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: http://www.abc.net.au/news/2013-12-07/comet-ison-debris-night-sky-fairweather-lorne-victoria-stars/5142380 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 _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l
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