RE: atmospheric extinction for low altitude flares T

From: Matson, Robert (ROBERT.D.MATSON@saic.com)
Date: Wed Jun 16 2004 - 17:41:57 EDT

  • Next message: Tom Wagner: "Re: atmospheric extinction for low altitude flares T"

    Hi Ed and List,
    
    My 2 cents on Tom's extinction question:
    
    >Two questions:
    >(1) Does H-A consider atmospheric extinction when making 
    >their flare magnitude predictions?
    
    Chris has already replied that it doesn't; I would add that
    it shouldn't.  My reasoning is that when you're determining
    the stellar magnitude of some celestial object (or in this
    case a manmade satellite), you ideally want to compare to
    other celestial objects in the same general part of the
    sky.  Particularly when making visual observations, the
    ability to distinguish small differences in visual magnitude
    diminishes rapidly with distance between the two objects
    being compared -- irrespective of differences in extinction.
    If two stars differ by only half a visual magnitude, I think
    many people would have difficulty telling the difference if
    those stars were more than 20 degrees apart.  But put them
    side by side and it's easy.
    
    The main point of predicting a visual magnitude is to give
    the viewer some expectation of how bright a pass will be --
    relative to nearby objects.  If an Iridium flare and Venus
    are both predicted to be at magnitude -4.5, and both are
    only 10 degrees above the horizon, then the two should
    appear the same brightness -- even though both would appear
    brighter if they were magnitude -4.5 at 40 degree elevation.
    
    Ed mentioned a remark in the Iridflar documentation which is
    actually unrelated to the extinction question:
    
    > However, in the Iridflar 2.21 documentation Rob Matson wrote
    > the following -- which I don't fully understand -- about
    > Iridflar (and SkyMap):
    
    >Improved calculation of satellite lighting condition.  Now 
    >calculates lighting the same way that SkyMap does -- 15 km 
    >or 20 km tangent height extinction (depending on season), 
    >with 0.2 or 0.1 degrees of refraction respectively.
    
    These remarks have to do with the determination of when a
    satellite transitions from being sunlit to going into the
    earth's shadow.  To properly calculate the lighting conditions
    at a satellite, you must account for refraction of sunlight
    by the earth's atmosphere, and choose a reasonable extinction
    height -- here defined as the tangent height below which no
    appreciable sunlight will pass through the atmosphere and
    back out into space.
    
    For spring-summer (determined by sun's declination and satellite's
    latitude), I use a 20-km tangent height cutoff.  At this altitude,
    visible light transmission is less than 5% for high volcanic extinction.
    Atmospheric refraction at that tangent height is approximately 0.1
    degrees.  For fall-winter, the atmosphere is less dense, so a lower
    15-km tangent height cutoff is used.  Atmospheric refraction at this
    altitude increases to roughly 0.2 degrees.  For either mode, when
    the highest point of the refracted sun (or moon, if computing moon
    illumination) reaches the designated tangent height, I consider the
    satellite to be eclipsed.
    
    --Rob
    
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