I received the following information from Curtis Haase, which I've summarized below. When I get a little time, I'll burrow into the internal workings of my own transit track computation and dig out the critical intermediate results (e.g., the ECI position of the ISS at 10:09:17 UTC), and post them for anyone who may find them useful, or can find any significant error in them. In the particular case of the simultaneous ISS / Venus transit, as observed by Tomás Maruska, failing to use UT1 in place of UTC would not make much difference in the predicted track- which was essentially west-to-east across the disk of the Sun- but instead would manifest itself as a small timing error (were the TLE, used for the prediction, itself accurate). ------------------------------------------------ In the transformation from an earth-fixed reference frame to a celestial reference frame, the proper "time" to use is UT1. (see page 162 of EXPLANATORY SUPPLEMENT TO THE ASTRONOMICAL ALMANAC, ed. by P. Seidelmann, 1992 -- this is the authoritative reference. The Astrodynamics text by Vallado is also very useful). The current value of UT1-UTC can be found in the IERS Bulletin A from the USNO: ftp://maia.usno.navy.mil/ser7/ser7.dat The current value of UT1-UTC is about -0.47 sec, so as you guessed, this makes a considerable difference in the position of the observer in a space-based frame. For an online reference on the coordinate transformations see http://maia.usno.navy.mil/conv2003.html Chapter 5 has the discussion of the transforms. Bottom line: for precise work involving earth-based observer, satellite, and planet, you should use: -- UTC for the satellite state vector -- UT1 to calculate the earth's rotation angle. Specifically, use UT1 to calculate Greenwhich Mean Sidereal Time (GMST); then add the equation of the equinoxes (EQEQ) to GMST to get Greenwhich Apparent Sidereal Time (GAST): GAST = GMST + EQEQ GAST is the "angle from Greenwhich" that is used the coordinate transformation. -- Dynamical time (i.e., apply the "delta T") for planetary positions. ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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