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

From: Russ Bessom (russbessom@desurf.com)
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!
Russ
----- Original Message -----
From: "Björn Gimle" <b_gimle@algonet.se>
To: "SeeSat" <SeeSat-L@satobs.org>
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
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!
> >
> ...
>
>
> -----------------------------------------------------------------
> Unsubscribe from SeeSat-L by sending a message with 'unsubscribe'
> in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org
> http://www.satellite.eu.org/seesat/seesatindex.html
>

-----------------------------------------------------------------
Unsubscribe from SeeSat-L by sending a message with 'unsubscribe'
in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org
http://www.satellite.eu.org/seesat/seesatindex.html
```

This archive was generated by hypermail 2b29 : Wed Sep 18 2002 - 03:38:42 EDT