Extracting Long/Lat from TLE's

From: Bruno Tilgner (Bruno_Tilgner@compuserve.com)
Date: Sun May 27 2001 - 13:36:50 PDT

  • Next message: Allen Thomson: "Re: Extracting Long/Lat from TLE's"

    Anthony Ayiomamitis wrote:
    >      How would one go about extracting a satellite's longitude and
    >latitude from the TLE's? I realize that this will be a function of time
    >but my question is restricted to geosats and, therefore, time as a
    >confounding variable should be immaterial. I realize that geosats
    >experience very small perturbations in their orbits but this is
    >something I can live with (for now).
    Don't underestimate the motion of so-called "geostationaries". If they
    are not exactly at the right orbital altitude they will drift more or
    or less quickly eastward or westward. However this is not the case for
    operational satellites. However, many operational satellites have
    significant orbital inclinations, so they will perform small loops
    accordingly. You either have to filter these satellites out or you must
    take time into account so as not to be off-target by several degrees.
    >      I am interested in writing some software to basically parse a
    >geosat TLE file and produce a list of corresponding longitudes and
    >latitudes. I would like to produce results, if possible, accurate to
    >within one and, preferably, two decimal places.
    This software exists already and is almost exclusively based on the
    NORAD SGP4/SDP4 algorithms. Whilst the SGP4 algorithm, applicable
    to LEO objects, yields very accurate results, SDP4 is flawed. It
    cannot predict accurately the daily loops. But in general it is
    capable of predicting the location of a geosat so that it can be
    found with a telescope.
    >      I am setting the stage for a mini-project involving the
    >photography of geosat constellations and would like to be able to
    >generate the above-mentioned results at a whim's notice so as to plan my
    >photo sessions and photo composition/framing.
    If I interpret your e-mail address correctly, you are located in Greece.
    You will be able to see two ASTRA and one HOTBIRD constellation.
    For your project you are not so much interested in longitude/latitude
    but in azimuth/elevation. If you want to get an impression of what can
    be done photographing geosats with a CCD camera through a telescope,
    I suggest you have a look at http://www.astroinfo.ch/aida/sposetti.
    I have made all the calculations necessary to obtain these photos and
    to interpret them.
    Bruno Tilgner
    Saint-Cloud, France
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