It came clear a while ago that non-appearances of Starlink train passes to the Sun-side of observers was due to the roll-angle of the spacecraft panel around the velocity vector. This became particularly apparent in the northern summer due to the Sun azimuth near north but the phenomenon had been noted in the southern hemisphere earlier this year. This has been confirmed by recent information from SpaceX. I had created a simple model of the spacecraft panel pointing axis assuming the panel long axis was the velocity vector and the roll axis was that needed to put the Sun on the normal through the long axis. The angle between this vector and the observed Starlink direction for a particular observer indicates how much of the sunlit side of the panel was observable (panel view angle). This gave panel view angles consistent with observed train non-appearances. Marco Langbroek's excellent observation and images of the L1.5 train from Leiden on 2020 April 21 provided a useful test of the model and of any potential deviations, hinted at in the recent SpaceX information. In the image below I plot the calculated glancing angles to the sunlit side of the panel (0=view angle in the plane of the panel) for two altitudes in Langbroek's image, 50 and 70degrees. The roll-angle is as calculated above. The trend of reducing glancing angle with Starlink brightness is correct. http://recole.org.uk/starlink/analysis-0.jpg The fit was best for a deviation of -9 degrees from the model, that is the actual roll-angle was 9 degrees less that the simple model. The panel azimuth angle was not changed. http://recole.org.uk/starlink/analysis-1.jpg Of course, SpaceX is now promising to change the roll-angle model to minimise brightness in potential observer twilight, i.e. maintain glancing angle at zero at those times. _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-lReceived on Sat May 02 2020 - 04:52:58 UTC
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