Updated FIA Radar 3 and KeyHole elements

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Mon Dec 09 2013 - 18:25:11 UTC

  • Next message: Ted Molczan: "RE: Updated FIA Radar 3 and KeyHole elements"

    This TLE is based on observations by Greg Roberts, Scott Tilley and myself.
    FIA Radar 3                                            1072 X 1088 km
    1 78817U 13072A   13342.79479970  .00000000  00000-0  00000-0 0    02
    2 78817 123.0084 232.6376 0010697 300.1695  59.8209 13.47777025    03
    Arc 20131206.76-1208.84 WRMS resid 0.021 totl 0.014 xtrk
    As the elements settle down to reliable values, it is interesting to compare the RAAN (right-ascension of ascending
    node) of the three FIA Radars in orbit. The following table states the RAAN at the epoch of the above TLE. Those of the
    other two s/c are computed from Mike McCants' most recent TLEs.
     FIA                    RAAN   Precession
    Radar   Desig  Cat #    deg      deg/d   
      1    10046A  37162  143.2120  3.10287
      3    13072A  78817  232.6376  3.13830
      2    12014A  38109  323.2252  3.10307
    As has long been noted, FIA Radars 1 and 2 are almost exactly 180 deg apart in RAAN. A few weeks prior to launch, Cees
    Bassa speculated that the RAAN of Radar 3 might exactly bisect those of the other two, on one side or the other. When
    the time of launch was released, that possibility became real. Based on the time of lift-off, and using the initial
    elements of the first two s/c as a guide, Radar 3's RAAN would have been roughly 85 deg greater (i.e. east) of Radar
    1's, and 95 deg less (i.e. west) than Radar 2's. Due to apparent yaw-steering by the Atlas V-501 launch vehicle, the
    initial bisection proved much closer to an even split: 89.426 deg east of Radar 3 and 90.588 deg west of Radar 2, per
    the above table. If the intent was an even split, that might not be a terrible result, but it is far less than the
    precision demonstrated by rockets for decades. Did the Atlas go off slightly off course? Was an even bisection not
    required? The answer to both probably is no.
    Radar 3 has entered a slightly lower orbit than its final orbit. Radars 1 and 2 did likewise, and then performed small
    manoeuvres to reach their final altitude several weeks after launch. Radar 3 no doubt will do likewise, but while it
    remains in its temporary lower orbit, its RAAN is precessing slightly faster to the east than those of Radars 1 and 2.
    The rate of RAAN precession is in part a function of orbital altitude - more precisely, of the semi-major axis.
    Differential rates of precession are commonly used to "rendezvous" with a target orbital plane, whether rendezvousing
    with another spacecraft, or fulfilling the orbital design requirements of a constellation of spacecraft.
    The precession rates are in the right-most column above. Radars 1 and 2 both precess at just about 3.103 deg/d east;
    Radar 3 at about 3.138 deg/d east. Radar 3 precesses faster that its sisters, at the rate of about 0.035 deg/d east.
    That difference is sufficient to increase the RAAN separation between Radar 1 and 3 to exactly 90 deg within (90 -
    89.4256) / 0.035 = 16.4 days from the epoch of the above TLE, or day 359.2062 = Dec 25 near 05 h UTC. That would be
    about 19 days after launch. Since its orbit is likely to be raised in small steps, or even temporarily lowered, the
    exact date to reach 90 deg separation is uncertain, but probably will be later, by perhaps one or two weeks.
    The Centaur
    Björn Gimle and Greg Roberts reported new observations of FIA Radar 3's Centaur stage. I had some difficultly with the
    residuals of Björn's data. Both seem a few seconds off in time, and the first one is considerably off in track. The
    second one may well prove fully reliable, but Björn reported some difficultly with the timing due to the cold weather,
    so I set it aside, preferring Greg's, Scott Tilley's and my earlier observations and Greg's later ones. It is possible
    that the orbit has changed in a way that makes Björn's observations seem less accurate than they are, and there is some
    evidence for that, in that the residuals of the earliest observations appear to have degraded somewhat as newer ones
    have been made. I suspect the cause is perturbations due to continuing venting by the Centaur stage. We have seen other
    Centaurs continue to vent propellants for a few days after launch, e.g. that of the NOSS 3-6 launch (12048N / 38770).
    Using the selected observations, I obtained the following result:
    FIA Radar 3 r                                            488 X 890 km
    1 78820U 13072P   13342.81287990 -.00006326  00000-0 -71422-3 0    06
    2 78820 120.4471 235.7285 0284836 340.6508  18.3910 14.61048186    06
    Arc 20131207.46-1208.86 WRMS resid 0.023 totl 0.017 xtrk
    The small negative decay term is consistent with the suspected venting (positive decay terms can also result). Greg
    alerted me to a photo of the main venting event (fuel dump), taken from Australia on 2013 Dec 06 near 11:00 UTC, not
    long after the secondary payloads were deployed. I have found another on the net, and hope to find more: 
    Analyzing such photos may shed further light on the evolution of the orbit, provided the time and location of the photos
    can be determined with sufficient confidence. Fuel dumps exert a small thrust that alters the orbit somewhat.
    Finally, I offer updated elements of two KeyHoles recently recovered by Greg Roberts.
    USA 129                                                  307 X 749 km
    1 24680U 96072A   13342.76255084  .00015082  00000-0  24218-3 0    03
    2 24680  97.5547  33.8311 0320001 354.8452   4.9507 15.11744450    02
    Arc 20131204.85-1208.82 WRMS resid 0.008 totl 0.007 xtrk
    USA 245                                                  271 X 982 km
    1 39232U 13043A   13342.83426663  .00028645  00000-0  33250-3 0    05
    2 39232  97.8249  44.7022 0507521 220.0704 136.2029 14.79901441    07
    Arc 20131206.87-1208.89 WRMS resid 0.013 totl 0.007 xtrk
    The decay terms are guesses. USA 245 appears to have raised its perigee about 14 km since it was last observed, in
    September, but that needs to be confirmed by additional observations.
    Ted Molczan
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