> The TLE I used is the one from the NASA spaceflight website which I got > about 2 hours before event time: > > ISS > 1 25544U 98067A 04030.38641742 .0002000 00000-0 20000-3 0 9012 > 2 25544 51.6275 312.2703 0007708 107.1260 253.0743 15.66615930 16547 That's the Mission Control Center TLE that my prediction was based upon: http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html There is the claim, at least, that: "You also have access to a Mission Control Center, or MCC, ephemeris during space shuttle missions and for the International Space Station. An ephemeris is simply a collection of state vectors in chronological order generated in the MCC. The state vectors that are generated in the MCC are far and above the most accurate available, and their use is highly recommended." http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/Vector.html According to Richard Clark of NOAO (who's monitoring 7 sites!): "In the absence of maneuvers those predicted TLEs should be considerably better than an SGP4 based prediction. Their models are more elaborate in both a more complete Earth gravity representation, and better atmosphere models. They are a numerical integration instead of an analytic solution. As long as the mission keeps to the timeline they're pretty good. But I've only used them in the context of shuttle missions. I always forget that they also do this for ISS. I've never ground truthed them (in the absence of maneuvers) with what eventually gets release from OIG." In other words- as had been my suspicion- the MCC does a numerical integration, then "dumbs the results down" to SGP4 model TLEs, for programs like mine to use. > Interestingly enough, there was a TLE from heaven's above which was even > closer to event time. > > ISS_HA > 1 25544U 98067A 04030.84074947 .00015969 00000-0 15407-3 0 1037 > 2 25544 51.6300 309.9574 0007833 109.2562 295.0015 15.66638907296617 Most likely, that came from OIG - NASA's Orbital Information Group: http://oig1.gsfc.nasa.gov/scripts/foxweb.exe/app01? > The ISS path from this TLE was not as accurate as the > one from NASA (although the time of the transit was closer -- 21:43:40.7 vs > 21:43:40.2 -- My estimate based on WWV was that the transit occurred very > close to 21:43:40.7): That seems to confirm my own impression- that the MCC ephemeris tends to give better positional data, while the OIG TLE (which is evidently based upon radar measurements made several times each day) will give better timing accuracy. It seems to be a bit of a paradox, since the timing should have a pronounced effect upon the position of the transit path! http://www.satobs.org/seesat/Jan-2004/0263.html Arnold Barmettler- the creator & operator of CalSKY- uses the MCC ephemeris only for "long range" predictions, and the OIG data for near-term predictions. I hope to do some after-the-fact comparisons, using photos such as yours, in the next day or so, to see if I can finally resolve the issue of MCC vs. OIG accuracy, in regard to both timing & position. Tom ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Sun Feb 01 2004 - 14:19:17 EST