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Lacrosse 4 Sunday 23 April 2006
Based on analysis of a flare of Lacrosse 4 on Christmas Eve 2005 I
estimated its proposed radar panel was at an of 25° and that it used XVV flight mode. I
also had 3 observations of Lacrosse 5 (see below) which suggested panel angles
between 29 and 36° in YVV flight mode. So I tried making predictions for Lacrosse
4 on the basis of 33°. The first
prediction that tied up with a clear night was on Sunday 23rd April 2006.
Amazingly weather conditions
were good at the designated time and it was 10°C in the observatory at 21:50BST.
Details of the prediction are show below:
Maximum altitude 77.2 at 23/04/06 20:59:47UTC
Minimum range km 705.845543583738
Sun Altitude -12.7
Ingress 23/04/06 21:03:19
Flare Prediction for Flight Mode XVV Panel Angle 33°
MA 0.28 at 23/04/06 20:59:24UTC
Alt=71.6
A flare duly occurred at almost exactly the predicted time, as analysis of the finder tape shows. But then the satellite showed strange cyclic
variations in brightness, some of which matched the flare itself. I have no
idea what caused this behaviour!
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One possible aspect for the flare. The incoming
sunlight (yellow) is reflected off the radar panel at rear. The panel is
rotated by 33° from the x-y plane about the y-axis. The reflected ray
(orange) is nearly coincident with the satellite observer vector (red).
The blue blob marks the location of the observer. |
Background
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During an observation of Lacrosse 5 on March 6th 2006 I noticed a mild flare
around 20:19:05. Here is a brightness trace taken from the finder camera video.
Shortly after the flare there is a drop in brightness which was probably
caused by cloud.
At 20:20:54 the brightness plummets. Again I'm not sure why but it
could have been cloud.
On the graph the blue line indicates the angle between the
observer-satellite vector and light reflected from the radar panel. I have
calculated the rotation of the panel required to bring this angle to zero
at the time of the flare.
A similar calculation is performed for the solar panel. Since the
result is nowhere near zero, this panel cannot have caused the
flare.
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This screen-capture from a VRML simulation of
the pass shows the instant of the flare. As the simulated telescope
view (inset) shows the possible radar panel has been positioned to cause it to
flare.
In the orbital view the incoming sunlight (yellow) is reflected from
the panel as the orange vector, which hits the ground quite near to my
location (red).
To achieve this I have a program which can calculate the rotations that
a panel needs to cause a flare for any location at any time. These
rotations are calculated for the x and y-axes of the satellite in the
specified flight mode (YVV here).
In reality I assume that the panel can ONLY rotate about the y-axis and
the good news is that at the observed time of the flare no rotation
about x was required whereas a rotation of 36 degrees was required for
y.
Two other flare observations for Lacrosse 5 reported to me suggest a panel angle between 29 and 36 degrees. If I assume say 33 degrees I
can predict future flares. Some are predicted for May. Watch this
space.
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