NASA Achieves Laser Communication With Lunar Satellite 99
New submitter EngnrFrmrlyKnownAsAC writes "Communicating with lasers has become the hot new thing. While most researchers are seeking faster throughput, NASA set its sights in a different direction: the moon. They recently announced the first successful one-way laser communication 'at planetary distances.' What did they send? An image of the Mona Lisa, of course. 'Precise timing was the key to transmitting the image. Sun and colleagues divided the Mona Lisa image into an array of 152 pixels by 200 pixels. Every pixel was converted into a shade of gray, represented by a number between zero and 4,095. Each pixel was transmitted by a laser pulse, with the pulse being fired in one of 4,096 possible time slots during a brief time window allotted for laser tracking. The complete image was transmitted at a data rate of about 300 bits per second.'"
Re:This is awesome (Score:5, Informative)
Because they can't reliably send individual bits. If you RTFA (I know, I know...) it shows that there is a fair bit of error and quite a few lost pixels. Rather than sending bits they send a pulse of a certain length per pixel, and if the edge of that pulse is distorted somehow they just lose some intensity resolution and don't end up with totally corrupted digital data.
It's kind of analogue. The timing method they use is a bit like PWM with one cycle per pixel, and actually there are far fewer than 4096 shades reliably transmissible, that is just the range they measure.
Re:This is awesome (Score:4, Informative)
The timing method they use is a bit like PWM with one cycle per pixel, and actually there are far fewer than 4096 shades reliably transmissible, that is just the range they measure.
It would actually be PPM [wikipedia.org] (pulse-position modulation).
Re:Fermi Paradox (Score:4, Informative)
https://en.wikipedia.org/wiki/Diffraction-limited_system [wikipedia.org]
It doesn't matter. Laser. Radio. Gama ray.Doesn't matter. At these distance the systems are, no matter how well focused, diffraction limited. Just like we can't build a mircoscope to see infinitely deep into the smal we cannot build a laser com with perfect focus. Diffraction wins. We can cheat a little, but not over these distances.
We could see laser flashes just as easily as hear radio waves from parabolic dishes.