Definitely STS-92; satellite phase function

From: Matson, Robert (
Date: Thu Oct 12 2000 - 18:06:50 PDT

  • Next message: Jim Nix: "Oct 13, ISS and STS 92 observed"

    Hi Tom,
    > Just happened to be outside and caught a pass of something very bright
    > and fast traveling from SSW passing through approx. 30 Degrees ESE 
    > headed to NE at about 19:15:00 EST. Sorry I don't have more exact times
    > and directions.
    This was definitely STS-92.  Not bad considering the sun
    was only 4 degrees below the horizon.  Predicted magnitude
    was about -2.  --Rob
    Which reminds me, I need to go into SkyMap and change the
    phase equation for satellite brightness.  For years (more
    than a decade?), it seems like everyone has been using a
    (1-COS(Phase)) factor to determine satellite magnitude from
    the standard magnitude (90 degree phase, 1000-km range).
    I used to have the correct phase function in there, but
    someone talked me out of it in the early 90's.
    While working on a related photometry problem, I had to solve
    the same double-integral that applies to this diffuse-reflector
    problem, and ended up "rediscovering" my original equation.  If
    any other satellite prediction programmers are using this cosine
    dependence on phase angle in their software (phase here defined
    as 180 for full, 0 for new), I'll be happy to supply you with
    the correct lambertian function.  The difference between these
    phase functions is not insubstantial, especially near opposition
    and at poor phase angles.  (At opposition, the cosine formula
    is too dim by 0.5 magnitude; at 45-degree phase, the old formula
    gives predictions that are too bright by 0.7 magnitude).
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