... it's insignificant compared to the power of The Force...
NASA has pulled off an impressive bit of space data transfer by beaming a 175 meg hi-def video from the International Space Station to the ground via laser. The Optical Payload for Lasercomm Science (OPALS) experiment used a 2.5W 1,550nm laser to beam the vid earthwards in just 3.5 seconds, a feat which "would have taken …
Such is usually the case when NASA says something is "equivalent to" or "as hard as" something else. Nonetheless it's rather an impressive achievement. Think about it for a second. They're hitting a small target from 250 miles away with a relative velocity of 17,100mph. While a human holding a laser pointer and training it on the end of a human hair while walking is basically impossible it's not a wholly inaccurate simile for what they've done here.
I disagree about the random movement. You see, we Southern Californians know about Wrightwood. It lies directly over the San Andreas Fault which periodically has very random movements! We have many earthquakes everyday, most of them being less than 3 magnitude which no one even feels. But I'm sure it affects the laser beam. I just wonder why they chose such a seismically active area for the equipment.
NASA's claim is wholly misleading. A 1 metre diameter receiving dish might well subtend approximately the same angle as the diameter of a human hair at 30 feet, but that's not the most important factor. What is far more important is the degree of divergence of the laser beam, which you can guarantee is far more than a metre by the time it hits the Earth's surface.
Human hair isn't a great standard measure, as the size varies a lot. However, if we take 2/1000th of an inch, it will subtend an angle of about 5 micro-radians. To a good degree of approximation, laser beam divergence depends on the minimum (waist) diameter of the beam and the (1,500nm) wavelength. If we take a reasonable beam "waist diameter" of 1mm, that gives a beam radial divergence angle of about 470 micro-radians. In other words, the degree of precision required is, perhaps, only about 1/100th of that claimed. Also, of course, the ISS moves in a rather smoother, and predictable manner than a human being walking.
To put this in perspective, it's reckoned that competition level target rifles can manage accuracies as high as 100 micro-radians, albeit, not hand held of course.
Plug in the minimum ISS orbital height of the 350km, and you get a beam diameter of about 160 metres, so the receiver only has to be in that area. Of course the transmission was likely at a considerably greater distance than the minimum ISS orbital height, but then that doesn't change the degree of accuracy required.
ps. sorry about the mixed units, but that's NASA for you, quoting wavelengths in nm and distances in feet. You'd have thought by now, having crashed a probe due to mixing up systems, they'd have stuck with the metric system.
I think that the performance improvement is oversold. It seems to me that the reception of this video over RF in the UK would've taken 10 minutes, whereas the reception of light from space, passing through the atmosphere would've taken from 2 days to 2 weeks, depending on your luck with the weather.
Artist's impression of OPALS' beam wandering. ==>
Artists seem to be such impressionable people. Perhaps the wrong kind of artist was chosen to render an impression. Next time, they should use, I don't know, a mime. Yes, a mime! At least the lack of sound would be accurate, because in space, no-one can hear you scream...
I'm more concerned that those hooligans on the International Space Station just lased the earth (though, in mitigation, they were aiming at California).
OT: Any word on what the video in question was? Inquiring minds want to know. My vote goes to "A video about a video being lased down to California".
>>Matt Abrahamson, OPALS mission manager at NASA's Jet Propulsion Laboratory, enthused: "It's incredible to see this magnificent beam of light arriving from our tiny payload on the space station."
Well, Matt, us mere meatbags will never know the pleasure of seeing wavelengths beyond 800nm, so you don't have to rub it in.
Cyborgs, they can act so f'n superior at times.
Looking at the NASA promo video - the laser was actually on the ground and the orbital payload was an ultra-precisely-wobbling mirror to modulate and reflect the carrier beam back to Earth...
P.S. Actually, it looks like both the ground and the flight systems use their own lasers, with the ground beam being used for aiming the flight laser...
Not wanting to spoil their enthusiasm, but considering that the German space research center managed a laser comms linke between two orbiting satellites back in 2008, and achieved a data rate of some 5.6 Gigabits/s, I'd say what NASA did just now is a bit old hat.
Of course, ESA has taken this a step further and they're now using laser comms terminals via a relay saetellite, so you don't even need to be overhead California anymore to laze someone there:
Of course, if the astronauts in the ISS had hand-pointe that beam, it'd be something completely different...
Known as the "Hall Effect"....reversing the polarity of a polarized light beam causes INSTANTANEOUS reversal of polarity at the source and at the terminal end. Having a positive and negative poles gives two positions for a digital transmission. Searching for ancient technology broadcasts using speed of light radio signals, like SETI has been doing for thirty years, is like looking for Native American smoke signals. Higher conscious life forms use a more rapid form of communication. See Cosmology at FauxScienceSlayer for more interesting expansions of your conscious.
I think the news helicopters have this one beat. They have a camera with a super long zoom lens that has to cope with vibrations in 3 dimensions, yet can zoom in from 500 meters altitude and lock on to a nearly vibrationless view of a person on the ground and easily tell if he's holding a pistol or a microphone. And they don't have any laser beam to guide them.
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