... 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
This archive was generated by hypermail 2b29 : Tue Sep 17 2002 - 14:27:40 EDT