Cosmos 2282 spinrate

From: Matson, Robert (
Date: Mon Sep 09 2002 - 14:58:23 EDT

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    Hi Paul, Ed and Björn,
    Paul wrote:
    > ... more specifically on 09sep I had:
    > at  53 flashes 20.8047
    > at 102 flashes 20.8042
    > at 181 flashes 20.8040
    > at 205 flashes 20.8041
    > to me this shows that I maxed out somewhere between
    > 53 and 102 meaning the overall uncertainty is somewhere
    > between 0.05 and 0.1 s, and PPAS seems happy with 0.1 s
    > & 0.001 s resolution which I seem to be getting as early
    > as 100 flashes.
    We're actually in a regime with these GEO flashers where
    it's possible to achieve accuracies higher than PPAS is
    probably interested in.  However, as you've discovered you
    can start to do some interesting things when measurement
    accuracy is high -- in particular, study the impact of
    spacecraft eclipses on spin rate.  Paul mentioned this:
    > ...but also wondering if the biggest shift occurs during
    > shadow or immediately after... eg it entered shadow at
    > 20.815 s and emerged closer to 20.804 s rather than
    > a constant drift (spin up) over 24 hours??
    Since the sun is the driver for pretty much all dead GEO
    spin-ups, significant changes can be expected during shadow
    time.  In other words, while the acceleration (or deceleration)
    may look fairly linear when analyzed at low temporal resolution
    (days), at high resolution (hours) you can see direct evidence
    of the sun being the driving force.
    The radiometer effect is a complex one involving optics,
    spacecraft materials properties (absorption, reflection,
    emission), geometry, and overall spacecraft structure.  When
    the spacecraft transitions to darkness, infrared emission
    effects dominate -- high emissivity surfaces (e.g. the
    "blacker" parts of the spacecraft) emit more IR photons
    than the reflective parts.  If these parts are not distributed
    in a symmetric fashion, then a net torque will result -- one
    which can either accelerate or decelerate any already existing
    spin.  Evidently in the case of Cosmos 2282, thermal emission
    effects are contributors to the spin up.
    When the satellite is sunlit, you still have thermal emission
    effects, but you also have visible photon absorption and
    reflection to add to the equation.  (A reflecting photon
    imparts twice as much force as one that is absorbed or
    emitted.)  Since the solar arrays are furthest from the axis
    of rotation (and usually represent the majority of the surface
    area), photon interactions with them will be the dominant source
    of torque on the spacecraft.
    It would help to know more information about the solar arrays
    on Cosmos 2282 (front- and back-side reflectivity/emissivity),
    but more importantly any non-symmetric features.  A symmetric
    solar array arrangement should not produce any torque since
    the contributions of each array would be cancelled by the
    other (I'm assuming there are two).
    If there are two arrays and they are mirror images of each
    other (same size, same reflectivity and emissivity, same
    distance from the axis of rotation, and parallel to one
    another), then you're left with asymmetries in the spacecraft
    itself as being the source of torque.
    My suggestion is to get split times every half-hour --
    especially around the times of shadow entry and exit.  This
    will probably show a stepping stair effect on the satellite
    spin-rate, with much of the increase occurring during
    eclipse.  As we approach autumnal equinox, the satellite is
    going to be spending more and more time in eclipse each
    night, so we're in the midst of the greatest acceleration
    at this time of year (and again in March).
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