**Previous message:**Marco Langbroek: "Re: one of the pair NOSS 3-1 AC is missing"**In reply to:**Ted Molczan: "Fobos-Grunt: analysis of orbit evolution"**Next in thread:**Paul Salanitri: "Re: Fobos-Grunt: analysis of orbit evolution"**Reply:**Paul Salanitri: "Re: Fobos-Grunt: analysis of orbit evolution"**Reply:**Chris Jones: "Re: Fobos-Grunt: analysis of orbit evolution"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

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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