RE: Question about STS-116 launch times

From: Ted Molczan (
Date: Fri Dec 08 2006 - 23:24:07 EST

  • Next message: Bob Christy: "RE: Question about STS-116 launch times"

    Ralph McConahy asked:
    > I plugged the STS-116 nominal (ISS In Plane) launch times 
    > from 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
    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:
    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
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