Re: significantly different predictions by different programs

From: John A. Dormer 2 (jad@texas.net)
Date: Mon Oct 30 2006 - 21:02:45 EST

  • Next message: Ed Cannon: "Re: significantly different predictions by different programs"

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    Here's a link which might shed at least a little light on some
    differences in the computational models:
    
    http://www.centerforspace.com/downloads/files/pubs/AIAA%202006-6753%20Revisiting%20Spacetrack%20Report%203.pdf
    
    I received the above from a friendly lurker on the list. It was quite
    enlightening, and made me understand some of the historical issues with
    things like picking the right epoch and time system to base the
    calculations on. It's 88 pages, but the layout puts some of the most
    interesting pieces in the first six pages.
    
    There's also the subject of the specific mathematical libraries used.
    Not representing a number properly and accounting for "roundoff error"
    can result in really bogus predictions. I confronted this myself in the
    past, and if I hadn't been writing programs to work on both a Palm Pilot
    and my Linux systems, I would not have noticed the 30-arc-minute-or-so
    discrepancies in star positions. This was for celestial navigation, so
    such a huge error was intolerable to me.
    
    Regarding Heavensat, Mike McCants wrote:
    > There are no RA and Dec labels on this display.  This is confusing.
    
    Maybe a note to the authors? Anyone with energy to write and release
    such a tool would probably desire it to be "right." Or so I'd hope. :)
    
    	John
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