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

From: Björn Gimle (b_gimle@algonet.se)
Date: Tue Sep 17 2002 - 14:08:08 EDT

• Next message: Joseph A. Dellinger: "story in space.com on "earth's new moon" J002E3"

```... 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 replies.

> And Bjorn, I'm confused.  If the sat is spinning one revolution and flashes,
> and it takes 4.66 seconds to complete another revolution and flashes again,
> wouldn't that be 12.88 RPM?  Or is the satellite showing me two reflective
> surfaces per revolution and I must divide by two?  Help!
>
...

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