Thanks Kevin, but... > WGS72 was the basis of the norad models we use. I (and CalSKY, apparently) use NASA Mission Control Center TLEs for the ISS (and shuttle, when it starts flying again). Evidently, the MCC uses numerical integration of highly accurate gravitational & atmospheric drag models, then "dumbs it down" to SGP4 model TLEs, so that programs that implement SGP4 can simulate the accuracy of NASA's numerical integration. The question is, "Does anybody know whether these state vectors (and for that matter, OIG TLEs) are tweaked for the WGS '72 geoid, or WGS '84?" (The slight difference between the 2 models will have an effect on the rate of orbital precession.) > WGS 84 is used by gps. I'd assume that most modern mapping programs (e.g., Microsoft Streets & Trips, or DeLorme StreetFinder) likewise use WGS '84, so it'd be desirable (if possibly overkill) to use WGS '84 when computing a transit track. It seems to me that, if TLEs still reference the WGS '72, one could nonetheless employ WGS '84 to compute the transit track, after using SGP4 (and implicitly WGS '72) to compute the position of the ISS. ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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