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:

click diagram to enlarge

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.