laser beam diffusion at satellite distances

Some colleagues were recently discussing the efficacy of using laser transmitters to secure communications amongst orbiting satellites.  In particular, they were under the opinion that point to point laser communications would be immune to eavesdropping.

Eventually, I couldn’t resist the urge to say, “Wait.  Stand back.  I’m going to do MATH.”

Some 1996 information from JPL described a typical space communications laser as having a beam diffusion angle of 3.5 microradians  (about 0.0002 degrees if I did the conversion correctly).

Assuming I did the rest of math correctly (it’s been awhile since I’ve done Trig calculations), the reference beam diffusion angle would result in a perfectly targeted laser completely covering a 10 Meter Target at a distance of ~1,770 miles.  If the sender and receiver are further than ~1,770 miles apart, an eavesdropper would only need to position themselves somewhere behind the receiver along a line of sight.

To put that in perspective, at the International Space Station’s Altitude, the average orbital distance (once around the world) is ~26,000 miles.

Basically, even a James Bond Super Villain with excellent cloaking technology aboard a massive constellation of close formation nano-satellites needs to assume their laser based communications are subject to eavesdropping.  Actually, the cloaking technology would only be useful so long as they maintained “radio silence”… the moment they starting transmitting laser emissions; they’d give their position away.


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