Geometry for flashes from DSP USA 39

From: Michael McCants (
Date: Wed Mar 26 2003 - 14:42:40 EST

  • Next message: Michael McCants: "Re: Geometry for flashes from DSP USA 39"

    Ed Cannon posted:
    >What hypotheses are offered to explain how
    >it can continue to flash at the same time each night?
    If I pick March 21, 2003 at 0400 UT, the sun is very nearly
    RA 0 Hr 0 Mn.  If I take the elset for 20066 and generate a
    prediction for latitude 0, longitude 145.5 west, I get an
    altitude of very nearly 90 degrees (since this is the location
    of the sub-satellite point) and an RA of about 6 Hr 12 Mn.
    When the flashes are seen at 0400 UT, the solar panels are pointing
    half way between the sun and the observer.  Since the difference
    in RA of 6 hours represents 90 degrees, it is clear that the angle
    of the solar panels relative to the long axis of the spacecraft is
    about 45 degrees.
    The solar panels are sending the reflected sunlight in a north-to-south
    (or south-to-north) path and the critical angle is the 90 degrees
    for the sun-satellite-observer angle, so it just does not make all
    that much difference whether the sun is at declination -20, 0, or +20.
    In this geometry, the sun-satellite-observer angle is changing
    because the satellite and the observer are turning at the daily
    rate of 1 degree every 4 minutes.  So the reflections from the
    panels will "sweep" across the Earth from east to west at a
    rate of 1 degree per 4 minutes.  One degree at a distance of
    22000 miles is about 500 miles (in projection), so if there was
    something distinctive about these flashes (like the Superbird A
    phase change time), a time difference from east coast to west
    coast could be measured just like Superbird A.
    The flashes from one of the two DSPs that Rainer Kracht is tracking
    (20929 and 26356) should be visible from the US in the morning
    sky under a similar 90 degree angle.
    Object 20929 is above longitude 38 west.  It crossed RA 6 Hrs
    about 20:50 UT.  He reported flashes from 20:55 to 21:07 and
    from 21:18 to 21:27 UT on Feb 25.  This DSP crosses 18 Hrs RA
    about 08:40 UT.  So flashes should be visible from the US in
    the morning sky near that time.
    Object 26356 is above longitude 8 east.  It is not visible from
    the US.  It crossed RA 6 Hrs about 17:40 UT on March 21.  Rainer
    saw fainter flashes about 18:16.  It crossed RA 18 Hrs about
    05:35 UT on March 21.  Since this object is near the meridian
    for Europe, its 90 degree flashes are probably only visible
    in the wintertime.  Possibly Greg Roberts could try for it
    now that it is fall in South Africa.
    Rainer reported that he observed bright flashes from this object
    at 22:50 for only two minutes.  The RA at 22:50 on March 21 is
    about 11 Hrs 15 Mn.  This gives a sun-satellite-earth angle of
    about 170 degrees and a half angle of about 85 degrees.  This
    would imply some kind of panel pointing almost straight at the
    Earth.  This would also imply that flashes could be seen at the
    RA of 12 Hrs 45 Mn, which would occur 90 minutes after the flashes
    he saw.  It would also imply flashes from the other DSPs both
    5 hours and 7 hours before/after their 90 degree flash times.
    Mike McCants
    USA 39
    1 20066U 89046A   03080.72784648 0.00000000  00000-0  00000+0 0    00
    2 20066   6.8620  52.8558 0060003 242.6560   0.0000  1.00273400    02
    DSP 15 (USA65)
    1 20929U 90095A   03056.00000000  .00000000  00000-0  00000-0 0    02
    2 20929   6.8243  60.4532 0007040 337.3686  78.3868  1.00272831    08
    DSP 20(USA149)
    1 26356U 00024A   03071.38900906 0.00000000  00000-0  00000-0 0    07
    2 26356   0.6979 317.6720 0004000  60.1177 299.8823  1.00262120    02
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