Re: Fobos-Grunt: analysis of orbit evolution

From: Paul Salanitri (paul.salanitri@gmail.com)
Date: Thu Nov 17 2011 - 13:11:58 UTC

  • Next message: Chris Jones: "Re: Fobos-Grunt: analysis of orbit evolution"

    A couple of graphics:
    
    Phobos Grunt : Gee-Wizz graph (delta of the apsides/mean)
    http://twitpic.com/7fbxv1/full
    Visibility guide for 2011-11-17 UTC  http://twitpic.com/7fc4ar/full
    
    Compare an active to the inactive Zenit 2nd stage : delta apsides
    http://twitpic.com/7ea8ne/full
    
    Paul S.
    
    On Thu, Nov 17, 2011 at 10:01 PM, Paul Salanitri
    <paul.salanitri@gmail.com>wrote:
    
    > The frustrating thing is that the Russian's know (they built it) whether
    > it is performing as expected in this contingency scenario (based on
    > observed behavior). If only we could get more words (even in Russian).
    >
    > Paul S
    >
    >
    >
    > On 17/11/2011, at 20:17, William Thompson <catbrusher@yahoo.com> wrote:
    >
    > > Is the Phobos probe itself alive and well?
    > > Maybe it doesn't "know" it's still attached to
    > > the escape stage, and is trying to hold its
    > > attitude and maneuver.
    > >
    > > --Bill Thompson
    > >
    > >
    > > ----- Original Message -----
    > >> From: Ted Molczan <ssl3molcz@rogers.com>
    > >> To: seesat-l@satobs.org
    > >> Cc:
    > >> Sent: Thursday, November 17, 2011 2:58 AM
    > >> Subject: Fobos-Grunt: analysis of orbit evolution
    > >>
    > >> T he orbit of Fobos-Grunt (11065A / 37872) continues to evolve in
    > unexpected
    > >> ways. To the previously noted low rate of
    > >> decay and possibly propulsive orbit change(s), we add the puzzling rise
    > of its
    > >> perigee altitude. These observations
    > >> relate to USSTRATCOM's TLEs, which remain the only orbital data
    > available to
    > >> the public.
    > >>
    > >> In an effort to determine whether the perigee rise could be explained by
    > >> gravitational perturbations not modeled by SGP4
    > >> (the orbital model of TLEs), I compared the actual orbital elements
    > against
    > >> those yielded by propagating the earliest
    > >> reliable TLE data using the STOAG (Semi-analytic Theory of mOtion under
    > Air drag
    > >> and Gravity) software. I propagated the
    > >> epoch 11313.39819696 TLE through day 11319, varying the area to mass
    > ratio, A/m,
    > >> by trial and error to match the actual
    > >> rate of decay of the semi-major axis.
    > >>
    > >> The value of A/m found to explain the observed rate of decay of the
    > semi-major
    > >> axis, 0.654 km/d, was 0.000634 m^2/kg,
    > >> one half the expected value of approximately 0.00129 m^2/kg, based on
    > the
    > >> object's known dimensions and mass. Despite
    > >> their forced agreement on the overall rate of decay, STOAG and the TLEs
    > differed
    > >> greatly regarding the evolution of the
    > >> perigee and apogee, as summarized:
    > >>
    > >>                    STOAG    TLEs    Diff
    > >>                    km/d     km/d    km/d
    > >> Semi-major axis   -0.654   -0.654   0.000
    > >> Mean perigee      -0.247   +0.386  +0.633
    > >> Mean apogee       -1.062   -1.694  -0.632
    > >>
    > >> It is evident that in addition to drag, some unknown force raised the
    > perigee at
    > >> the rate of +0.633 km/d, but lowered
    > >> the apogee at nearly the identical rate.
    > >>
    > >> It should be noted that running STOAG with A/m set to zero, reduced the
    > overall
    > >> rate of decay to zero, and revealed
    > >> negligible non-drag perturbations of perigee and apogee of -0.025 km/d
    > and
    > >> +0.025 km/d, respectively; therefore, the
    > >> observed evolution probably does not have a natural cause.
    > >>
    > >> The analysis also revealed that the argument of perigee is precessing
    > at more
    > >> than 1.3 times the expected rate. Since
    > >> the STOAG rate is close to the value predicted by SGP4, the observed
    > rate
    > >> probably is a side-effect of the decreasing
    > >> eccentricity.
    > >>
    > >> Comparison of the rate of precession of the RAAN revealed close
    > agreement among
    > >> STOAG, SGP4 and the observed TLE values,
    > >> which tends to confirm the uniqueness of the findings with respect to
    > the
    > >> argument of perigee.
    > >>
    > >> I have plotted the above results on two graphs, available here:
    > >>
    > >> http://satobs.org/seesat_ref/phsrm/Fobos-Grunt_orbit_evolution_v1.pdf
    > >>
    > >> The unexpected low rate of decay and the unusual perturbation affecting
    > >> eccentricity and argument of perigee probably
    > >> are manifestations of the same unknown force acting on the orbit. The
    > STOAG
    > >> model is sufficiently complete to rule out
    > >> natural forces that could have such a large effect over so short a time;
    > >> therefore, I am left with the possibility of
    > >> thruster firings or venting. The former seldom have such a large effect;
    > >> however, on an NSF forum, contributor Patchouli
    > >> wondered whether the Fregat-derived propulsion stage could be firing
    > thrusters
    > >> to settle propellants - an interesting
    > >> idea that I intend to follow up.
    > >>
    > >> I remain open to other possible explanations. I doubt that errors in
    > the TLEs
    > >> could explain all of the observed effects,
    > >> but could the perturbation be creating some side-effects in the
    > elements that
    > >> are more apparent than real?
    > >>
    > >> In closing, I caution that my experience using STOAG (and programs like
    > it) is
    > >> extremely limited. I ran a few tests to
    > >> familiarize myself with its operation and accuracy. I was impressed
    > with its
    > >> ability to accurately propagate USA 193
    > >> (06057A / 29651) from 2007 Jan 1 until just prior to its unnatural
    > demise in
    > >> 2008 Feb, using a value of A/m well within
    > >> 10 percent of what I believe to be correct. It also handled the object's
    > >> frozen orbit very well, with argument of
    > >> perigee remaining in the vicinity of 90 deg at all times. The program
    > appears
    > >> not to propagate mean anomaly, which
    > >> somewhat limits its usefulness, but it appears to be helpful in
    > evaluating
    > >> long-term perturbations of LEO orbits. A
    > >> description of STOAG, as well as source code and binaries is available
    > here:
    > >>
    > >> http://www.asu.cas.cz/~bezdek/density_therm/pohtd/
    > >>
    > >> I would be interested to learn the results of similar evaluations, using
    > >> semi-analytic and numerical models.
    > >>
    > >> Ted Molczan
    > >>
    > >>
    > >> _______________________________________________
    > >> Seesat-l mailing list
    > >> http://mailman.satobs.org/mailman/listinfo/seesat-l
    > >>
    > > _______________________________________________
    > > Seesat-l mailing list
    > > http://mailman.satobs.org/mailman/listinfo/seesat-l
    >
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