Re: Bright flare from NOSS 3-4 and double flare from FIA-Radar 1

From: Vladislav Gooba via Seesat-l <seesat-l_at_satobs.org>
Date: Tue, 26 May 2015 15:15:48 +0300
```Hello Bjorn,
satellites with rotating elements. Is your variant works on them too?
Sincerely

--- Исходное сообщение ---
От кого: "Björn Gimle"
Дата: 26 мая 2015, 13:27:57

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> 2015-05-26 12:21 GMT+02:00 Björn Gimle :
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Unless the satellite is controlled, all reflecting surfaces are usually fixed.
There are essentially three modes of rotation:
E: Fixed relative to nadir and orbit plane, or velocity vector
T: Tumbling after recent explosion, meteor hit or leakages
S: Rotation around the axis of greatest inertia.

Type T is really difficult - I can't imagine solving it.

E is very well handled by Simone Corbellini from FPAS report submitted to http://www.satflare.com/fpas_reader.asp

I have solved many S cases with simple Excel calculations. When you know the RA/Dec of the Sun and a satellite flash,
compute the unit vectors x,y,z (length 1). For the flashes, change all values to opposite sign, then take the average of all x,y,z pairs. This gives the normal (right angle) vector to the front side of the reflector - convert it to RA/Dec. You need to collect flash observations in several directions, and/or from different parts of the world. Plot the normal vectors' RA/Dec on a star chart (with as small distortion as possible, e.g. a stereographic projection centered near the average of thevectors), or use the "Solver" in Excel to find the rotation axis as the point where the distances (d) to the normal vectors are as equal as possible (minimum standard deviation). With a current Sun position, the rotation axis, and the (average) distance (d) from it you can compute new flash positions.

Some complications:
Large solar panels often has a flat back side which can be sunlit and reflecting when satellite is uncontrolled.
In the S cases this occurs as a circle at angle 180-d.

In S cases, the rotation axes precesses over months, usually at near constant declination. You may estimate it by dividing long periods of observations into two close groups, and/or include the precession value in the Solver computations.
In all situations, there may be more than one reflecting surface !

S objects affected by drag and magnetic field precess faster, and not necessarily along declination.
Rotating cylindrical objects flash anywhere along the cylinder, when its long axis crosses the plane perpendicular to the Sun-Satellite-Observer plane.

--------------------------------------------------------
Björn Gimle, COSPAR 5919
59.2617 N, 18.6169 E, 51 m
Satellite observation formats described:
http://www.satobs.org/position/IODformat.html
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2015-05-24 23:34 GMT+02:00 Vladislav Gooba via Seesat-l :

> May 24 at 00:47 UT+3 I have observed very quick flares from NOSS 3-4 (A) #31701 and (C) #31708. The flares was -3 mag with duration shorter than second. Flare of (C) happened at 00:47:00+-1s. Mirror angles of (C) for Heavensat are 60 51.5, elongation from Sun at the moment of flare was 96.5.
> May 24 at 22:54:10 UT+3 +-10s I have observed single flare with two maximums from USA 215 or FIA-Radar 1 #37162. Flare duration was about 2 seconds, with magnitude of first maximum -1, second 0. Mirror angles for Heavensat are 45 243, elongation from Sun at the moment of flare was 125.
> One more question. How we can calculate flares from rotating surfaces such as solar panels, with unknown pattern of rotation? E.g. for fixed mirrors all the conditions is off-nadir and yaw angles, what is needed to predict flares from rotating mirrors?
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Received on Tue May 26 2015 - 07:16:38 UTC

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