RE: Fobos-Grunt: no manoeuvring evident in TLEs after 2011 Nov 20 UTC

From: Ted Molczan (
Date: Tue Nov 29 2011 - 13:07:08 UTC

  • Next message: Robert Holdsworth: "RE: Fobos-Grunt: no manoeuvring evident in TLEs after 2011 Nov 20 UTC"

    Manoeuvring was last clearly evident in the USSTRATCOM TLE of epoch 11324.94169617. It appeared to have ended by the
    epoch 11325.19110916 TLE, and appeared not to have resumed as of the epoch 11333.12745729 TLE (2011 Nov 29 03:04 UTC). 
    Orbit evolution for the latest 5-day period (11328.24537235 - 11333.12745729):
    Assuming solar activity remains near the low end of normal, STOAG propagates the orbit to decay on 2012 Jan 16.
    Uncertainty +/- 10 days, based on the rule of thumb of 20% of time remaining until predicted decay.
    I have been watching some recent small variations between the TLEs and theory, that are too small for me to attribute to
    manoeuvring, but perhaps worth noting. For example, the unusually high rate of precession of the argument of perigee was
    one of the manifestations of the manoeuvres. The theoretical rate, per the SGP4 model was about 4.05 deg/d, but during
    the manoeuvres it averaged ~5.21 deg/d, as shown on the following plot:
    The red triangles are derived from the TLEs, calculated as the mean rate of precession between pairs of TLEs about 2
    days apart. Shorter intervals yielded similar, but more scattered (noisy) results. This clear evidence of manoeuvres
    ended late on Nov 20 or early Nov 21 UTC, and was fully reflected in the 2-day average by Nov 23 UTC. For the next few
    days, the 2-day mean rate of precession was nearly identical to the theoretical rate, but then it began to rise on Nov
    26, reached a peak of 4.39 deg/d the following decay, then declined, and returned to nearly the theoretical rate as of
    the latest TLE. The effect appears to have been real, but I suspect it was more likely due to a gradual correction of
    the elements than a manoeuvre. During this period, the rate of orbital decay decreased slightly, and the rate of
    decrease of the perigee distance was slightly less than predicted by STOAG for the same mean rate of decay, but these
    effects appear to be normal variations.
    Ted Molczan
    Seesat-l mailing list

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