and
extended tests of the software must be done on the ground.
The transition of Ao-40 to the three-axis mode will thus still take some time.
Some steps of the strategy can take place only with a favourable solar angle.
The suitable Phase begins in November 2002.

AMSAT DL journal Sept. 2002, franc sperber, DL6DBN

Hope this will give an overview of the 3 axis stabilization work progress as per posting on AMSAT-BB



 

 
{PRIVATE "TYPE=PICT;ALT=Stacey Mills, W4SM"}






Luc Leblanc VE2DWE (AMSAT 33583)
Coordonnateur AMSAT pour le Québec
Quebec AMSAT coordinator
SITES WEB:http://www.sorel-tracy.qc.ca/~luclebla/
          http://www.qsl.net/ve2dwe/
C.P.341
Sorel-Tracy QC.
Canada
J3P 5N6


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