Re: Geodetic model used by SGP4

From: Thomas Fly (
Date: Mon Jan 05 2004 - 11:48:35 EST

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    Hi Arnold,
    Thanks for the information. The solar panels on the ISS are about 78 meters
    across, so apparently one can ignore the difference between WGS'72 and WGS'84
    (except maybe for cruise missile "applications").
    Given my somewhat limited data, I've found that the MCC data (up to about 3 days
    since the latest update) tends to give better positional accuracy than OIG,
    though the timing accuracy of 3-day old MCC predictions may be off by several
    seconds- while the current OIG timing will be a second or better.
    Certainly one advantage of using the MCC data is that, under normal conditions,
    you're forewarned of any manuevers, such as the one scheduled for January 8,
    20:03:51 UTC.
    ----- Original Message ----- 
    From: "Arnold Barmettler"
    To: "Thomas Fly"
    Sent: Monday, January 05, 2004 9:46 AM
    Subject: AW: Geodetic model used by SGP4
    Dear Thomas,
    the numerical difference between coordinates in WGS84 and WGS72 is in the
    order of 20 meters for the tropics and and goes down to about 5 meters over
    the poles (checked up to 1000 km altitude, the altitude difference is below
    1 meter).
    From my work with remote sensing satellites, I think that none of the
    published TLEs are that accurate... Hence I do not perform a datum shift to
    WGS84 for the satellite calculations within CalSKY (it involves also quite a
    few trigonometrical functions and multiplications), but assume the state
    vectors as WGS84.
    BTW, I wasn't sure on the quality of the TLEs from MCC versus those from
    OIG. After John's Mars odyssey last December I know that the MCC data is
    virtually useless for the final transit line calculation. Thereafter, I
    changed my ISS TLE update scheme: I update them from OIG every 3 hours. The
    last set is kept valid for 24 hours, afterwards the remaining TLEs from MCC
    are used for longer-term predictions. Hence future orbit boosts are still
    accounted for, but the short-term accuracy is much better.
    Best regards
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