Hi Arnold, > that's too much of an error. My point exactly. Also, I'm not sure that I understand it, unless it's just that I never tried before to compare my results with yours, to this level of accuracy. Particularly interesting is the fact that your results suggest that the MCC TLE was extremely accurate in this case. > deltaT = ET-UT = 64.5 sec I calculate deltaT in my WorldView program, but at present, I don't actually do anything with it. Rob Matson's SkyMap program gives a value of 67.74 sec for deltaT. deltaT, of course, should be used to adjust UTC in the computation of the position of the Sun & Moon, etc., whereas (unadjusted) UTC should be used for the SGP4 computation. > 208m above WGS84 ellipsoid (164m above sea level) There's not a huge amount of difference between 164 and 208 meters (44 meters * cos(63.2)= 20 meters or ~ 1/6 Venus perpendicular to the line of sight), but 208 meters was what I used in SkyMap. > Are you also using topocentic coordinates for Sun and Venus? I'm currently using DE404, and may include DE406 soon (difference in position I think will be smaller than 1")... My WorldView program computes the ground track from the ECI position of the ISS (from the SGP4 computation) and the ECI position of Venus (in this case). For the computation of the positions astronomical bodies, I use Mark Huss' Java AstroLib, which is a translation from Bill Gray's Project Pluto C code, and is based upon VSOP. In a test of accuracy based upon the most recent total eclipse of the Sun by the Moon, I have high confidence in its results. I don't know the internal workings of SkyMap, but SkyMap generally confirms my computed ground track to high accuracy, given the same TLE and my computed observer site & time. At the very least, the relative positions of Venus & the Sun on this date are known to very high level of accuracy, so it shouldn't be difficult to determine the source of the discrepancy. Tom ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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