Ralph McConahy asked: > I plugged the STS-116 nominal (ISS In Plane) launch times > from Spaceflightnow.com into a spreadsheet, then created a > column that displayed how much time earlier each day the > launch will occur compared to the previous day (see below). > Comparing the day by day "Earlier by" times, it is > interesting how it is equal every other day, with a +/- > ~3m10s alternating from one day to the next. Could someone > explain why the delta-time jumps like this instead of a > constant change from day to day? If lift-off always occurred at the time when the launch pad rotated through the plane of ISS's orbit (or at some fixed time relative to that), then the time of lift-off on successive days would change at a nearly constant rate, determined by the rate of nodal precession of the target orbit - about 24m08s per day earlier in the case of ISS's current orbit. As Ralph has found, the daily rate of change of shuttle launch time to ISS deviates from 24m08s. Primarily, this is to allow for variations in the angular separation between the shuttle and ISS, within the orbital plane at the time of launch. Several years ago I discovered that the difference between the coplanar time (i.e. the time of passage of the launch pad through ISS's plane) and the preferred time of lift-off, is a linear function of ISS's argument of latitude at the coplanar time. Argument of latitude is the angular distance measured within the orbit plane from the ascending node to the object; mathematically, it is the sum of the true anomaly and the argument of perigee. For 13 of the 16 launches beginning with STS 86, I found an almost perfect linear relationship (r^2 = 0.99924), resulting in the following regression line: offset = 5.6783 - 0.00906283 * U, where offset = coplanar time - preferred lift-off time, minutes U = ISS's argument of latitude at coplanar time, deg Here is a small program which uses this formula to estimate STS to ISS launch times, typically achieving accuracy to within about 10 s: http://satobs.org/seesat_ref/Shuttle/STSLnch.zip Using recent ISS elements issued by NASA/JSC: 1 25544U 98067A 06342.53518261 .00020000 00000-0 20000-3 0 9005 2 25544 51.6341 8.6043 0023926 311.5507 48.3605 15.75545710 20678 the program estimates the following launch times for Pas 39B: Launch Coplanar Lift-off Date UTC UTC ----------------------------------- 2006 Dec 10 01:52:07 01:47:30 2006 Dec 11 01:27:59 01:25:00 2006 Dec 12 01:03:51 00:59:15 2006 Dec 13 00:39:43 00:36:45 2006 Dec 14 00:15:35 00:11:00 2006 Dec 14 23:51:26 23:48:30 2006 Dec 15 23:27:18 23:22:46 2006 Dec 16 23:03:09 23:00:16 2006 Dec 17 22:39:00 22:34:31 2006 Dec 18 22:14:51 22:12:01 2006 Dec 19 21:50:42 21:46:16 Ted Molczan ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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