Re: GEO Sats., figure-8

From: Rick Baldridge (
Date: Wed Mar 07 2001 - 22:56:39 PST

  • Next message: Russell: "MAR7.OBS"

    > > >They're not perfectly still over one spot, but instead move in a
    figure-8 (analemma) over the course of a day.
    > > This is frequently stated but still it is wrong!
    > > Only a satellite in an orbit with an inclination of several degrees and
    a very small eccentricity would theoretically do that. I would be very
    surprised if there was any such orbit in real life!
    > >
    > > Rainer
    Geosats can make many fairly amazing tracks as viewed from the ground,
    ranging from circles to lines to sine-waves to analemmas, depending on the
    satellite's Mean Motion, Eccentricity, Inclination and Argument of Perigee.
    A satellite with a Mean Motion (revs per day) of 1.0027 is most likely
    "station-keeping", staying truly geosynchronus and coming back to the same
    Longitude on Earth once each day.  If it's inclination is small as well,
    then it's GEOSTATIONARY as well.
    Most "defunct" geosats begin taking on higher inclination orbits (due to
    Lunar perturbations) and these have Figure-8 shaped orbits when viewed from
    a fixed location if they also have a small Eccentricity.  The cross-over
    point of the Figure-8 will vary with the Argument of Perigee orbital
    Several examples of the different orbits geosats can attain are posted at:
    &res=high     GEOS 9 (circle) and DBS-1
    &res=high    High Inclination Analemma!
    &res=high    3-degree Inclin./ Low Ecc.
    &res=high    Typical High Inclination (defunct) Geosynchronus Satellite (not
    Plots are courtesy Rob Matson's SKYMAP (v6.4) Program and show several
    geosat "orbits" from a fixed location on the Earth for a 24-hour period.
    There are many other shapes possible!
    Campbell, CA  USA
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