Monday, 22 February 2010

Chasing satellites through clouds and unusual brightness behaviour of the STSS Demo 2

The evening of Saturday-Sunday 20-21 February saw a very dynamic weather situation. Fields of clouds came and went very rapidly: the sky could go from clear to clouded to clear to clouded again in a matter of minutes. It made it a big gamble whether a particular object would eb visible or not.

As it came to be, I hauled a nice batch of positions on several objects: Lacrosse 5 (05-016A), the Lacrosse 5 r/b (05-016B), the STSS Demo 2 (09-052B) and the NOSS 3-2 (03-054A & C) duo. I also photographed the NOSS 3-4 duo but the image was too much hampered by cliuds tp reliably measure it. I lost amongst others Lacrosse 4 and the STSS Demo 1 to clouds (the latter a pitty, as it was predicted to pass right through the Pleiades).

Most of the images have some clouds on them: some extensively. Below are a few pictures: from top to bottom they show the Lacrosse 5 r/b amidst clouds; The NOSS 3-2 duo passing between the Pleiades and the Hyades; and the STSS Demo 2 passing near capella and the three Goats:

click images to enlarge

The STSS Demo 2 appears to show an unusual brightness variation in the first 2 seconds of the trail (the left part of the trail in below negative image), consisting of what appear to be a series of even spaced modest glints. Note the dashed appearance of the first part of the trail:

click image to enlarge

Below is the brightness profile over the trail (grey small crossmarks are individual pixel values, the solid line is a 3 point average), and below that is a graph of the time between brightness maxima visible in the profile.

click diagrams to enlarge

Note in the second diagram how the time between maxima is very constant, at about 0.13 seconds, during roughly the first 2 seconds . After that, it begins to wildly vary. As the first diagram shows, the amplitude of the brightness variations is larger in those first 2 seconds too. In fact, after those first two seconds the variation is largely or completely random pixel variation.

The first 2 seconds of the trail are quite different in character from the rest of the trail though: a clear constant, larger amplitude pulsing behaviour. This is very interesting. A second image obtained on the STSS Demo 2 during the same pass showed a quite constant brightness.

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