R: Re: all my flares for Heavensat

From: satrack@libero.it
Date: Fri Jul 13 2012 - 19:36:05 UTC

  • Next message: Björn Gimle: "Re: Re: all my flares for Heavensat"

    Bjorn is absolutely right! In order to predict flares it has to be known how
    the satellite surfaces are oriented with respect to a coordinate system fixed
    with the satellite body (i.e. its shape: front surface, lateral surface and so 
    on), and  
    how the satellite is kept oriented with respect to its orbital plane (attitude 
    control).
    
    In the case of Iridium satellites we know that they have three main reflecting
    surfaces (Main Mission Antennae) that have fixed angles with respect to the
    satellite body (they are not mobile). Usually, their orientation is specified 
    by two 
    angles, namely "Tilt" and "Angle". In addition we know that their attitude 
    control
    keeps the satellite Z-axis oriented toward the Earth's centre and another
    axis (lets say X-axis) toward the direction of motion (i.e. in the orbital 
    plane), and
    consequently the third axis (Y-axis) perpendicular to the orbital plane.
    
    Only now, that we know its shape and how it is oriented at a given time 
    (attitude
    control) it is possible to compute where the sun light is reflected.
    
    In the case of satellites out of control (no more operative) their orientation 
    is no
    more controlled and they may start rotating. If the precession of the rotation 
    axis
    is small and the satellite fast rotates, it is still possible to predict where 
    the
    reflected light is projected (visible as "flashes") if both the orientation of 
    the rotation
    axis and of the reflecting surfaces (with respect to this axis) are known. 
    
    
    Visual SAT-Flare Tracker 3D can manage both operative and non operative 
    satellites:
    
    1. In case of operative satellite, it can use different attitude controls, 
    such as: 
    Vertical-Along Track  (Iridium-Like), Vertical-Along Sun and Yaw-Steering and 
    others.
    
    However, only the first two are available on the current public version of the 
    (offline) software.
    
    The Yaw Steering is used, for example, by the online version to predict flares 
    of 
    some metereologic satellites, such as the METOP-A:
    
    http://www.satflare.com/track.php?q=Metop
    
    2. In case of rotating satellite, the program is able to compute the 
    satellite's
    rotation axis if the user can provide 3 observed flares generated by the 
    same surface. The program does not take into account the precession.
    
    However, It has been sucessfully employed to model some tumbling Iridiums. 
    The task is challenging because to obtain good results several flashes have to 
    be 
    collected over a short period (e.g. 2-3 days).
    
    I used this function in the past to compute the rotation axis of Iridium 33 
    and
    to predict its flashes:
    
    http://digilander.libero.it/SATrack/iridium33.html
    
    and to compute the orientation of the Phobos Grunt axis:
    
    http://www.satflare.com/track.php?q=phobos&sid=3
    
    Instructions and examples on how to use these functions of SAT-Flare are 
    available in the help file included in the program archive:
    
    http://www.satflare.com/download.php
    
    Best Regards,
    Simone
    
    
    
    >----Messaggio originale----
    >Da: bjorn.gimle@gmail.com
    >Data: 13-lug-2012 15.53
    >A: "Jon Mikel"<jonmikelk@gmail.com>
    >Cc: <seesat-l@satobs.org>
    >Ogg: Re: all my flares for Heavensat
    >
    >Pardon my unfamiliarity with HeavenSat (and SatFlare), but the list looks
    >too simple.
    >
    >Any flare calculation must assume that the surface(s) is/are fixed relative
    >to a coordinate system, whose axes MAY be in the direction of travel and to
    >the local vertical (and orthogonal to those), like the Iridium antenna
    >panels.
    >But most GEO satellites' solar panels point in the equatorial plane, close
    >to the Sun's RA; Iridium solar panels are sometimes fixed and sometimes
    >turn to the Sun, depending on the Sun's position relative to the orbital
    >plane.
    >Others may be fixed in stellar space, but turning solar panels. A flashing
    >satellite has a rotation axis slowly precessing, and surfaces usually have
    >a fixed angle relative to this axis (requires three angles)
    >
    >Does HeavenSat only suppose one of many possible reference frames ?
    >2012/7/13 Jon Mikel <jonmikelk@gmail.com>
    >
    >> Here all the angles (for flares.dat)
    >> Note: The title of the flare contains the magnitude of the flare, and a "t"
    >> if the satellite makes more than 1 flares (t for tumbling)
    >>
    >> Lacrosse 5
    >> mag-2       42.200º      87.000º
    >> mag+1      33.200º      88.300º
    >>
    >> Lacrosse 4
    >> mag0        34.000º      85.400º
    >> ....
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