Fobos-Grunt: decay estimate

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Fri Dec 30 2011 - 05:54:45 UTC

  • Next message: Bob Christy: "Pocketsat3"

    The mean rate of decay of the semi-major axis of the twenty-eight (28) USSTRATCOM TLEs from epoch 11358.77915233 to
    11363.71577446 (2011 Dec 24 18:42 - Dec 29 17:11 UTC) was 1.7623 km/d. 
    
    Using the STOAG propagator and actual space weather during that period, I found that the mean rate of decay could be
    accounted for using A/m = 0.0012054 mē/kg and Cd = 2.2.
    
    Using those values with predicted space weather, STOAG propagates the orbit to decay on 2012 Jan 15. Estimated
    uncertainty is +/- 4 days, based on the rule of thumb of 20 percent of the time remaining to decay.
    
    I noticed a small error in Celestrak's recent compiled values of 10.7 cm solar flux, such that observed values were
    taken to be 1 A.U.-adjusted values, from which erroneous observed values were estimated. It is too soon for the site to
    have responded to my alert, but assuming my finding is confirmed, and that I have not erred in my corrections, then the
    A/m values I have reported to-date have been nearly 3 percent too low. That is much less than the cumulative effect of
    the uncertainties in modelling of atmospheric density, orbit propagation, and space weather forecasting; therefore, it
    did not materially affect my decay estimates. This report reflects my corrections of the data in question.
    
    Below is the plot of estimated A/m since the orbit manoeuvres ceased, through Dec 27. Most of the points are at ~2 day
    intervals, and typically span the preceding ~2 day period. For Cd = 2.2, the mean A/m was 0.0012875 mē/kg; those values,
    with predicted space weather, would advance the estimated date of decay by about one day, to Jan 14.
    
    http://satobs.org/seesat_ref/phsrm/Fobos-Grunt_area_to_mass_ratio_evolution_v5.pdf
    
    I extracted the actual and predicted space weather data required by STOAG from data provided by Celestrak.com, which
    compiles data issued by official sources (mainly NOAA) and presents it in a convenient format:
    
    http://celestrak.com/SpaceData
    
    http://celestrak.com/SpaceData/SpaceWx-format.asp
    
    Ted Molczan
    
     
    
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