Re: USA 81: Complex variations in brightness. Main photometric period at 0.41s

From: Bob Christy (
Date: Wed Aug 01 2012 - 15:01:52 UTC

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    Back in 2006, the late Ivan Artner derived a 1.2s periodicity on the 
    S-band radio signal. On another channel, at VHF, I observed that there 
    was occasionally a 0.4s periodicity in the signal strength that used to 
    appear suddenly and then died away over perhaps 30-60s. The peaks 
    varied, with the strongest occurring every three cycles (1.2s again).
    At the time, I compared notes with a visual observer who confirmed that 
    the radio and optical characteristics were similar. He wrote 
    ".....sometimes it can appear steady in brightness for a couple of 
    minutes but usually there is a small variation in mag, then at times, 
    perhaps two or three times on a good pass, it goes in to what I call 
    'twinkle mode' for perhaps 10s; these rapid flashes are too fast to time 
    but around 2 or 3 per second.....".
    Bob Christy
    On 01/08/2012 15:23, wrote:
    > Browsing through Russell Eberst observations, I noticed this one of USA 81 (1992-23 A) made on 25 July 2012 :
    > "9202301 223903.08 004342+591855 5.8 7.1 0.3 R"
    > That's interesting because on the same satellite pass I took 2 series of photos of USA 81 from Paris suburbs (48.67 N 2.13 E) while my friend Alain Grycan took an additional photo from Toulouse ( 43.62 N 1.46 E). The satellite was then photographed using a 200mm teleobjective during 35 seconds, more precisely from :
    > 22h41m29s to 44s , on two 10-sec photos, aimed at LAC constellation from Paris, about 2-3 minutes later than the timing by Russell.
    > 22h 42m 22s to 27s , in one 5-sec photo, in CAS from Toulouse.
    > 22h43m23s to 38s , on two 10-sec photos, aimed at EQU from Paris.
    > In all our 5 photos the light variation is rather complex, as its pattern is somewhat changing, however a periodicity from maximum to maximum is obvious at 0.4 s.
    > Hence we don't confirm the 0.3s period derived by Russell a few minutes before.
    > For most of the pass, a repeating pattern every 3 periods (1.2s) is striking :
    > - either as a succession of 3 equal maximum distant by 0.4s from each other
    > - or a succession of 2 primaries and 1 secondary, all distant by 0.4s from each other
    > - or a succession of 1 primary and 2 secondaries, all distant by 0.4s from each other
    > On the last series of photos in EQU the pattern is rather :
    > - a succession of maxima distant by 0.2s (half-period). Maxima being not all of the same intensity.
    > A more accurate period value might be about 0.41s .
    > IMPORTANT CONCLUSION : As shown by our photos, the pattern of variation may change rather quickly so that a safe way to ascertain a period for this object is to do it using photography (or video). Visual determinations may lead to deriving spurious values for the periodic light variation, I guess.
    > Alain Figer
    > 48.67 N ; 2.13 E ; 170 m a.s.l.
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