RE: Model of Starlink flares - some progress

From: Daniel Deak via Seesat-l <>
Date: Mon, 11 May 2020 19:16:56 -0400
Hi Richard,

My brother caught on video a series of bright flares from the Starlink-5
satellites (I think) on a 15-minute span on April 25th. He told me they were
almost as bright as Venus. I will try to get more info for time and location
and come back to you as soon as possible. Approximate coordinates of the
observation site are : 45.576 N, 72.012 W.

Daniel Deak, CFEI
Pompier, enquêteur certifié
SSI de Saint-Cyrille-de-Wendover

-----Message d'origine-----
De : Seesat-l <> De la part
de Richard Cole via Seesat-l
Envoyé : 11 mai 2020 05:50
À :
Objet : Model of Starlink flares - some progress

I have been working on a model of Starlink visibility based on a
Sun-pointing model used in the low-drag configuration below 550km. This is
discussed at the link below and is termed here Model-A.

I have used the same model to analyse various flare events gleaned from
personal contacts, reddit posts and SeeSat-L posts. The analysis looked at
simple modifications of Model A to try to explain the flares.

Model-A was modified (to Model-B) to allow larger variations in roll-angle
round the velocity vector and also deal properly with Starlink locations at
large distance from the observer. The Sun and view angles to the panel was
analysed to indicate when specular reflections might be expected, at
multiple positions across the sky.

Model-A does not predict flares at high observer altitudes, since a
Sun-pointing panel (even one aligned to the velocity vector) will reflect
sunlight in the Sun-orbit plane, which is at low altitudes (though under
study to see if there are observable events at those altitudes). Model-B
uses large roll-angle to allow reflecting surfaces at orientations that
might give flares at higher altitudes (where the angle of ray deviation has
to be larger to reach the observer).

I deal with four reported flare cases which are charted here (fig 1):

Case 1: Italy 17th April - (unknown observer)

The observation is shown in a nice video here:

This shows multiple Starlinks crossing the virtual image of the Sun created
by some reflective structure on each Starlink, all aligned in the same
direction by the same attitude control law. I have used the brightest flare
for the analysis in fig 1.

A rough match to the flare is given by a roll-angle deviation of 30 degrees
(the same deviation is used for cases 2 and 3).

Case 2: Sydney 5th May (Reddit user ChodaGreg)

At lower altitude.

Case 3: Leiden 21st April (Marco Langbroek)

The same 30 degree roll-angle deviation give a reasonable fit to the flares
near Pollux.

I am reasonably confident that the basic model is correct, that is the panel
is sun-facing as described in the article. The detailed analysis of the
Leiden data using Model-A would break down if a large roll-angle applied to
the whole spacecraft.

Therefore, I am suggesting there is some sub-structure that is tilted 30
degrees from the main structure around the long axis of the whole panel,
that is the short axis of the spacecraft block itself. Possibly this is the
sub-panel holding the dish antennas at ends of the panel (used for s/c
control and telemetry, not the user links). These panels are required (in
this scenario) to articulate the dishes in deployment, to point the dish
towards the ground. The images of ejection of the spacecraft from the Falcon
show the dishes are pointing in the other direction at that time.

I note the published sunshade design for Visorsat may shade the area of
these dishes, suggesting SpaceX is seeking to mitigate any effect of these
panels in the final orbit configuration. A polished panel only 15x15cm
acting as a mirror can give a bright flare on the ground. 
Published images of a Starlink stack prior to launch show a lot of shiny
aluminium panels.

Case 4: Arizona 4th May (Austin Godber)

Austin reported a number of Starlinks showing bright flares in sequence,
with the Starlinks visible before and after the flares.

This one is different. Model-A predicts this pass should be totally
invisible from Austin's location as the 'open book' panel would be pointed
away from the observer due to high Sun azimuth. Model-B needs a roll-angle
of 67 degrees (downwards) to get a flare at the right sky flare direction.

This pass was over southern California. I suggest that in accordance to
recent information from Space-X, the roll-angle has to be changed to allow
communications with the ground and so in case 4 the spacecraft baseplate is
pointing nearly to the nadir and these flares are from the main baseplate,
not the dish support panels (if that is what is flaring in Cases 1-3)

More work is needed to understand the deviations of observations from
Model-B, predict some flares (and try to observe them) and apply the model
to Starlinks at 550km.


Richard Cole

mob:  0771 858 8940
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Received on Mon May 11 2020 - 18:19:50 UTC

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