91082a recovered

From: Ted Molczan (molczan@home.com)
Date: Thu Sep 07 2000 - 11:44:55 PDT

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    I took advantage of a clear sky to recover 91082A, which had not been tracked
    for two months due to bright twilight. Here are search elements derived from
    DMSP B5D2-6      6.4  1.7  0.0  6.4 v 5.43
    1 70000U          00251.03553819  .00000300  00000-0  15595-3 0    03
    2 70000  98.8164 279.9606 0017257 168.1428 191.9191 14.14660000    09
    I have used an arbitrary search element designation, because most of the
    elements are estimates. Nevertheless, I do expect predictions to be accurate to
    well within 1 min of time for the next few days.
    The recovery was made during a planned search, +/- 5 minutes of the predicted
    time of the most recent elements:
    1 21798U 91082A   00186.10578752  .00000300  00000-0  15630-3 0    01
    2 21798  98.8240 215.9779 0012004 349.7602  10.2397 14.14578513    09
    Despite making appropriate allowances for Earth's rotation during the search
    period, I very nearly missed the object, because its path was somewhat lower
    than predicted. Fortunately, it passed just within the bottom of my field of
    view, running about 1.9 min early.
    Finding my reference stars proved difficult, because I could not initially
    account for the difference in the predicted path. Finally, a look at the
    historical elements provided the clue:
    The object's inclination has decreased steadily since it was launched in 1991,
    from 98.9246 deg to less than 98.85 by mid 1999. This decline continued in
    2000, with the epoch 00186.1 elements giving a value of 98.824 deg.
    I reasoned that the inclination had continue to decline to the present, and
    found that decreasing it moved the track closer to my observed path, which
    helped me to identify my reference stars.
    I was concerned that I needed to change the inclination a bit more than seemed
    reasonable, which led me to question whether or not the inclination of the
    available elements was accurate, so I analyzed of set of 10 points observed
    over the period 1999 Dec 27 to 2000 Jan 16, by Russell Eberst, Jim Nix, Peter
    As my starting point, I used elements that had been issued earlier this year:
    1 21798U 91082A   00016.76419049  .00000280  00000-0  14632-3 0    06
    2 21798  98.8330  48.6891 0013004 114.2226 245.7773 14.14410533    03
    Using Elcor, I obtained this result, having WRMS residuals well under 1 arc
    1 21798U 91082A   00016.76419049  .00000289  00000-0  15118-3 0    05
    2 21798  98.8164  48.6486 0017257 114.2552 245.7809 14.14410862    09
    The differences are small, but this result confirmed that the inclination was
    lower than had been thought, by about 0.017 deg. That small change took out
    most of the error in the predicted track that I observed last night. The
    difference was sufficient to produce a 0.47 deg error in the RAAN over a period
    of about 8 months.
    I propagated my epoch 00016.76 result to the current epoch, for use as the
    basis for the search elements at the top of this post, allowing Elcor to
    correct only the RAAN and the Mean Anomaly.
    The inclination almost certainly has continued to decline, so I expect further
    observation and analysis to reveal a somewhat lower value than the 98.8164 I
    have used above.
    I observed the following points:
    21798 91 082A   2701 G 20000907010236250 17 25 1536045+095647 38 S
    21798 91 082A   2701 G 20000907010310500 17 25 1507888+184923 18 S
    The object passed well outside both pairs of reference stars, so I doubled my
    estimated error, and I would not be shocked if they prove even less accurate
    than stated.
    I also timed a close appulse of 37 Epsilon in Serpens Caput. At 01:02:14.86
    UTC, the object passed roughly 0.1 to 0.2 deg above and to the left of that
    star, as seen in my field of view. The position-angle relative the celestial
    co-ordinate system was about 10 o'clock. I did not reduce the appulse to a
    point, but I note that my orbit solution is in very good agreement, based on
    plotting the path in an atlas.
    I believe that 91082A's gradually declining inclination may warrant more
    intensive observation. Based on my limited understanding of perturbations, I
    associate declining inclination with resonant orbits - those which repeat their
    groundtrack daily or every two days. This object roughly repeats its
    groundtrack every 7 days. Is that sufficient to account for such a pronounced
    decline in inclination?
    Ted Molczan
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