This is a summary of about 5 years experience in looking for Gorizonts
and Radugas, and probably applies to many other near GSO objects.
Things I say here may not be always true, and I would welcome any comments
corrections and updates.
This summary applies to those no longer in operational use, which means more
than 5 to 10 years old, with mean motion not 1.0027 rev/day, or inclination
more than 5 degrees. Disused GSO satellites are often pushed into "graveyard
orbits" which are near GSO. Whether they are pushed or not, the inclination
will drift away from zero at about 1 deg/year.
Such objects will generally be rotating slowly, about an axis that changes
little with time (a few deg per year?). Any nearly flat surfaces will at any
instant reflect the sun into a cone of light maybe a few degrees across.
As the satellite rotates this cone will move in a circle through space.
If an observer is inside the cone, he/she will see a flash or flare of light.
Typically the interval between flashes is usually between 1 and 3 minutes,
indicating that the satellite rotates in either the same interval, or twice
that interval (if flashes are coming from two opposite surfaces, for example
Superbird A = 89041A = #20040).
Flashes are almost always at regular intervals.
The brightness of the flashes is usually between mag 4 and mag 6, and
they usually last for 1 or 2 seconds (for me with a usual mag limit of 8 or 9).
The satellite does not move during the flash.
The rotation of the flash cone through space gives a ring of sky in which
flashes would be seen. For an observer, the satellite is moving very slowly in
azimuth and elevation, and about 1 deg per 4 minutes in an approximate great
circle across the stars.
This great circle may cross the ring near its centre, in which case there will
be two periods in the day/night in which flashes could be seen.
Or the great circle may cross the ring near its edge, in which case there will
be one longer period of visibility.
Only in rare cases will the satellite flash throughout the night.
My experience is that the period for which flashes are seen is usually about
1 hour.
On later days, the flashes appear in the same RA/dec area, which may not
be the same time of night.
There are several ways to improve your chances of finding a particular Gorizont.
(1) If someone has seen it recently (say within a week), then look when the
satellite is in the same part of the sky.
(2) I look for 5 continuous minutes per hour (or maybe half hour).
If you know the flash interval (eg from PPAS) then look for twice
the flash interval (in case you miss one flash).
(3) Choose times when the satellite is in a part of the sky which you can
easily locate: time searching for the right part of the sky is time
wasted.
(4) If you are looking for several Gorizonts, then often two or more will
be close together in the sky, and you can look for two or more at the
same time (binoculars are usually better than telescopes for this).
If you see something flashing, then try to get 2 or 3 times and positions,
so the identity of the satellite can be checked. I have often accidentally
seen a different Gorizont, and once a rocket near apogee of a GTO orbit.
Mike Waterman mike.waterman@marconi.com or mike.waterman@web-hq.com
Site Yateley = COSPAR 2115 = 51.3286N 0.7950W 75m.
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This archive was generated by hypermail 2b29 : Tue Apr 01 2003 - 13:43:34 EST