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AO-40: Hibernation #1 - Sun Angle & Attitude



Dear All,

AO-40 performs very well according to our expectation.

The current attitude table as of Apr 23 is:

ATTITUDE MEASUREMENTS:

  Orbit   alon alat
----------------------
  202     142        *          *    alon based on ES stop(s)   
  203     138        *          **   camera gives alon and alat 
  204     136        *
  205     133        *
  206     132        *
  207     128   14   **
  208     127        *
  209     125        *
  210     121        *
  211     119        *
  212     116        *
  213     114        *
  214      -     -
  215     109        *
  216     106   11   **
  217      -     -
  218     102        *
  219      -     -
  220      98        *
----------------------
        -2.43°/perigee
RMS fit: 0.52°



ATTITUDE ESTIMATE and TEMPERATURE MEASUREMENTS:

      Attitude   | Batteries (deg.C)|   Tanks (deg. C)  |  (deg. C)  Panels
Orbit lon/lat SA | Main2 Aux1  Aux5 | MMH-3 N204+  NH3-2| IHU   EPU    2-5
-----------------+------------------+-------------------+-------------------
200          -41 | 29.8  33.1  21.1 | 19.3  17.3   31.8 |  9.4  19.9   9.0
201          -43 | 31.1  35.1  21.9 | 19.9  18.6   33.1 | 10.0  21.2   7.1
202  141  16 -46 | 33.8  37.1  22.6 | 20.6  19.3   35.7 | 10.0  21.9   1.8
203  139  15 -48 | 35.1  39.0  23.2 | 21.2  20.6   37.1 | 10.0  22.6
204  136  15 -51 | 36.4  39.7  23.9 | 21.2  21.2   38.4 | 10.0  23.2  -2.5
205  134  15 -53 | 37.1  41.0  23.9 | 21.2  21.2   39.0 | 10.0  23.2  -4.5
206  131  14 -55 | 37.7  41.7  23.9 | 21.2  21.2   39.7 |  9.4  23.2  -6.4
207  129  14 -57 | 37.7  41.7  23.2 | 20.6  20.6   39.7 |  8.7  22.6  -8.4
208  126  14 -60 | 37.1  41.0  21.9 | 19.9  19.3   39.0 |  8.1  21.9 -10.4
209  124  13 -61 | 37.1  40.4  21.9 | 18.6  17.9   38.4 |  6.7  20.6 -13.7
210  121  13 -63 | 36.4  40.4  21.2 | 18.4  17.3   38.4 |  6.1  19.9 -15.0
211  119  13 -65 | 35.7  39.7  20.6 | 16.6  16.6   37.7 |  5.4  19.3 -17.0
212  117  12 -66 | 35.7  39.0  20.6 | 16.6  16.0   37.7 |  4.8  18.6 -18.3
213  114  12 -67 | 35.1  39.0  19.3 | 15.3  15.3   37.1 |  4.1  17.9 -19.6
214  112  12 -67 | 35.1  39.0  19.9 | 15.3  14.7   37.1 |  4.1  17.9 -19.6
215  109  11 -67 | 35.1  39.0  20.6 | 16.0  14.7   37.1 |  4.1  17.9 -19.6
216  107  11 -67 | 35.1  39.0  20.6 | 16.0  14.7   37.1 |  4.1  17.9 -19.0
217  104  11 -66 | 35.1  39.0  19.9 | 16.0  14.7   37.1 |  4.1  18.6 -18.3
218  102  10 -65 | 35.1  39.0  20.6 | 16.0  15.3   37.1 |  4.8  18.6 -17.6
219   99  10 -64 | 35.1  39.0  20.6 | 16.6  15.3   37.1 |  4.8  18.6 -16.3
220   97  10 -63 | 35.7  39.0  21.9 | 17.3  16.0   37.7 |  6.1  19.9 -13.7
221   95   9 -61 |                  |                   |
222   92   9 -59 |                  |                   |
223   90   9 -57 |                  |                   |
224   87   8 -55 |                  |                   |
225   85   8 -52 |                  |                   |
226   82   8 -50 |                  |                   |
227   80   7 -48 |                  |                   |
228   78   7 -45 |  2001-04-29      |                   |
-----------------+------------------+-------------------+--------------------
Rate of change of alon: -2.428°/perigee
Normal precession:      -0.161°/perigee
--------------------------------------
Mystery effect:         -2.267°/perigee at 2.046 rpm and Hp ~ 320 km
--------------------------------------


James Miller G3RUH also provided the following information:



______________________________________________________________________________

                      Hibernation  #1  Analysis
                      ~~~~~~~~~~~~~~~~~~~~~~~~~
                               -or-

                       Did the Cameras Fry?
                       --------------------

                               by

                        James Miller G3RUH

                           2001 Apr 24


  Executive Summary
  -----------------
  During hibernation #1 (2001 Jan/Feb) the AO-40 Sun angle reached
  approximately 60°, and not 90° as was once feared.  Thus the cameras
  did not get direct sunlight onto their imagers.  The analysis is based
  on solar-panel temperatures.  The "Mystery" torque is outlined.

Introduction
------------
The IHU-2 camera pictures are now much degraded, so we need to explore what
might have happened to it.  One possibility was that during hibernation #1
(see notes for explanation of "hibernation") the Sun angle exceeded 80°,
admitting sunlight directly into the camera.

A Time There Was
----------------
Recall that the Sun angle measurements (see notes) just /before/ hibernation
#1 suggested that the s/c attitude direction was around ALON/ALAT 258/-15;
this lies exactly on the Ecliptic (Sun's orbit), implying that half way
through hibernation #1 we'd get a Sun angle of +90° which would fry the
camera.  At the time this was a very real concern.

Hibernation #1 was expected to take from Jan 08 to Apr 05 (12 weeks); to
everyone's surprise, it lasted only until Feb 24 (7 weeks).  So we had a
mystery on our hands.

Sun angle measurements taken immediately /after/ hibernation #1 suggested
the attitude was now 208/-26.  This, too, lies exactly in the Ecliptic
plane.  So surely at some time the Sun angle must have passed through 90°?

The Wisdom of Hindsight
-----------------------
Hibernation #2 has been very benign, and has enabled this problem to be
explained.

During hibernation #2 we have had measurements of ALON every orbit, from the
Earth sensors, and some IHU-2 camera data.  And we find that contrary to the
fundamental precepts of a spinning P3-type spacecraft (see notes), AO-40's
attitude direction is moving in inertial space!  The measurements show
that ALON is changing by -2.46°/perigee due to some "Mystery Effect".

So this ALON drift must also have happened during hibernation #1.  And the
effect of this is that attitude determination by Sun Angle evolution, which
assumes an invariant attitude with time, gives misleading results.  This
means that since during hibernation #1 the attitude direction was changing
all the time, it could not possibly have been in the Ecliptic plane, and
therefore the Sun angle did not pass through 90°.

Taking its Temperature
----------------------
So, once again, what was the worst Sun angle during hibernation #1, and can
we estimate the attitude trajectory based on indirect measurements?

To answer this we have to look at spacecraft temperatures.  The most
sensitive indicators of Sun angle are the solar panels.  During hibernation
#1 (orbits 89-149) we only had telemetry just after perigee, roughly
MA 0-18.  At this time eclipses ended at MA=5, and there was also Earth
warming which has quite a significant effect when the spacecraft is closer
than about MA=15.  So getting representative solar-panel temperatures would
not be a simple matter.

To deal with the vast amount of data I [JRM] wrote a program that looks at
each orbit's worth of telemetry from orbit 89 to orbit 149 [from Paul
Willmott's magnificent archive - see notes], and calculates the average
temperature of the four panels 2+3+4+5 taken together, during each MA
interval for which there was any available telemetry.

>From this we find that the solar panel temperatures during the nadir of
hibernation #1 (orbits 113-127), when as far as possible from the Earth and
from eclipse, was typically -11C to -13C.

Temperatures to Sun Angles?
---------------------------
Can this temperature be converted to a Sun angle?  Remember, at the time the
Sun angle was in unknown regions  > +45°  for which we have no direct
calibration data.

What we do have now, however, is solar-panel temperatures vs Sun angle for
the vastly better characterised hibernation #2.  The difference is that in
hibernation #1 the Sun angles were positive (Sun lights the s/c top), and
during hibernation #2 Sun angles are negative.

So we make a bold assumption, that the solar-panel temperature profile is
symmetric about zero Sun angle.  That is, we suppose that the panels are
heated the same amount for positive Sun angles as for negative Sun angles.
it may not be a perfect assumption, but it shouldn't be too far out.

In hibernation #2, a temperature of -11C to -13C corresponds to a Sun angle
of -60°.  Thus we propose that during hibernation #1, the worst Sun angle was
of order +60°.

   SA(max) = +60° means that the IHU-1 camera, and importantly, the SCOPE
   cameras were not exposed to damaging direct sunlight.


So What Really Happened?
------------------------
We can build on this result to deduce the attitude history for hibernation #1.

A Sun angle of +60° around orbit 120 implies an ALAT of +7°.  If we also
assume this value of alat at entry and exit of hibernation, then we find the
following attitudes for the spacecraft:

        |  Orbit  SA   alon  alat
  ------+-------------------------
  Entry |   89    46   241    7
  Nadir |  120    60   227    7
  Exit  |  149    46   214    7
  ------+-------------------------
  Hibernation #1 attitude table

There is more to be milked from this data.  From the table, the rate of
change of ALON is seen to be -0.435°/perigee.  Now the rate of change in ALON
due to normal precession of the orbit plane is -0.161°/perigee.  Therefore
the "Mystery Effect" accounts for an additional drift in ALON -0.274°/perigee.

Now let us compare hibernation #1 and hibernation #2:

                Observed  Natural   Mystery
                 change    change   Effect    Spin
                 ALON      ALON      ALON     rate
     Orbits      °/p       °/p       °/p      rpm
    -----------------------------------------------
     89-149     -0.44     -0.161   -0.274    17.60
    202-218     -2.45     -0.161   -2.289     2.05
    -----------------------------------------------

Intuitively you would expect the rate of drift in ALON to be inversely
proportional to spin rate.  Astonishingly, comparison of the last two columns
shows this to be true.

All based on a few solar-panel temperatures.  Isn't hindsight wonderful?

Homily
------
This analysis clearly illustrates that despite being in orbit nearly 6 months
we are still feeling our way with AO-40.  Much is uncalibrated, and things
often cannot be explained until long after the event using telemetry elements
that might have seemed insignificant at the time.

There are NO answers to simply look up from the manual; there isn't one.
We're writing the manual as and when it happens.

                                    o
                           ------------------
                                    o
Notes
=====
Hibernation
-----------
"Hibernation" is a term applied to periods when the Sun angle exceeds
45° and the Sun sensors cannot function, depriving us of attitude control via
the magnetorque system.  Thus the s/c has to endure a cold spell of
unfavourable Sun angles;  hopefully not too unfavourable.

There have been two periods thus far:

  Hib. Date         Orbits
 ---------------------------
   1   2001 Jan/Feb  89-149
   2   2001 Apr     202-228
 ---------------------------

Sun Angle
---------
This is the angle between the Sun's direction and the spacecraft's spin
equator.  Thus a Sun angle of 0° maximally illuminates the solar panels,
whilst a Sun angle of 90° is directly onto the top or bottom of the spacecraft
and gives no illumination at all.  The management goal is to keep the Sun
angle between +/- 45°.  This is not always possible.

The "Sun Sensor" measures this angle, provided it is the above range.

Spinning Spacecraft
-------------------
The principle of conservation of angular momentum requires that the direction
of the spin axis remains constant in inertial coordinates unless acted upon
by an /external/ torque.  With P3 spacecraft the only torque of consequence
supposedly comes from the magnetorquers, when they're in use.  Any other
change in attitude direction must therefore be due to some other source of
torque.

The attitude direction can also be changed by an internal re-distribution of
angular momentum from, in particular, momentum wheels.

AO-40 Telemetry Archive
-----------------------
To get archive data:

  ftp://ftp.amsat.org/amsat/telemetry/ao40

To deposit data, any data, however poor, ZIP compress and send to:

  mailto:ao40-archive@amsat.org 

<end>
______________________________________________________________________________


73s 
Peter DB2OS 
for the AO-40 Command Team

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