Flaring Geosats

rkresken@esoc.esa.de
Wed, 1 Sep 1999 09:43:42 +0100

Dear SeeSaters

In April, Paulo Cacella and Alcir Carra from Brazil reported seeing a group
 of
satellites without optical aid that turned out to be the cluster of Hotbird
 geostationary
direct tv satellites. I found this quite remarkable and tried to figure out
 how
this could be. I think that such flares of OPERATIONAL  three-axis
stabilised
geostationary satellites should be visible at any given location during a
couple
of days around the equinoxes. This phenomenon should be limited to
satellites that are not higher than about 20 to 25 degrees above the
observer's
horizon DURING PENUMBRAL ECLIPSE.
Since the geostationary autumn eclipse season starts today, I want to ask
you
to look for such flares. I am working on a prediction program is which
currently only based on guessing and assumptions and is not yet ready to be
published (maybe spring equinox). For this purpose, your observations would
 be
highly appreciated.
 The huge solar arrays (40 or 50 square meters is a typical value) act than
 as a point mirror and project a inverted image of the partially eclipsed
sun
 on to the surface of the earth and they can be seen flaring to maybe
 mag +2 (flashes from tumbling satellites of this class like Superbird A
 or TDF-1 get that bright). The season for such flares just started today
 (01/09/99). The belt of visibility moves north as the sun moves southwards
 in the sky and the optimum dates should be the following:

latitude    optimum date
-80          01/09
-60          04/09
-40          09/09
-20          15/09
0            23/09
+20          30/09
+40          07/10
+60          12/10
+80          15/10

The flares should occur during shadow entry shortly after sunset and during
shadow exit shortly before sunrise. For planning observations, I propose
the following procedure for observation attempts:

- Get a compilation file with tles of geostationary satellites, e.g. from
OIG
- Identify ANY operational satellite (small drift and inclination ) that is
 close
  to the eastern horizon (evening) or western horizon (morning)
- use a prediction program (I use good old Traksat) to find eclipse start
(evening)
  or end times (morning)  and the associated position of the satellites at
that time
 in terms of right ascension and declination.
Observe this position when it is between the horizon and about 20 or 25
degrees elevation
and some satellites should flare as this spot scan across their position on
 the geostationary
arc.
In theory, direct sunlight reflection should only be possible on two or
three nights around the
optimum date, but Paulo Cacellas observation (10 days after optimum time)
suggests
that atmospheric stray light and/or attitude tolerances may cause flares
before or after this
spell.
Please be aware that these statements are based on minimum observational
data, but
I think that these observations could be quite interesting and
opportunities are quite rare,
so give it a try! It would appreciate feedback from you and both positive
or negative observation
reports!

regards,
Rainer Kresken