RE: ISS orbit adjust ?

From: Ted Molczan (seesat@rogers.com)
Date: Wed Aug 03 2005 - 23:21:18 EDT

  • Next message: Leo Barhorst: "LB obs 2005 Aug 3"

    Gerhard HOLTKAMP wrote:
    
    > They seem to boost their orbit in small increaments on a daily basis
    > as long as the Shuttle has enough fuel to spare. This proves 
    > frustrating for people like me who try to figure out how the chances
    > are to see a sun transit on the weekend when the ISS and Discovery
    > are about to separate
    
    NASA's current set of predicted elements now appear to take into account these
    small boosts, as evidenced by the seemingly erratic evolution of mean motion of
    successive TLEs: some days increasing, other days decreasing.
    
    Each TLE yields an accurate position near its epoch, but we need a way to
    account for manoeuvres between successive TLEs. 
    
    As Jim Oberg reported (http://www.msnbc.msn.com/id/8814642), the unusual boosts
    are due to thruster firings, thus the small increments that you noted, Gerhard.
    This suggests a way to improve the prediction accuracy of NASA's TLEs: model the
    small boosts as a pseudo-drag term.
    
    Unfortunately, NASA's predicted elsets contain only approximate drag terms,
    which are seldom revised. Lately, for ISS they are:
    
    ndot/2 = 0.00016071 rev/d^2
    B* = 0.12697e-3 er^-1
    
    I have computed more realistic values for the appended versions of NASA's TLEs.
    
    I computed the ndot/2 of each TLE as the mean value to the subsequent TLE. For
    example, for the epoch 05214.82045700 elset:
    
    ndot/2 = 0.5 * (15.71559505 - 15.71587498) / (05215.83786716 - 05214.82045700)
    
           = -0.00013757 rev/d^2
    
    It is negative because the boosts will raise the orbit between those two epochs.
    
    Of course, ndot/2 is of no use to the vast majority of us who use the SGP4
    orbital model, which uses the B* drag term. Some years ago, I derived a formula
    which accurately relates B* and ndot/2, which I used to compute the B* in the
    appended elsets:
    
    http://www.satobs.org/columbia/SGP4decayterms.rtf
    
    As a test, I used my version of the epoch 05214.82045700 elset to compute the
    position of ISS at the epoch of the subsequent elset, and found agreement to
    within a fraction of one second (0.6 s early). NASA's B* would have resulted in
    a 2.4 s early prediction.
    
    Same test of my epoch 05216.79167367 version agreed to within 0.02 s of the
    subsequent elset's epoch. NASA's version would have been 3.2 s early.
    
    Of course, the thrusters' small boosts probably occur sporadically, and are not
    of equal magnitude, so modelling their effect as linear is imperfect, but
    hopefully most of the time it will be better than nothing.
    
    One important note of caution: since my decay terms look forward to the next
    TLE's epoch, the appended TLEs should not be used for predictions earlier than
    their epoch, except when high precision is not required.
    
    Ted Molczan
    
    The following TLEs are as downloaded from NASA.gov on 2005 Aug 03 UTC, except
    for heir decay terms, which have been modified as described above.
    
    http://spaceflight1.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orb
    it/ISS/SVPOST.html
    
    
    1 25544U 98067A   05214.82045700 -.00013757  00000-0 -10457-3 0  9005
    2 25544  51.6445  22.1174 0002239 210.6479 149.4552 15.71587498  1151
    
    1 25544U 98067A   05215.83786716  .00014800  00000-0  11263-3 0  9011
    2 25544  51.6439  16.9117 0001998 211.5799 148.5243 15.71559505  1311
    
    1 25544U 98067A   05216.79167367 -.00038477  00000-0 -29248-3 0  9023
    2 25544  51.6437  12.0311 0001901 219.3452 140.7572 15.71587737  1461
    
    1 25544U 98067A   05217.80909304  .00016540  00000-0  12615-3 0  9037
    2 25544  51.6446   6.8253 0001776 221.7200 138.3827 15.71509442  1626
    
    1 25544U 98067A   05218.82653588 -.00024030  00000-0 -18301-3 0  9044
    2 25544  51.6440   1.6203 0001787 230.1338 129.9667 15.71543100  1780
    
    1 25544U 98067A   05219.84397518  .00013715  00000-0  10467-3 0  9057
    2 25544  51.6440 356.4145 0001465 233.5800 126.5227 15.71494202  1941
    
    
    
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