The flash period of USA 81 on June 2nd (UPDATED with February 24 results)

In my previous post I reported on a bright stroboscopic flash display by the SIGINT satellite USA 81 (92-023A) on June 2. I now had some time to analyse the images. Below are diagrams showing the brightness behaviour in the two images that captured the event.

click diagrams to enlarge

In the first image, the median period between succesive peaks is a very neat 0.20 seconds (average: 0.18s; modus: 0.19s). The same period also is present in the series from the second image, but with more "noise" in terms of either extra "peaks" or missing peaks. Below images shows it as diagrams of the delta time between peaks, and the deviation of these to the 0.20s period established by the first image:

click diagrams to enlarge

The results compare with similar results I obtained in February this year, which for some reason I never published on this blog. These are diagrams from my February 24, 2010 observations (two images), when it showed a series of sharp glints in the zenith, similar to those of June 2nd, with a period of ~0.41s (multiple of the 0.20s of June 2nd). Lower in the sky, it changed to a slower cycle of less sharp peaks with a period of 1.26s.

click diagrams to enlarge

The brightness behaviour hence is quite similar to that for it's older sister ship USA 32 (see here).