
NASA Laser Communication System Sets Record with Transmissions to and from Moon - derpenxyne
http://www.nasa.gov/press/2013/october/nasa-laser-communication-system-sets-record-with-data-transmissions-to-and-from/#.Ume155Q4W5s
======
ck2
How huge is a 1mm beam when it hits the moon (and visa-versa) ?

I wonder if wolframalpha can tell me...

Actually, I don't even know how to ask it the proper question.

Can anyone make this formula work in wolfram?

[http://laserpointerforums.com/attachments/f51/17952-calculat...](http://laserpointerforums.com/attachments/f51/17952-calculate-
expansion-laser-beam-schematic_laser_beam.jpg)

update:

I think I figured this out as

    
    
          ((λ * L) / d)* 2
    
          (((630nm in meters)*(380000km in meters)) / (1mm in meters)) * 2
    

Which would be a nearly 500km wide for a 1mm beam on earth from a red laser at
630nm

The shortest wavelength laser I can find online is experimental violet at
400nm which would make a 300km beam on the moon.

~~~
DanBC
Have you seen this "What If?" post by xkcd?

[http://what-if.xkcd.com/13/](http://what-if.xkcd.com/13/)

It doesn't talk about the size of the beam as it hits the moon, but it's
interesting noodling none-the-less.

~~~
ck2
The end of that had me in tears. I think I like the "what-if" section more
than the comics!

Gems like

 _Unfortunately, the laser energy flow would turn the atmosphere to plasma,
instantly igniting the Earth’s surface and killing us all._

and another article

 _It 's a shame humans wouldn't live this long, because at this point,
something really neat would happen._

~~~
b_emery
That one links to this: [http://what-if.xkcd.com/8/](http://what-
if.xkcd.com/8/) which devolves into a hilarious analysis of 'how to get 7
billion people out of Rhode Island'.

~~~
ck2
I remember reading that one and the hilarious part is, it is not how it is
supposed to start off.

He seems at first disappointed other sites have attempted to answer the
original question, so in his own wonderful style he tries to solve the other
weird engineering aspects of it.

He seems cut from some of the same cloth as Douglas Adams but with advanced
math knowledge.

~~~
b_emery
Agreed, he must be one of the 10 most creative people on earth.

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peeters
The article cites 622 Mbps down, 20 Mbps error-free up. Is the implication
that the 622 Mbps down is before error correction?

Either way, it's pretty incredible. Even if the 622 Mbps down isn't completely
error free, but maybe 99%, you just need to use a video compression algorithm
that is error tolerant.

~~~
pilom
Why is it easier to transmit from the moon to earth? Here we have more power
and larger/better/faster/more tuned lasers than the space vehicle does. Seems
like getting earth's transmit rate way up would be relatively easy while
getting the vehicle's transmit rate up would be really really hard.

~~~
beloch
I don't work in this field, but here's an educated guess (or two).

Lasers are not perfectly coherent, parallel beams. They're close but, over the
distance between the Earth and the moon, even the most powerful, carefully
adjusted and perfectly focused laser will expand much like a flashlight beam.
Take the laser pointer you use for presentations and shine it across the room
and you'll see a substantially larger spot size than if you shine it on your
hand. You get the idea. Getting a strong signal means you need a lot of area
in your receiver. A big dish will help at either end, but it's harder to send
a big dish to the moon.

Another huge factor is going to be atmospheric distortion. The atmosphere is
full of eddies and currents of air at different temperatures and relative
motions. Light going through it gets refracted somewhat chaotically. If you
try to lock a free-space laser signal in over a distance of even just a few
tens of kilometers through air, it's actually a bit of a challenge because
that (rather large) spot will jump all over the place. Light traveling from
the Earth to the moon goes through atmosphere at the source and gets bent and,
by the time it reaches the moon, will be all over the place. Light traveling
the other way travels through vacuum until it's very close to its destination.
Consider trying to shine a laser pointer up out of a pool of choppy water and
hit a distant target vs trying to hit a specific spot in the pool from that
target. It's a lot easier if the random bending happens nearer to the
destination! There are a lot of things you can do to overcome this, but they
probably all reduce the speed at which you can transmit.

Edit: I just read this: [http://www.spaceflight101.com/ladee-lunar-laser-
communicatio...](http://www.spaceflight101.com/ladee-lunar-laser-
communication-demonstration.html)

The sending and receiving modes are completely different. The moon craft sends
polarization encoded data using a continuous beam. This is received on the
ground by highly sensitive single-photon-detectors hooked up to multiple large
scopes. Sensitive, but bulky and components have to be cryo-cooled. However,
this allows for the use of a relatively weak laser at the moon and
polarization modulation means you can probably get pretty good bandwidth,
provided the atmosphere isn't so turbulent that it requires you to check your
states too often. (Polarization will be randomly transformed by the
atmosphere, so you'd determine which polarization is which periodically as a
part of communications. This is presumably why they have multiple receivers
that are probably measuring in different bases so each bit from the moon can
be tomographically reconstructed.)

The ground-station sends pulse-position modulated (aka time-bin) encoded data
with a fairly powerful laser. Say you wanted to signal a friend yes or no
without talking. You could synchronize a pair of stop-watches and, at an
agreed time, you throw a ping-pong ball at his head. If you want to say yes,
you throw it right on time. If you wan to say no, you throw it a bit late.
Obviously, your ability to distinguish early from late limits how fast you can
send data this way. If it was a windy day with random gusts, the ping-pong
ball would arrive a bit randomly, so you'd need to make the delay required to
declare a ball "late" somewhat longer. Atmospheric distortion probably limits
bandwidth to the moon for this reason.

Both polarization and time-bin encoding can be made to work both ways I
suspect, but there might be reasons to choose one over the other for sending
vs receiving. It does seem like the receiving station for a polarization
encoded signal might be more bulky due to requiring multiple detectors
operating on different polarization bases. I'd love to ask the folks at NASA
about this! It might also be they just wanted to test both methods, since this
really is an experiment more than anything.

~~~
dysfunction
And fortunately in most use-cases, we'd prefer a spacecraft to have more
upload bandwidth to Earth than download bandwidth from Earth. The largest data
NASA would send to the craft would probably be software updates, megabytes or
a few gigabytes at most, while the craft might want to send thousands of
images and even videos, gigabytes or terabytes of data, limited only by its
energy budget and length of transmission window. (Note: I don't work in this
field either).

------
joshdance
There is an xkcd that seems awfully appropriate here - [http://what-
if.xkcd.com/13/](http://what-if.xkcd.com/13/) \- shooting lasers at the moon.

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erid
The latency must be huge though.

~~~
rodh
I'd imagine TCP, as a protocol starts to fall over with moon-to-earth
latencies.

Or would it? Perhaps simply increasing the retransmission timer would give us
interplanetary internet. I wonder.

~~~
drcross
[http://en.wikipedia.org/wiki/Interplanetary_Internet](http://en.wikipedia.org/wiki/Interplanetary_Internet)

~~~
noselasd
Code from NASA here, [http://sourceforge.net/projects/ion-
dtn/](http://sourceforge.net/projects/ion-dtn/)

------
freifunk_berlin
Background information: [http://www.spaceflight101.com/ladee-lunar-laser-
communicatio...](http://www.spaceflight101.com/ladee-lunar-laser-
communication-demonstration.html)

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cdurr
Should be "laser" with the quotes. Also, LLCD isn't catchy - the project
should be renamed to "The Allan Parsons Project".

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Kabacaru
So fine for torrenting crappy for gaming?

~~~
VLM
I'm annoyed that gaming is defined solely as multi-player FPS twitch games.
Astronauts / colonists into chess, go, poker, real role playing games, pretty
much anything that happens in a casino, single player of any sort, wouldn't
really notice.

~~~
knowaveragejoe
Besides, the speeds mentioned here would be more than enough for multi-player
twitch FPS too.

~~~
jewel
A game like an FPS doesn't need much speed (they can work with modem-level
bitrate). They instead depend on having low latency, or ping time. Since the
moon is so far away, it takes a signal 1.3 seconds to travel to or from it.

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2close4comfort
That is better than the connect at my house. I am moving there!

~~~
milliams
The latency would probably kill you though.

~~~
throwaway9101
Or the cold.

~~~
2close4comfort
Or lack of atmosphere...but I digress. Plus I am sure that latency would be
better than earth based sat transmissions

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zaroth
Would it help at all to signal over RF to a low-orbit relay, and then go
optical just between two points in space? Much less atmosphere to contend
with...

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s800
622mbps is interesting. Same as OC-12c... Something COTS?

~~~
jpmattia
> _Something COTS?_

Probably outside of the transmission/receiver. Choosing a conventional speed
allows them to use plenty of off-the-shelf electronics (FEC chips, etc) and
then focus on the transmission.

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dghughes
I remember reading on reddit or maybe it was here on HN that a laser is not a
thing it's a method i.e. LASER, we use LASER not a laser.

~~~
aggie
Language evolves. Unless you're writing a technical document it's usually best
to go with what is commonly used and understood.

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devx
Couldn't we set-up a laser system to detect asteroids from farther away? Or
would that be pretty inefficient?

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jbarrec
That's some lag!

