Updated KeyHole elements

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Mon Apr 20 2009 - 23:00:17 UTC

  • Next message: Marco Langbroek: "SATOBS ML (4353), 20 Apr 2009 (Pt. 1): USA 189, USA 129"

    Derek Breit, David Brierley, Russell Eberst, Peter Gertson, Bruce MacDonald,
    Pierre Neirinck, Mike Waterman, Brad Young, and I, contributed observations used
    to determine one or more of the following element sets.
    
    USA 129                                                  293 X 939 km
    1 24680U 96072A   09109.83927812  .00003607  00000-0  57315-4 0    07
    2 24680  97.8317 163.5889 0462230 273.6120  81.2362 14.83287457    06
    Arc 20090418.09-0419.85 WRMS resid 0.024 deg
    
    USA 161                                                  308 X 960 km
    1 26934U 01044A   09109.99860360  .00003615  00000-0  73200-4 0    07
    2 26934  97.9090 222.0736 0465151 180.4717 179.6123 14.77546055    05
    Arc 20090411.95-0420.01 WRMS resid 0.022 deg
    
    USA 186                                                 257 X 1013 km
    1 28888U 05042A   09109.89384167  .00008659  00000-0  79966-4 0    02
    2 28888  97.8886 173.6996 0538748 352.6618   6.6926 14.77300822    02
    Arc 20090411.85-0419.90 WRMS resid 0.040 deg
    
    Peter Gertson expressed interest in "USA 129 particularly about how it's
    manoeuvres relate to its mission":
    
    http://satobs.org/seesat/Apr-2009/0207.html
    
    The design of the KeyHole constellation enables frequent repeat coverage of
    targets, without need of special manoeuvres. 
    
    The constellation consists of a western and an eastern orbital plane,
    respectively occupied by USA 186 and USA 161. 
    
    USA 129, which was replaced in the western plane by USA 186, remains in orbit in
    a nearby plane, on an extended mission. Likewise, USA 116 (95066A / 23728)
    remained in orbit long after USA 161 replaced it in the eastern plane, until its
    apparent de-orbiting in Nov 2008.
    
    Each individual KeyHole ground track nearly repeats after 4 days. Passes of the
    eastern plane KeyHole lag those of the western KeyHole by a couple of hours, and
    pass somewhat east of its ground track. The eastern KeyHole nearly retraces the
    track of western plane, after two days.
    
    KeyHoles manoeuvre two or three times per year, to maintain their orbits against
    perturbations due to atmospheric drag and solar gravity.
    
    The western plane KeyHole typically operates in a lower perigee orbit than that
    of the eastern KeyHole; as a result, it decays more rapidly, and the temporal
    spacing between passes of the two slowly changes. When the time lag of the
    eastern KeyHole exceeds ~2.5 h, the western KeyHole raises its orbit about 7 km,
    enabling the eastern KeyHole to begin gaining on it, until the drag imbalance
    causes them to once more separate, eventually triggering another reboost.
    
    Since drag affects mainly apogee height, the reboosts to counteract it normally
    occur at perigee, and affect only the mean motion and eccentricity.
    
    Solar gravity is a more complex perturbation, which affects all sun-synchronous
    orbits, because their plane maintains a nearly constant orientation relative the
    sun, enabling the perturbation to build up over time. Unlike drag, it affects
    all of the elements, most notably the inclination. The inclination required for
    a sun-synchronous orbit is a strong function of mean-motion and inclination, and
    to a lesser degree of eccentricity. When solar gravity causes a primary
    KeyHole's inclination to stray outside approximately 97.85 - 97.95 deg, a
    corrective manoeuvre is made. Such manoeuvres are combined with reboosts against
    drag, and are scheduled to occur within a few days of the precession of the
    perigee through the equator. The equator is the only location at which a single
    manoeuvre can alter inclination, without altering the plane of an orbit.
    
    Ted Molczan
    
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