RE: VisorSat Phase Function

From: Richard E Cole via Seesat-l <seesat-l_at_satobs.org>
Date: Mon, 5 Jul 2021 13:28:34 -0000
I have taken the liberty of plotting a similar graph to Anthony’s using the Starlink magnitude model I developed some months ago. My plot and Anthony’s original are shown at:

 

https://www.dropbox.com/s/28e6tr0b7utlm40/phase-angle-plot.jpg?dl=0

 

The axes are the same. I have used a Sun-depression angle of 15 degrees, with an observation cut-off elevation also of 15 degrees.

 

There is some agreement on the general trend, but with an amount of scatter. I had modelled the brightness at high phase angles as a reflected beam from the large, polished-metal Earth-pointing face of the spacecraft with a very strong power-law drop-off in the angle from the centre of the beam. The baseplate is virtually a 4x3m mirror in space; the centre of the beam misses the Earth but its effect can still be seen many degrees off-axis. 

 

Regards

 

Richard Cole

 

From: Anthony Mallama <anthony.mallama_at_gmail.com> 
Sent: 04 July 2021 14:20
To: Seesat-L <seesat-l_at_satobs.org>; Richard Cole <richardc_at_recole.plus.com>
Subject: VisorSat Phase Function

 

VisorSat visual magnitudes recorded by Jay Respler and myself during the first half of 2021 have been analyzed. They were adjusted to a standard distance of 1,000 km and then plotted against phase angle as shown in this graph: https://photos.app.goo.gl/sncE9yANwr8vzcSY7.

 

An unexpected result is that brightness is greater at phase angle 140 degrees when the satellites are back-lit by the Sun than at 90 degrees when they are side-lit. This characteristic indicates strong forward scattering of sunlight.

 

The graph also shows that VisorSats are brighter at phase angles smaller than 90 degrees. That result is not unexpected since the satellites are opposite to the Sun at small phase angles and are front-lit.

 

Tony Mallama

 

 

 

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Received on Mon Jul 05 2021 - 08:29:48 UTC

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