Model of Starlink train brightness and comparison to observation

From: Richard Cole via Seesat-l <>
Date: Sat, 2 May 2020 09:51:49 +0000
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.

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.

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.

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Received on Sat May 02 2020 - 04:52:58 UTC

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