Mike McCants wrote: > Of course our (Alan and me) past discussions were concerned with > decaying objects. The question was: does the elset of the decaying > object (almost always, but not always, at its south->north equator > crossing) represent observations made subsequent to that time > or could they possibly represent an extrapolation into the future? > I believe that our conclusion was that they are never extrapolations > into the future. > Only NORAD knows. :-) Not exactly... there's some unclassified stuff published on what they do. Tracking stations measure the early/late error and the off-track (plane) error compared to predictions made from their own ephemeris software, not elsets. If the errors are acceptable, the satellite is considered "correlated." If the errors are too large or the object is completely unknown, it is considered an uncorrelated target (UCT). Once a UCT is found, ground stations attempt to obtain tracking data for a minimum of 5.5 percent of one orbital period. This is their rule of thumb for the minimum data needed to generate a good set of elements. Correlated objects are always observed from multiple ground stations to sample different parts of the orbit. The observed track is far less than is done for UCT's. The mulitiple observations from multiple stations are used to correct the previous element set using differential corrections. These working 'elements' are state vectors plus perturbation terms. Two-line mean elsets are made after the state vector elements are produced. The only exception to the use of multiple ground stations is for objects near decay. Then they use what observations they can get (the Austin landfall object being an example). For most objects in the catalog there is no correlation between the 2-line elset epoch and any given ground observation. In a sense you could say that most elsets are 'predicted' because the elset position at epoch is never the raw observed position. The near decay objects appear to be highly correlated to ground observations only because one or two stations are contributing to the analysis. Jim Varney jamesv@softcom.net