Re: Geodetic model used by SGP4

From: Thomas Fly (tfly@alumni.caltech.edu)
Date: Sun Jan 04 2004 - 10:57:30 EST

  • Next message: Alan Pickup: "Decay watch: 2003 Jan 4"

    Thanks Kevin, but...
    
    > WGS72 was the basis of the norad models we use.
    
    I (and CalSKY, apparently) use NASA Mission Control Center TLEs for the ISS (and
    shuttle, when it starts flying again). Evidently, the MCC uses numerical
    integration of highly accurate gravitational & atmospheric drag models, then
    "dumbs it down" to SGP4 model TLEs, so that programs that implement SGP4 can
    simulate the accuracy of NASA's numerical integration.
    
    The question is, "Does anybody know whether these state vectors (and for that
    matter, OIG TLEs) are tweaked for the WGS '72 geoid, or WGS '84?" (The slight
    difference between the 2 models will have an effect on the rate of orbital
    precession.)
    
    > WGS 84 is used by gps.
    
    I'd assume that most modern mapping programs (e.g., Microsoft Streets & Trips,
    or DeLorme StreetFinder) likewise use WGS '84, so it'd be desirable (if possibly
    overkill) to use WGS '84 when computing a transit track.
    
    It seems to me that, if TLEs still reference the WGS '72, one could nonetheless
    employ WGS '84 to compute the transit track, after using SGP4 (and implicitly
    WGS '72) to compute the position of the ISS.
    
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