Re: Geosat Flare Question

From: Rainer Kresken (rkresken@t-online.de)
Date: Sun Oct 19 2003 - 07:40:32 EDT

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    Hey all,
    
    May I answer your second qustion first:
    
    >2.) I've calculated the ring of geosats to be at -5.45
    >degrees declination for my latitude. So when the
    >earth's shadow is south of the celestial equator
    >(before autumnal equinox), the eclipsing should be
    >optimal for me. But Rainer's table of optimum dates
    >for latitudes shows the farther south before equinox
    >and farther north after equinox. Could someone explain
    >this better?
    
    Geostationary satellites flare because of specular reflections
    from their solar arrays. 
    This image shows attitude of such a satellite more or less correctly 
    http://www.btinternet.com/~c.vowles/Sat/images/astrap5.jpg
    
    Important is that the rotation axis of these arrays is perpendicular
    to the equator plane of the earth. In the course of one 24h orbit,
    the arrays are steered to follow the sun while the antennae of the
    satellite remain pointed to the earth.
    Since the plane normal of the arrays is always in the equator plane,
    sunlight is reflected to northern hemisphere when the sun is south of the 
    equator and vice versa.
    
    >1.) He states that best time to see these flares is
    >when the geosat is less than 25 degrees above the
    >horizon. Why? I was looking up at nearly 50 degrees,
    >straight south when I saw them, middle of the night.
    
    I came up with this idea because the earth would eclipse the satellite
    when the flare would be brightest IF the satellite would follow
    the above attitude rules perfectly. A very bright fare would then only
    be possible when the observer was close to the edge of the earth as
    seen from the sat. That means as well that the sat would be close to the 
    horizon as seen from the observer. 
    
    In real life this turned out to unrealistic, mostly because the arrays
    are usually not orientated precisely as described above. Since this posting,
    many observations by seesaters revealed that bright flares are possible
    all through the night close to the optimum date according to the list 
    I posted.
    
    Regards,
    
    Rainer
    
    
    
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