Tracking and elements of the SLDCOM satellites

From: Ted Molczan (
Date: Mon Feb 26 2007 - 11:40:48 EST

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    In recent years, several hobbyists have been tracking the Titan Launch
    Dispensers, which were the specialized upper stages used to deploy the second
    generation NOSS.
    Originally, they were intended for Shuttle launch, and so were named Shuttle
    Launch Dispenser:
    Photos are available here:
    There have been open source references to SLDCOM, an apparent contraction of the
    dispenser's original name and the description of the payload it hosts, which is
    believed to be experimental communications, so that is the name we use for them.'s John Pike reported on SLDCOM's communications payload in
    The approximate orbit of the SLDCOMs emerged in the late 1990s: 63.4 deg, 1200 x
    11600 km, 270 deg argument of perigee, as I mentioned here, in this Sep 2000
    At the time, I had a fairly good idea of the probable SLDCOM orbital planes,
    based upon their launch dates, hobbyist tracking of 91076A and 96029A while
    still in LEO, and information filed by the U.S. with the U.N.; however, several
    more years passed before they could be tracked.
    By 2002, Greg Roberts had improved the sensitivity of his video tracking system,
    enabling him to track the objects from his excellent vantage point (near
    perigee) in South Africa. Greg is a major contributor of observations, as are
    David Brierley and Peter Wakelin. Peter mainly uses his CCD camera for these
    faint objects, which has been another key enabling technology. A few hobbyists
    make telescopic observations, including Mike McCants, who also has generated
    almost all of the 2-line elements from the team's observations.
    The objects were first tracked as unknowns, but could readily be identified by
    referring to the aforementioned estimated orbital planes. 91076A's use of a 5.5
    rev/d orbit instead of a 6 rev/d orbit was a surprise. It was identified by a
    process of elimination, since 90050A and 96029A were accounted for.
    A further surprise came in the spring of 2003, when 96029A went missing after
    several weeks out of sight of observers. We guessed that it had manoeuvred to an
    orbit similar to that of 91076A. 
    I believed there might be some advantage to tracking and studying the objects
    quietly - at least until 96029A could be tracked again - so at my request, we
    delayed making the elements and observations public.
    Tracking of 96029A resumed in 2006, which was found to have manoeuvred to the
    5.5 rev/d orbit, as we had suspected.
    Also in 2006, a piece of debris turned up, which Mike McCants identified as
    having originated from 91076A. It is fairly strongly perturbed by SRP, revealing
    a high area to mass ratio. Also, it is intrinsically bright, so perhaps it is a
    piece of thermal blanket. We eventually added both pieces to the list of
    unknowns, as 90064 and 90065, respectively; however, from here on 96029A will be
    referred to by its real ID.
    The latest orbits of all four SLDCOM objects being tracked are as follows:
    SLDCOM 1         0.0  0.0  0.0  5.1 v
    1 20641U 90050A   07030.66518297  .00000000  00000-0  00000-0 0    04
    2 20641  63.4203 113.1674 4028367 268.3743  91.6257  6.02570819    07
    SLDCOM 2         0.0  0.0  0.0  5.3 v
    1 21775U 91076A   07048.63525490  .00000000  00000-0  00000-0 0    01
    2 21775  63.3559 235.6202 3325559 266.6278  93.3722  5.53394918    07
    SLDCOM 2 deb     0.0  0.0  0.0  4.1 v
    1 90065U 06629B   07056.67525164 -.00000076  00000-0 -67469-1 0    08
    2 90065  63.3134 225.2698 3277609 271.3766  88.6234  5.53955279    03
    SLDCOM 3         0.0  0.0  0.0  5.6 v
    1 23893U 96029A   07023.74104674  .00000000  00000-0  00000-0 0    09
    2 23893  63.4683   7.8791 3315345 264.7452  95.2548  5.52844075    06
    To-date, no debris from 91076A has been officially catalogued, so we will
    continue with the 90065 / 06629B designation for the piece that we are tracking.
    90050A's ground track repeats daily, after 6 revs. 91076A and 96029A originally
    were in similar orbits, but subsequently manoeuvred so that their ground tracks
    repeat every 2 days, after 11 orbits.
    Ted Molczan
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