Re: ISS occulting.../precession

Date: Thu Dec 04 2003 - 14:23:54 EST

  • Next message: Thomas Fly: "Re: Nodal regression"

    Thomas Fly asks:
    "So, what would account for such a huge precession of the plane of orbit?"
    The oblateness of the Earth. Think of it this way: the Earth is wearing a belt equal in thickness to about 1/300 of its diameter. When a satellite is headed south towards the equator from a middle northern latitude, it experiences an extra force towards the equator from this belt. That pulls it towards the equator so it crosses a little ahead of schedule (compared to the fixed orbit you would have around a spherical Earth). On the return pass, it experiences another force towards the equator so again it crosses early. Orbit after orbit, the crossing point shifts back. The net effect is a slow steady regression of the orbital plane.
    Most orbit calculations obsess a little much over the 'fifth decimal place' and SGP4 and all that (and for some calculations, you really need that). You can get excellent satellite position predictions by including only three effects: precession of the node, precession of the perigee, and decay of the semimajor axis. You can get positions accurate to a few tenths of a degree with about fifteen lines of code (for the calculation itself --more for file reading and so on). Errors increase as you get farther from the epoch of the orbital elements, but that doesn't matter so much anymore since we can always get fresh elements.
    Frank E. Reed
    75% Mystic, Connecticut
    25% Chicago, Illinois
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