Greg posted: >HIGHFLY: > Time alt azi > 23:40 37 260 > >GUIDE 8 (SDP4) >Date Alt Azim >20 Mar 23:40 39.8 265.2 Note that although these two predictions differ by 3 degrees in altitude and 5 degress in azimuth, this difference is really a difference of about 50 seconds in the time of prediction. In other words, a Highfly prediction for 50 seconds later would match the Guide prediction or a Guide prediction for 50 seconds earlier would match the Highfly prediction. I loaded SkyMap 6.6 onto my work computer and verified that its SDP4 prediction for 21:40 UT on March 20 for Greg matched this Guide prediction. Then I used an option for SkyMap and changed the prediction model from SDP4 to SGP. Then I verified that the SkyMap prediction matched the Highfly prediction. So my conclusion is that SGP (also SGP4) differ from SDP4 in this case by about 50 seconds in time. Since this object passes by the same place in the sky at nearly the same time every night, it was easy enough for me to change the date of prediction from 3/20 back to 3/12. This allowed me to compare SGP and SDP4 for an time very near the epoch of the elset. The 50 second difference between the two models remained the same. So it appears to be a fundamental discrepancy between the two models. I have never used SDP4 - I always use SGP or SGP4 - even for the objects that are higher than the SDP4 limit. The particular elset for 90027 that Greg used was an elset that I generated with my SGP4 programs. But if Spacecom was using SDP4 to generate elsets for eccentric objects and if a 50 second discrepancy between Spacecom elsets and the model in Highfly was to be expected, you would think that I would have noticed this some time in the last 15 years. :-) Mike McCants ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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