RE: Perigee & Apogee Height

From: Ted Molczan (
Date: Sat Dec 14 2002 - 17:48:23 EST

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    James wrote,
    > right in front of my nose! I have the SGP4/SDP4 orbital model 
    > code which of
    > course recovers the semimajor axis and perigee from the input 
    > elements. I
    > just change Perigee = (aodp*(1 - eo) - ae)*xkmper to (aodp*(1 + eo) -
    > ae)*xkmper in order to recover the Apogee (I think!?)
    That is what I use when I want something more accurate than the method I posted, but don't care about the perturbations due to
    Earth's oblateness.
    The value of eccentricity that appears in the tles, e0, is stated for argument of perigee of zero (regardless of the actual of
    argument of perigee), so using it, provides perigee and apogee relative the equator. 
    If the eccentricity at the epoch of the elements is required (i.e. corresponding to the actual argument of perigee), then I suggest
    calling SGP4 with the time since epoch set to zero, and taking the square root of the value ELSQ, which is computed early in the
    Short Period Preliminary Quantities section.
    Ted Molczan
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