Ted Molczan had noted some while back that: "the OIG elset closest in epoch to that of the MCC had a slightly lower RAAN... the RAAN of subsequent OIG elsets moved toward that of the MCC elset." http://www.satobs.org/seesat/Jun-2004/0258.html Tomás Maruska's extraordinary video of the simultaneous ISS / Venus transit provided a basis for a detailed analysis of predictions based upon these 2 brands of TLEs - it was particularly fortunate that the epoch of one OIG TLE represented a time delay of only about one minute, relative the transit itself. http://www.satobs.org/seesat/Jun-2004/0313.html (subsequent, more careful measurement revealed the discrepancy to be about 1.64' of arc, rather than 2.2') I did a significant amount of analysis of Tomás' photo here: http://iss-transit.sourceforge.net/MissionAccomplished.html Using Rob Matson's SkyMap 6.6 (which confirms my current WorldView ground track), the OIG TLE "ISSd," whose epoch was nearly coincident with the transit time, predicted a path that was nearly dead-center from Tomás' GPS-measured position; the predicted ground-path center line corresponding to that TLE was about 78 meters north of the actual ground path. Not coincidentally, it turns out, CalSKY's prediction, using the appropriate MCC TLE, is virtually identical to that OIG path: http://eclipse.astronomie.info/transit/venus/isstransit/historicalimage.html The most frustrating aspect of developing my WorldView program has been collecting all the bits and pieces of arcane knowledge that are required to accurately complete the puzzle. From http://celestrak.com/columns/v04n05/index.shtml (and reiterated in http://www.stk.com/pdf/STKandSGP4/STKandSGP4.pdf ) "What is the reference frame of the resulting coordinates? "This question is a bit more technical than most we have covered. To be precise, the reference frame of the Earth-centered inertial (ECI) coordinates produced by the SGP4/SDP4 orbital model is true equator, mean equinox (TEME) of epoch. "In layman's terms, this simply means that the Cartesian coordinates produced by the SGP4/SDP4 model have their Z-axis aligned with the true (instantaneous) North pole and the X-axis aligned with the mean direction of the vernal equinox (accounting for precession but not nutation)." In fact, it appears that OIG TLEs assume True Equator / True Equinox, while MCC TLEs assume Mean Equator / Mean Equinox. Taking this into consideration, and given the small error of the resulting predictions, then even the wandering of the earths' axis becomes significant (i.e., true vs. mean equator): http://www.iers.org/iers/earth/rotation/polmot/figure1.html (1" of arc in latitude is 1/60th nautical mile, or about 100 feet) As an aside- now that WAAS-enabled GPS units compute positions allegedly accurate to a few meters- there is a question as to where the earth's poles are assumed to be! http://www.defenselink.mil/photos/Feb1996/960120-N-1259S-002.html http://www.npr.org/news/healthsci/antarctica/southpole.gallery10.html http://www.confluence.org/photo.php?visitid=808&pic=ALL Another consideration is that the MCC TLE page http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html assumes that the difference between UT1 - UTC = 0.00 seconds (i.e., UTC doesn't differ from International Atomic Time: http://tycho.usno.navy.mil/systime.html ) One thing that's been lacking, in the relatively few good opportunities provided by Tomás, John Locker, and a handful of others* to fully facilitate the analysis of transits is precision timing. But Christmas isn't far off... http://www.geocities.com/kiwi_36_nz/kiwi_osd/kiwi_osd.htm * not including myself!... evidently, I'm permanently jinxed by Murphy's Law when it comes to capturing the ISS on video! ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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