The flashing behaviour of USA 81 (1992-023A)

The video footage above was shot by me on 25 July 2017 and shows the classified satellite USA 81 (1992-023A). It is flashing rapidly, at ~5 flashes per second.

USA 81 is one of two satellites (the other is USA 32; the attempted launch of a third one failed) on which little is known, but that were probably ELINT gathering satellites, probably in the Singleton/SBWASS program. It was launched from Vandenberg AFB on 25 April 1992 with a Titan 23G.

With its earlier sistership USA 32, USA 81 is renowned for the light shows it puts on. It has a very rapid and at times very conspicuous flash cycle of sharp specular flashes, like a disco-ball: the video above shows an impressive example of this. How pronounced the flashing is depends on the viewing angle: the video above catches a brief period, halfway the footage, where it becomes very pronounced.

The flashing is very regular and specular, with a main period of 0.2 seconds and a secondary period half of that, 0.1 seconds.

The diagrams below show this periodicity well. They are the result of an analysis of (part of) the video above with LiMovie and PAST - the analyzed part of the video was the second part, after the camera repositioned, when flashing was most pronounced.

The first diagram shows the observed very specular flashes (blue) and a fitted double sinusoid of 0.2 and 0.2 seconds (red, dashed). They fit very well:

click diagram to enlarge

The two periods are also well discernable in the Lomb periodogram and the fast Fourier analysis below:

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click diagram to enlarge

The earlier sister ship of USA 81, USA 32 (1988-078A), shows the same brightness behaviour (but with a main periodocity of 1.2 seconds) with the same periodicity. This points out that the flashing is intentional.

There are two options to explain the flashing behaviour. One is that the satellite is spin-stabilized. The other is that the satellite could have a large and shiny rotating element, for example a large rotating antenna.

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.

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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:

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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).

USA 202 and other geostationaries, and a stroboscopic show by USA 81

I am awfully behind with reporting on my observations.

In deep twilight on June 2nd, I observed USA 81 (92-023A) briefly attaining easy naked eye magnitudes in Bootes, and firing off a rapid series of flashes. The trails on both images covering this episode of bright stroboscopic behaviour partly run off the image, as I re-aimed in (too much of) a hurry. I still have to analyse the flash period. But it was impressive to see:

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Apart from two Lacrosses, I also targetted the geostationary satellite Milstar 5 (02-001A) again. below image shows it together with the commercial geosats Galaxy 11 and Intelsat 802:

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I also imaged another classified geostationary satellite, USA 202 (09-001A), and ELINT satellite (probably a Mentor/Advanced Orion). I had not realised it was so bright, so initially I thought the faint object on below image was USA 202 and the brighter one the commercial geosat Thuraya 2. However, the brighter object is USA 202, as it turns out (hence, the questionmarks still in below image can be removed). The satellite is at an altitude of only 17.5 degrees in the south-southeast for mu location.

click image to enlarge