Re: Almost time for flaring GEOSATS

From: Richard Clark (rclark@LPL.Arizona.EDU)
Date: Fri Mar 02 2001 - 20:17:38 PST

  • Next message: Tony Beresford: "Re: GEO Sats."

    Part of the sat (including the antenas) must have an Earth pointing
    attitude, but the solar arrays must be tracked. Many sats will use
    only one degree of freedom sun tracking-- in right ascension. There is
    no tracking in declination. Hence the flares are seen near eclipse
    near the times of equinox while the obserer's hemisphere is in the
    winter half of the year. This may not be the case for all sats, but it
    is for quite a few... the flares have been observed during the
    observing seasons predicted by this model.
    
    Here is a unified version of the table.
    
    latitude  |solar|     days from
              |decln|     equinox
    0           0           0
    20         3.45        8.8 +- .1
    40         6.27       16.15 +- .2
    60         8.06       20.9 +- .3
    80         8.69       22.65 +- .3
    
    The uncertainty comes from the fact that the rate of change in
    declination is not exactly symmetric before and after the equinox, and
    also very slightly different between march and september equinoxes.
    
    The ideal value of solar declination for any latitude is given by
    
                   -.1511 sin(lat)
    tan(decln) = ------------------
                 1 - .1511 cos(lat)
    
    Richard Clark
    rclark@lpl.arizona.edu
    
    On Fri, 2 Mar 2001, Cees Bassa wrote:
    
    > I've tried to compute these days for myself but I got different
    > results.
    >
    > ... [northern-southern hermisphere got swapped]
    >
    > Now I'm wondering if all geostationary satellites keep their solar
    > panels fixed, i.e. they don't track the sun. It seems very plausible,
    > but I can imagine that it is a little effort to let the panels track
    > the sun. Are there any geosats with solar panels that do track the
    > sun?
    > 
    > Here is my list for the March 20, 2001 Equinox, with and without tracking 
    > solar panels:
    > 
                 Without     With Tracking
    > Latitude   Month/Day   Month/Day
    >   80.0000    2 26       4 12
    >   60.0000    2 28       4 10
    >   40.0000    3  5       4  6
    >   20.0000    3 12       3 29
    >    0.0000    3 21       3 21
    >  -20.0000    3 29       3 12
    >  -40.0000    4  6       3  5
    >  -60.0000    4 10       2 28
    >  -80.0000    4 12       2 26
    > 
    > So for me here at latitude +52 the optimum for non-tracking
    > satellites has just passed, and clouds are keeping me inside for way
    > too long now.
    > 
    > Regards,
    >          Cees Bassa
    
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