Re: Flash period vs RPM; was Re: ATLAS CENTAUR R/B (#10779U)

From: Russ Bessom (
Date: Wed Sep 18 2002 - 03:31:03 EDT

  • Next message: Greg Roberts: "PPAS obs 12 Sep 2k2"

    You lost me Bjorn, Zing! right over my little bean!
    ----- Original Message -----
    From: "Björn Gimle" <>
    To: "SeeSat" <>
    Sent: Tuesday, September 17, 2002 8:08 PM
    Subject: Flash period vs RPM; was Re: ATLAS CENTAUR R/B (#10779U)
    > ... and there is the synodic effect: If it IS doing 12 rpm, then
    >  it is rotating 360*12 deg/min or 6*12 = 72 deg/s, and the
    >  reflection up to twice that speed.
    >  Since the satellite moves (a few degrees) during these 6 seconds,
    >  it may send the reflection to you a fraction earlier, or later.
    >  Now if you observe a pass where the axis is nearly perpendicular
    >  to the orbit and to your line-of-sight, and the Sun somewhere
    >  near the plane of these two vectors, and the satellite happens to
    >  move at 0.72 deg/s, you would see a 0.5 % increase or decrease in
    >  the visual period, so it would have a flash period of 5.025 or
    >  4.975 s. (If the rotation is slower, the relative effect is
    >  correspondingly larger).
    >  If you can observe one pass culminating "moving left", and the
    >  next one "moving right", and see a larger difference between
    >  their flash periods (at culmination) than what a similar estimate
    >  gives, you are pretty sure that you have been timing fractional
    >  rotations.
    >  If the geometry is less than ideal, it is more difficult to draw
    >  conclusions, but you should use a stopwatch with 50/100/300
    >  memories, and plot the observed times vs. the computed ones,
    >  using the less distorted period at the start or end of the track,
    >  to see a stretched "S" or "Z" shape displacement of the flash
    >  times (like an arctan() or arccot()  function.
    >  In a long "Determination of Rotational Axis" project several
    >  years ago, Bart de Pontieu (SeeSat founder) showed that with
    >  accurate "simultaneous" observations from two locations, and/or a
    >  lucky pass of the rotation axis near one observer, the true
    >  position of the axis, and thus the speed and direction of
    >  rotation, could be computed.
    > > Next time, I'll wait till the next morning to give details  ; )
    > ...and I should think twice, at least, instead of sending incomplete
    > > And Bjorn, I'm confused.  If the sat is spinning one revolution and
    > > and it takes 4.66 seconds to complete another revolution and flashes
    > > wouldn't that be 12.88 RPM?  Or is the satellite showing me two
    > > surfaces per revolution and I must divide by two?  Help!
    > >
    > ...
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