AEHF 1 Orbit Evolution

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Sat Aug 13 2011 - 02:36:41 UTC

  • Next message: Derek C Breit: "RE: AEHF 1 Orbit Evolution"

    It has been a while since I posted about AEHF 1 (10039A / 36868), which continues to fire its Hall Current Thrusters,
    nearly one year after the failure of its liquid apogee engine system left it far short of its intended orbit. In June,
    the USAF announced an approximately one month extension of the schedule, in the interest of fuel economy. The target
    date to arrive at the planned 4.8 deg synchronous orbit is Oct 3rd.
    
    Mike McCants has led a successful observing campaign to keep track of AEHF 1, despite its constantly changing orbit.
    Last October, when the HCT firing began, I doubted that we could keep track of the object much longer, and I recommended
    to Mike that we not expend any further effort on it, but he relished the challenge, and wanted to see how long the group
    could hang on to it.
    
    About every five days or so, Mike analyzed the recent trend in the evolution of the orbital elements, and projected them
    forward for the next five days. He circulated the estimated elements to interested observers, who attempted to observe
    as circumstances permitted. During this period of constant manoeuvring, Mike was joined by observers Derek Breit, Scott
    Campbell, Kevin Fetter, Tim Luton, Greg Roberts, Peter Wakelin and Brad Young. Mike and Peter together account for about
    two-thirds of the more than seventy successful observing sessions, but everyone's contribution was essential to success.
    
    As of the latest observations, on August 8, the orbit was 5.1 deg, 35393 km x 44091 km, with period 1643 min, per Mike's
    TLE:
    
    AEHF 1                                               35394 X 44093 km
    1 36868U 10039A   11220.17869798  .00000000  00000-0  00000-0 0    08
    2 36868   5.1154 277.6131 0942962 231.7380 135.0000  0.87651600    04
    
    I have used the more than 140 hobbyist generated TLEs to plot the evolution of the apogee and perigee height, orbital
    period and inclination since launch. There are two nearly identical sets, the second of which includes the approximate
    planned orbital evolution, had the liquid apogee engine system operated as intended:
    
    http://satobs.org/seesat_ref/misc/AEHF_1_orbital_evolution_v1.pdf
    
    http://satobs.org/seesat_ref/misc/AEHF_1_orbital_evolution_v2.pdf
    
    The plots include a projection of the likely remaining evolution to reach the planned orbit, which the spacecraft
    appears to be on target to achieve by Oct 3rd.
    
    For anyone interested in contributing to the tracking, I will close with the predicted orbit message Mike issued earlier
    today:
    
    At the current rate, the perigee will reach the desired target of
    about 35800 Km in another few days.  One of these days they are going to
    have to start decreasing the orbital period more rapidly.  When they
    start to do that, it will be pretty early.  It will also take longer to
    drift around the Earth and that will also make the predictions more uncertain.
    
    1 36868U 10039A   11224.29911168 0.00000000  00000-0  00000-0 0    04 35642
    2 36868   5.0280 277.3219 0832267 231.9146   0.0000  0.88471600    07
    1 36868U 10039A   11225.42941792 0.00000000  00000-0  00000-0 0    06 35696
    2 36868   5.0073 277.2286 0804537 232.0078   0.0000  0.88701600    01
    1 36868U 10039A   11226.55679329 0.00000000  00000-0  00000-0 0    05 35748
    2 36868   4.9877 277.1377 0776640 232.1011   0.0000  0.88941600    08
    1 36868U 10039A   11227.68112655 0.00000000  00000-0  00000-0 0    04 35797
    2 36868   4.9689 277.0532 0748580 232.1851   0.0000  0.89191600    03
    1 36868U 10039A   11228.80230840 0.00000000  00000-0  00000-0 0    06 35843
    2 36868   4.9506 276.9776 0720399 232.2533   0.0000  0.89451600    04
    
    Use the tle preceding your time of prediction.
    
    The additional data on line 1 is the perigee height, which you may need to trim off, depending on your ephemeris
    generator.
    
    Ted Molczan
    
    
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