Re: Flashing geosats

Hi all,

Last night I observed flashes from the de-activated geostationary
satellite GOES 10 [24786/97019A] again. I used the CMOS camera and
85mm F/1.4 lens to record video frames at 10Hz between 19:56 and
22:12UTC (April 1, 2021). The flash pattern that was also observed on
March 30, 2021 repeated, but with about a half hour delay.

These are the time series of the satellite brightness for both nights.
The satellite flux was extracted using a simple method and probably
can be improved upon.
https://ibb.co/yFvZG62
https://ibb.co/WKGCFmt

The ~29 second rotation period is clearly visible, with the second
group of flashes being offset by about half a rotation. For March
30th, the rotation period was 29.115 seconds, while on April 1st it
was lower, at 28.990 seconds. By folding the time series of both
nights at these rotation periods, the brightness as a function of
rotation angle and as a function of time can be plotted, as shown in
these plots:
https://ibb.co/gtkxYh0
https://ibb.co/HXMfS42

On both nights, the flashes near 270 degree rotation angle (arbitrary
definition here), start first, being bright for about 10 to 15
minutes, after which the flashes from the 90 degree rotation angle
become brighter. The observations show that there is a weak leading
and trailing flash around 80 and 100 degree rotation angle, which is
infact visible for much longer. These components on the long
observation of April 1st, also seem to shift slightly in rotation
angle, possibly due to changes in the viewing angle.

There is some interesting physics going on, both in terms of geometry
between the satellite, the Sun and the observer, as well as the
rotation vector of the satellite and the weaker flashes being visible
for longer.

I'd be curious to hear from those who observed Superbird A and other
satellites how this behavior compares.

Regards,
   Cees
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