Hello all, Bruno Tilgner inquired about the comparative accuracies of propagation models when applied to geostationary satellites -- in particular the failure of said model(s) to reproduce the observed relative and absolute motions of satellites in the ASTRA cluster located at 19.2 E longitude. He went on to say, "Since I suppose virtually all software uses the same NORAD model as the prediction engine, I begin to have serious doubts about the applicability of the SDP4 model to geostationary satellites." As Bruno knows, very few programs actually have SDP4 coded. And any programs that have coded SDP4 using the source code in the widely available Spacetrack Report #3 are flawed. Note that the equations there are not flawed -- just the coding. I corrected these coding mistakes back in March of this year, incorporating them in SkyMap 5.11. At that time, SkyMap became the first publically available program with SDP4 coded as originally intended by the designer. In March and April I exchanged hundreds of messages with a number of individuals explaining how to "fix" SDP4. As far as I know, only Ed Wright of JPL had sufficient perseverance to make all the necessary changes, and his results are now compatible with those of SkyMap. My notes also indicate that Bruno, Rainer Kracht, Jim Varney and Curtis Haase made, at the very least, partial fixes that eliminated a rarely-occurring discontinuity caused by a modulo-2 pi problem -- a problem which we've referred to as the "Lyddane bug". (As an historical footprint, I'd like to mention that it was Rainer who discovered this discontinuity and traced it to the Lyddane modification section of SDP4.) But unless the solar-lunar offsets are handled correctly, SDP4 will not work as the author originally intended. If anyone wants to know how to fix the SDP4 source code, let me know. The Lyddane fix is easy; the solar-lunar offset problem is a bit more involved, but it is the more important fix of the two since it affects the results of all high-altitude satellites. Finally, as a side note I'd like to mention that at least one other organization created its own corrected version of SDP4 -- Goddard Space Flight Center. Their Lyddane correction is essentially identical to my own, and their S-L offset correction solution produces the same results as mine. David Irwin did a great bit of sleuthing back in April and located Goddard's source code on the web: http://seawifs.gsfc.nasa.gov/SEAWIFS/SOFTWARE/src/bobdays/sgp4sub.f I haven't confirmed that this link is still accurate; if it isn't, I still have this file on my computer somewhere. --Rob