Re: [satellitetracker] RE: Predict the satellite over my location

From: Björn Gimle (
Date: Sun Oct 13 2013 - 20:46:46 UTC

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    Two major errors in this description :
    1. The Epoch does not have to be for a northbound crossing, though it
    usually is very close ( a program named SatEvo can compute the accurate
    2. The RAAN is not longitude on Earth, but in the sky (divide by 15 to find
    RA as HH.hhhhh). To find the current East longitude of the node you must
    subtract the siderial time at Greenwich meridian at the time you can
    observe the satellite. There is also a precession of nodes (about 5 deg
    W/day for ISS)
    In the example, 24.2879 78.9001 are the longitude of the perigee from the
    node, and how long the satellite has moved from its last (or current)
    perigee position - expressed in degrees !
    So for a circular orbit, these two values add up to nearly 360, IF the
    Epoch is close to the node time.
    For a highly eccentric orbit, this is far from the truth (Kepler's equation
    of motion.
    Also, the Mean Motion is no. of perigee passes/day - due to precession of
    the perigee the number of node passes/day is slightly lower for ISS and
    other low inclination orbits, higher between 63.4 and 116.6 degrees.
    2013/10/13 <>
    > **
    > (drat...Yahoo's new layout caused me to lose a very long reply... i'll try
    > to recreate it...)
    > fatemeh: John's reply included a reference to
    > As that describes, the TLE has an Epoch, with defines when the satellite
    > crosses over the equator, heading northward.
    > The "Right Ascension of the Ascending Node" is the longitude on the
    > earth's surface where that crossing is happening... i.e. the satellite is
    > straight overhead then and there.
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