Re: Longterm Propagation of Geostationary Satellites

From: Mike McCants (mmccants@io.com)
Date: Fri Jan 19 2007 - 14:08:50 EST

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    On Jan. 16, Gerhard Holtkamp posted:
    
    >In other words SDP4
    >represents the effects of the geopotential more accurately than SGP4.
    
    Yes.  SDP4 includes the effects of the longitude-dependent harmonics
    in the Earth's gravitational field and the 12-hour and 6-hour harmonics
    will change the mean motion for drifting near-geosynchronous objects.
    SGP4 assumes a constant mean motion and that assumption is usually not
    correct for such drifting objects.
    
    >Of course it doesn't know about the frequent longitude station keeping
    >maneuvers performed on operational satellites that keep them at their
    >nominal positions and that's why predictions a few months into the future
    >will go wrong.
    
    Yes.  For objects that perform station-keeping, the assumption that
    the mean motion is constant is correct.  So SDP4 is using an incorrect
    assumption.  To propagate an old elset for such an object to a current
    epoch, you can use my "int2" program (which assumes that the mean motion
    is constant if the drag term is zero).
    
    >With regard to the Classified list however you should use SGP4 rather than
    >SDP4 as to the best of my knowledge those elements are generated with the SGP
    >model (correct me if I'm wrong).
    
    I generate elements for drifting classified objects using the "int3"
    numerical integration program.  So the correct procedure is to use that
    program to propagate an old elset to the current date and then use any
    prediction program to generate predictions.  Since the prediction time
    will be very near the epoch of this propagated elset, it does not matter
    which algorithm you use to generate the prediction.
    
    However, I have now converted the Fortran and C++ versions of the new
    "fixed" "combined" SGP4/SDP4 code into subroutines that I can use and
    I have created a new version of "FitElem" that uses SDP4 for such objects
    and it is very likely that I will switch to that version in the near future
    and that means that the elsets that I generate for such drifting objects
    will be compatible with SDP4.
    
    Mike McCants
    
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