Intrinsic brightness error strikes again

From: Matson, Robert (ROBERT.D.MATSON@saic.com)
Date: Tue Apr 17 2001 - 17:31:35 PDT

  • Next message: Tom Wagner: "Saw Daytime Iridium Flare"

    Hi All,
    
    Jonathan mentioned the following from the Heavens-Above
    website:
    
    > Satellite Details
    > Orbit:  683 x 690 km, 68.0 
    > ...
    > Intrinsic brightness (Mag):  3.6(at 1000km distance, 50% illuminated) 
    > Maximum brightness (Mag):  1.9 (at perigee, 100% illuminated) 
    
    Not to beat a dead horse, but these two brightness values
    are not self-consistent.  If 3.6 is the intrinsic (aka
    standard) brightness at 1000km range, 90-degree phase),
    then the peak brightness at perigee and full-phase would
    be:
    
    3.6 - 2.5*LOG(Pi/0.683^2) = 1.53
    
    "50% illuminated" is a very misleading description since
    what is really meant is that the observer-to-satellite-to-
    sun form a 90 degree angle.  In any case, 90-degree phase
    is not half as bright as full-phase; it's only 31.83% as
    bright (factor of 1/pi).  This is the source of error in
    the maximum brightness calculation.
    
    Cheers,
    Rob
    
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