Latest update of ISS/Venus transit page

From: Thomas Fly (thomasfly@j2ee-consultants.com)
Date: Tue Jun 01 2004 - 21:21:15 EDT

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    http://iss-transit.sourceforge.net/IssVenusTransit.html
    
    The transit path seems to be fairly well defined at this point- the weather's a
    murkier question, as usual :-(
    
    I nearly have GTOPO30 Digital Elevation Model working in my WorldView program
    (it's been about 80% done since January!), which will allow the program to look
    up the actual elevation above sea-level (to 30" resolution in latitude &
    longitude, or ~1 km at the equator).  The transit path isn't strongly affected
    by elevation above sea-level, of course, when the angle of elevation is large.
    However, if you're shooting at a small target (e.g., a planet about the angular
    size of the ISS itself), it's desirable to minimize all the errors one can.
    
    In that regard- and for the sake of accuracy for those who'll be trying to make
    the observation- I think it'd be useful to have a discussion about all the
    factors that go into computing the ISS/Venus transit track.  So, a bit later,
    I'll be posting some of my own results, to see if anyone else can confirm or
    refute them.
    
    The final results are already posted on the page given above (computed for an
    elevation above sea-level of 820 feet / 250 meters), if anyone wishes to test
    them against this or that program.
    
    More usefully, I hope, I also intend to post some of my intermediate results,
    such as the ECI position of the ISS that I compute at some conspicuous times;
    e.g., 5:32:55 UTC on June 8, "Contact II" when the disk of Venus is first
    entirely within the disk of the Sun (as seen by a demon at the center of the
    Earth).
    
    I did a lot of work back around Christmas time to figure out the effects of ray
    bending in the Earth's atmosphere (the results are on the ISS Transit source
    code page, for anyone that may be interested)... a lot of work for not much.
    
    For the ISS at least (and even more so, for satellites in higher orbits), I
    found that ray-bending can be completely ignored at elevation angles greater
    than about 37, where it amounts to only about 6 meters (in other words,
    ray-bending at that angle of elevation has the effect of raising your elevation
    above sea-level by 6 meters, were the Earth's atmosphere suddenly to become
    non-refractive).  At the horizon & sea-level, ray bending amounts to a couple
    kilometers, however (were it not for ray-bending, the light from the "rising"
    sun would pass a couple kilometers over your head, rather than being bent down
    to your eyes).
    
    So the critical factors affecting the final accuracy (at reasonably high angles
    of elevation) boil down to 1) the position of the satellite at a given instant,
    2) the (apparent) position of the astronomical object (i.e., accounting for
    light delay), and 3) the compution of the line connecting the two, and its point
    of intersection on the real Earth's surface, which presumably will be modeled by
    the WGS84 ellipsoid (though my code still uses WGS72 constants!).
    
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