ISS glints

From: Matson, Robert (ROBERT.D.MATSON@saic.com)
Date: Fri Dec 15 2000 - 19:21:29 PST

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    > Actually, the X-axis is =parallel= to the ground with the Soyuz
    =trailing=.
    
    > Q. One more question about the attitude with the "LVLH".  I assume it's
    PMA 
    > forward - Z1 going to nadir - PMA-3 going to zenith, straight into the 
    > flight direction?
    
    > A. No.  PMA-3 is facing Earth.  P6 is sticking up, facing the zenith and 
    > wings flat.  It's in the attitude you saw in the fly-around video when the
    
    > (STS)97 crew undocked from the station.  It's in exactly the same
    attitude.
    
    This means the main arrays have surface normals that are
    pointing approximately toward nadir/zenith (i.e. the arrays
    are parallel to the velocity vector).  If so, then solar
    glints off the main arrays can only occur when ISS is
    extremely low in the southwest in the evening, and about
    to enter the terminator, or extremely low in the southeast
    in the early morning having just exited the terminator
    (northern hemisphere viewing).
    
    There are 2 pairs of smaller solar arrays that appear to
    have their surface normals tilted slightly forward of nadir
    (slightly in the ram direction).  If so, then these arrays
    CAN produce glints.  The only favorable glint geometry for
    winter in the northern hemisphere would be a descending node
    pass in the early morning when ISS is traveling northwest to
    southeast.  The amount of the tilt of these arrays would
    determine where in the pass that ISS glints.  For a very
    shallow tilt, the glint occurs when ISS is past culmination
    and setting in the southeast.  For a slightly greater tilt,
    the glint occurs earlier when ISS is closer to culmination.
    With a little more tilt still, the glint would occur before
    culmination while ISS is still rising out of the NW.  Based
    on the picture at:
    
    http://spaceflightnow.com/ops/stage4a/001209snapshots/withearth.html
    
    it appears that the forward tilt is somewhere around 20-30 degrees,
    which would favor glints at fairly high satellite elevation angles.
    Whether ISS needs to pass northeast of your zenith or southwest
    of your zenith would depend on how high your latitude is.  If you're
    pretty far north, then the descending ISS pass would need to
    culminate southwest of your zenith to produce a glint; at some magic
    north latitude (which changes with the seasons), a perfect NW to SE
    zenith path produces the correct geometry; south of this latitude,
    the ISS culmination needs to be in the NE.
    
    Note that the current geometry does not permit glints off these
    smaller solar panels in the early evening, only the morning.
    
    Best,
    Rob
    
    P.S.  If someone can provide the tilt angle, I can make an adjustment
    to IRIDFLAR to provide glint timing information for ISS (but only a
    relative brightness estimate until enough observations have been
    made).
    
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