Updated elements

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Thu Sep 20 2012 - 11:16:26 UTC

  • Next message: Brad Young: "BY C 092012"

    On 2012 Sep 19 UTC, Russell Eberst and Scott Tilley made new observations of objects from the NROL-36 NOSS launch.
    
    Both payloads were a few seconds early, revealing that they had lowered their orbits slightly. 12048P remains the
    leader, but their rate of separation has decreased. I expect that 12048A eventually will manoeuvre lower to overtake
    12048P, followed by a manoeuvre by 12048P to match the altitude of 12048A, once they are at the standard operational
    spacing. At about that time, 12048A will manoeuvre to shift its RAAN about 0.2 deg lower than that of 12048P. At that
    point, they are likely to be in a slightly lower orbit than the operational altitude of the NOSS constellation, enabling
    them to drift to the relative position that optimizes the constellation, whereupon they will manoeuvre to match the
    operational altitude.
    
    Space Track TLE queries reveal a gap at 38773, which falls between objects 12049B and 12050A, and may be the second NOSS
    3-6 payload. I will wait for the confirming catalogue entry before adopting it.
    
    NOSS 3-6 (A)                                           1013 X 1199 km
    1 38758U 12048A   12263.46739731  .00000000  00000-0  00000-0 0    03
    2 38758  63.4283  27.5604 0124892 180.5104 179.5813 13.40570619    08
    Arc 20120919.18-0919.48 WRMS resid 0.012 totl 0.007 xtrk
    
    NOSS 3-6 (P)                                           1010 X 1202 km
    1 79603U 12048P   12263.46712754  .00000000  00000-0  00000-0 0    03
    2 79603  63.4252  27.5502 0128027 179.1345 180.9925 13.40607252    04
    Arc 20120919.18-0919.48 WRMS resid 0.018 totl 0.015 xtrk
    
    NOSS 3-6 r                                               460 X 773 km
    1 38770U 12048N   12263.47756416  .00001637  00000-0  12617-3 0    04
    2 38770  64.6705  23.4498 0223360 293.1266  64.6443 14.83662424    07
    Arc 20120916.19-0919.49 WRMS resid 0.035 totl 0.013 xtrk
    
    About the Increasing Eccentricity of the NOSS Orbit
    
    An interesting characteristic of the NOSS orbit, is its gradual increase in eccentricity, due to the effects of the odd
    zonal harmonics of the geopotential on orbits near the "critical inclination", i.e. 63.43 degrees. For example, compare
    an early TLE of a NOSS 1-1 payload with a recent one:
    
                                                           1091 X 1128 km
    1 08835U 76038C   76219.49464155  .00000393  00000-0  81749-3 0   269
    2 08835  63.4426 339.4386 0024903  72.4670 287.9030 13.39535601 13112
    
                                                            377 X 1652 km
    1 08835U 76038C   12263.18168741  .00007000  00000-0  50214-3 0    03
    2 08835  63.2940  31.1383 0862214 103.4938 256.5062 13.65445550    04
    
    The orbit has been stretched, such that the perigee has decreased by hundreds of kilometres and the apogee has increased
    by about the same amount. Decay due to the lower perigee accounts for the decrease in mean altitude.
    
    I have long wondered whether the increasing eccentricity was intended, or a tolerated side-effect? Recently, I came
    across the abstract of a 1978 report that reveals it was the latter, and totally unexpected:
    
    http://www.dtic.mil/docs/citations/ADA073975
    
    "A cluster of three satellites showed large, unpredicted orbital perturbations shortly after launch. There was immediate
    concern for the success of its mission which depends on the stability of the cluster geometry. The Naval Surface Weapons
    Center found the zonal harmonics of degree five and seven to cause most of the observed perturbations. Subsequently, the
    three orbits were computed for the time frame June 1976 to July 1984. It was also shown that the cluster configuration
    remains unaffected by the large orbital perturbations, and the mission appears safe."
    
    So, the increasing eccentricity is a bug, not a feature.
    
    The 3rd gen NOSS employ a partial mitigation. They start out in a 1000 x 1200 km orbit, with argument of perigee near
    180 deg. The latter causes the eccentricity to initially decrease to near zero, while the argument of perigee shifts to
    90 deg, which takes about four years. Over the subsequent four years, the eccentricity increases, as the argument of
    perigee moves toward zero deg, and the orbit is once again 1000 x 1200 km. This strategy maintains a reasonably circular
    orbit for ~eight years, which probably is the nominal mission life. The orbit continues to become more eccentric, so any
    operation beyond 8 years will be with a gradually increasing eccentricity.
    
    Ted Molczan
    
    
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