Fw: geo sats to internal power = time for geo sat flares

From: Bjoern Gimle@GlocalNet (Gimle@GlocalNet)
Date: Tue Sep 02 2008 - 06:17:25 UTC

  • Next message: Russell Eberst: "2008SEP1-2.OBS"

    ...and geostationary is the key --
    
    For flaring to occur, the satellite must be stabilized, ie usually 
    operative.
    99% of these have close to 0.0 degrees inclination (some military sats have 
    ~10 degrees)
    and a MeanMotion of ~1.0026
    Thus, these satellites move in a band on the celestial sphere, whose 
    declination depends on your latitude. (For me at 60N this is -8 degrees)
    And the shadow outline is roughly an 18-degree circle, centered on the Sun's 
    Right Ascension + 12h, and the opposite declination, plus this offset.
    So for me today, with Sun at +8d, the center is around -16d, and these 
    satellites pass in the upper part of this circle.
    
    But most of these appear to have their panel normals pointing in the 
    equatorial plane, so the reflections, appearing FROM declination +8, now 
    reach the Earth at approximately 60S.
    
    To locate the flaring sats with naked eye, or small binoculars, this rough 
    info is sufficient.
    Since stars pass the satellites so slowly, they can be difficult to 
    recognize, without comparing the "stars" you see to a sky chart.
    But a common digital camera, mounted on a tripod, with 30-60s exposure, 
    easily shows them as dots among the trailing stars.
    You can read more on my page http://www.algonet.se/~b_gimle/geoflare.htm, 
    which have links to
    http://www.algonet.se/GeoFlare_10150017_crop One of my first photos
    http://www.algonet.se/~b_gimle/programs/GeoFlare.xls Excel spreadsheet to 
    compute RA/Dec of shadow outline.
    http://www.algonet.se/~b_gimle/gif/Iotacet.gif shows SkyMap graphical 
    prediction (horizontal projection) of satellites entering and leaving the 
    shadow.
    http://www.algonet.se/~b_gimle/gif/1010_RA.gif shows in RA/dec, including 
    multiple outputs from my spreadsheet, how the shadow moves during a night.
    and others.
    
    ----- Original Message ----- 
    
    > Geometry is the key --
    >
    > First, it is the solar panels that are causing the flare (unlike the 
    > Iridiums where it is antennas). Solar panels, by their very purpose have 
    > to point toward the sun. When the sun reflects off a solar panel of a 
    > geosat it will only reflect back in the general direction of earth near 
    > your local midnight. In the summer or winter, the sun is above or below 
    > the equator, so the reflection from the sun bouncing off the solar panels 
    > will not beam exactly toward the earth but past the earth over the north 
    > pole or below the south pole. During those months the geosat receives sun 
    > 24x7 because the earth's shadow is never cast on the satellite (no 
    > eclipse).
    >
    > But just because the sun-to-satellite reflected "beam" off the solar panel 
    > isn't being reflected toward earth doesn't mean you can't see the geosat 
    > at night. In fact, with a telescope you can see many geosats on any clear 
    > night--year around. That's because the sun is reflecting off parts other 
    > than the solar panels. It's only when you get a direct reflection off a 
    > large solar panel that you see a flare. Which brings us back to the 
    > geometry...
    >
    > For a couple of weeks on either side of each equinox, the earth's shadow 
    > eclipses the sun as seen from the satellite (lasts about 70 minutes). 
    > During those times, from your location, just before the satellite goes 
    > into shadow it will be at the best angle for you to see a flare. Remember, 
    > the solar panel is pointing back at the sun. But the reflection itself, as 
    > it travels back in the direction of the earth, spreads out. The 
    > sun-satellite-observer angle will be such that you will catch some of the 
    > reflection (a "flare") just before the satellite goes into shadow. 
    > Obviously, when it is in shadow, you can't see it. When it emerges out of 
    > shadow you will again have an opportunity for a flare.
    >
    
    
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