Re: ISS Transit time trials

From: Thomas Fly (tfly@alumni.caltech.edu)
Date: Sat Aug 14 2004 - 14:57:52 EDT

  • Next message: Daniel Deak: "Fast flashers TLE, August 14"

    Very nice John!
    
    I'm curious as to what mapping program you use... I've also been very impressed with the aerial color photos that are
    available to you.  In the U.S., generally all that's freely available are B&W USGS photos, such as this of my digs:
    http://terraserver.microsoft.com/image.aspx?t=1&s=10&x=2096&y=19320&z=17&w=2
    
    I'm about centered in the photo- my neighbor 2 houses to the north (up) has a swimming pool in his back yard, that's
    evident as a black rectangular, bordered by white.
    
    A somewhat interesting website that also displays USGS aerial photos of the U.S. is this:
    http://www.terrafly.com
    
    I've been replacing old wooden windows with hi-tech vinyl ones, and I've got 3 more to go, so now that the rain from
    Hurricane Charlie has ended (not much of an effect, where I am), I guess I need to get at it.
    
    It would be helpful to me, for later analysis, if you included a couple of my ground track points, on either side of
    your obs point.
    
    A------- B----- C-----  D--- E----- F--- G------- H-------- I----
      3.3 N  14 Aug 103153  39.1  124.3  341  53.4324   -3.1221   2.0
      5.3 NE 14 Aug 103154  39.2  124.4  340  53.4450   -3.0081   2.0
    
    Using decimal notation, your location was 53.4380 N, 3.0639 W, and according to my track, the ISS should have passed
    the center of the Sun at about 10:31:53 + (3.0639 - 3.0081) / (3.1221 - 3.0081) =
    10:31:53 + 0.0558 / 0.1140 = 10:31:53.49
    
    I didn't digitally dissect your gif, but just eyeballing it, that appears to be about 0.5 sec early (factoring in 235
    msec latency for your webcam).
    
    The MCC TLEs apparently are computed to allow the simplifying assumption that UT1 - UTC = 0.00 (UT1 supposedly is the
    proper time to use in the SGP4 computation), and (coincidentally or not) UTC is currently leading UT1 by about 0.5
    seconds:
    ftp://maia.usno.navy.mil/ser7/ser7.dat
    http://tycho.usno.navy.mil/systime.html
    http://www.boulder.nist.gov/timefreq/pubs/bulletin/leapsecond.htm
    http://home.tiscali.se/pausch/comp/time.html
    
    
    The day-of-year time was 227.397153, and the nearest MCC TLEs would have been:
    
    1 25544U 98067A   04226.42819699  .00020000  00000-0  20000-3 0  9012
    2 25544  51.6324  33.5657 0005255 107.0130 253.1608 15.70811150  7316
    
    1 25544U 98067A   04227.44609185  .00020000  00000-0  20000-3 0  9023
    2 25544  51.6314  28.3608 0005243 110.3838 249.7887 15.70791748  7479
    
    (My prediction actually would have used the first one.)  OIG reports:
    
    1 25544U 98067A   04227.08328711  .00017992  00000-0  14584-3 0  4409
    2 25544  51.6326  30.2103 0005561 113.4008 353.7675 15.70830660327417
    
    1 25544U 98067A   04227.53163875  .00018164  00000-0  14708-3 0  4418
    2 25544  51.6324  27.9180 0005523 115.2669   9.0526 15.70848506327485
    
    I recently received some information from Jon Weaver at the MCC, who's responsible for the software that computes the
    MCC
    TLEs.  Aside from indicating that these TLEs assume True Equator / Mean Equinox (per SGP4 gospel), he had some other
    interesting information, in reply to one of my questions:
    
    "What drives the" (usually) "Monday and Thursday updates is the ISS Trajectory Flight Controllers receive updated Orbit
    Determination vectors. The source of these is either the Russians or USSTRATCOM. They then anchor their ephemeredes on
    these fresh OD vectors and the ephemeris accuracy hangs in good enough 'operations-wise' such that two updates a week
    are sufficient.
    
    "Special updates occur when the vehicles dock with the ISS (which makes minor perturbations to the orbit), when ISS
    performs orbit adjust or collision avoidance maneuvers, etc."
    
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