Fobos-Grunt: decay estimate

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Sat Jan 07 2012 - 19:22:37 UTC

  • Next message: alberto rango: "4542 SATOBS 07 JAN 2012."

    Today's STOAG-based decay estimates are later than yesterday's by about one day, primarily due to a significant downward
    revision of the forecast solar flux for the coming days, per the USAF's latest forecast, issued 2012 Jan 06 21:00 UTC:
    
    http://www.swpc.noaa.gov/ftpdir/latest/45DF.txt
    
    The forecasts are updated at least daily, at about the same time of day. The forecast values may be stable for several
    days, then suddenly change significantly, much like terrestrial weather forecasts.
    
    1. Main Estimate
    
    The mean rate of decay of the semi-major axis of the twenty-nine (29) USSTRATCOM TLEs from epoch 12002.53447468 to
    12007.51163696 (2012 Jan 02 12:50 - Jan 07 12:17 UTC) was 2.7011 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 = 0.0012592 mē/kg and Cd = 2.2.
    
    Using those values with predicted space weather, STOAG propagates the orbit to decay on 2012 Jan 16 about 16 h UTC.
    Estimated uncertainty is +/- 2 days, based on the rule of thumb of 20 percent of the time remaining to decay.
    
    Since the decay is drawing closer, I have begun to include the approximate time of day of the estimated decay, but that
    is only to help reveal trends. My estimated uncertainty is still a couple of days, and the exact hour of decay will only
    be known with certainty a few hours before the fact. It should be noted that STOAG propagations are output at intervals
    of no less than one day, and terminate when the semi-major axis falls below 140 km; therefore, I have been using Satevo
    to estimate the time remaining to decay after the end of the STOAG data.
    
    2. Alternative Estimates
    
    I have produced a few alternative decay estimates to demonstrate their sensitivity to methods and input data.
    
    Reducing the span of the historical TLE basis for A/m to three days (epoch 12004.50254709 to 12007.51163696), during
    which the rate of decay of the semi-major axis was 2.8060 km/d, reveals A/m = 0.0012123 mē/kg. STOAG propagates the
    orbit to decay on 2012 Jan 16 about 23 h UTC.
    
    Below is the plot of A/m estimated from historical orbital and space weather data since the orbit manoeuvres ceased,
    through Jan 06. Most of the points are at ~2 day intervals, and typically span the preceding ~2 day period. For this
    update, I recalculated all previous A/m estimates to account for the recently released final kp index values for Nov
    2011, and changes/corrections to various other recent data. For Cd = 2.2, the mean A/m was 0.0012494 mē/kg; with those
    values and predicted space weather, STOAG propagates the orbit to decay on Jan 16 near 18 h UTC.
    
    http://satobs.org/seesat_ref/phsrm/Fobos-Grunt_area_to_mass_ratio_evolution_v10.pdf
    
    The above plot of A/m reveals no obvious long-term trend. Some of the apparent variation in A/m is due to imperfections
    in the orbital elements, orbital model and atmospheric density model; some of it may be due to actual changes in the
    object's orientation with respect to the velocity vector. In making decay estimates, the unpredictability of space
    weather (which is the basis of most atmospheric density models) adds to the uncertainty.
    
    I also estimated the decay using Alan Pickup's Satana and Satevo programs in combination, which yielded Jan 16 near 12 h
    UTC, based on the same span of historical orbital elements and 10.7 cm solar flux (mean = 136) as my main forecast.
    Unlike STOAG and Harro Zimmer's special perturbations propagator, Satevo does not employ forecast space weather, so its
    accuracy depends on the space weather not changing. Space weather cannot be forecast with great precision, so this is
    not as great a disadvantage in practice as it is in theory.
    
    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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