NASA Laser Communication System Sets Record with Transmissions to and from Moon

http://www.nasa.gov/press/2013/october/nasa-laser-communication-system-sets-record-with-data-transmissions-to-and-from/#.Ume155Q4W5s

by derpenxyne • 13 years ago

144 79 13 years ago

nasa.gov

2close4comfort4• 13 years ago

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

Ellipsis753• 13 years ago

To NASA or the moon? I wouldn't think there's much good stuff to download from the moon but if you're on the moon then I'm sure there's plenty of cat videos you can download from Earth over that kind of connection.

2close4comfort4• 13 years ago

With connectivity about 600x better than the average rural broadband connection. The direct application to purely to solve a problem it is great to see what we can do. That said it is kind of a bummer to see that once commerce gets involved that 1.5 dsl is good enough to claim your check from the FCC...but leaves a lot to be desired when trying to interact with the Modern world.

milliams• 13 years ago

The latency would probably kill you though.

throwaway91013• 13 years ago

Or the cold.

2close4comfort4• 13 years ago

LAZERS RULE!

erid• 13 years ago

The latency must be huge though.

paulgerr• 13 years ago

2.6 second round trip for light, so pretty huge ping times.

nraynaud• 13 years ago

That's why I always found moon people to be lame at quake!

rel• 13 years ago

I worked at NASA (AMES) on a project to put small mobile robots on the moon. Our biggest goal was to create a tele-operated mode where from earth we could actively roam around the moon. The hardest part was that factoring in a 4 second delay from our commands to the network stacks to the rover on the moon, and then that video from the moon back to our computers and then creating good judgement. A 4 second delay seems insignificant but is in fact detrimental to intuitive controlling.

gprasanth• 13 years ago

Unless, our technology advances to always do stuff a couple of seconds in advance by knowing our intents.

toomuchtodo2• 13 years ago

Or we figure out how to use quantum entanglement or gravity as information transfer mechanisms.

rodh• 13 years ago

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.

arnarbi• 13 years ago

interplanetary internet

Winternet!

drcross• 13 years ago

http://en.wikipedia.org/wiki/Interplanetary_Internet

TylerE• 13 years ago

Moon to earth should be workable enough...average speed of light roundtrip will be about 2400ms.

stonemetal2• 13 years ago

TCP uses RTT time to try to guess network congestion. Having worked on software that almost always had satellite hops, it sucks on long links. Primarily what you notice is that you get limited bandwidth because the TCP stack is throttling to prevent congestion. Not sure what RTT is to the moon and back but you would want to use something like TCP Hybla that does network congestion control a different way.

http://en.wikipedia.org/wiki/TCP_congestion-avoidance_algori...

agashka• 13 years ago

wouldn't it be less then RF? Radio Frequencies travel at the speed of sound, while lasers would be light-speed, from my understanding. I'd really love some insights on this!

EDIT: I still need to learn a lot on signals!

robin_reala• 13 years ago

Radio waves are EM waves, same as light. They all travel at c.

zargon• 13 years ago

Radio is not sound, it is an electromagnetic wave that travels at the speed of light.

robodale• 13 years ago

oh boy.

16s• 13 years ago

Speed of light, not sound.

rzimmerman• 13 years ago

Still not as bad as Voyager, where the ping time is an entire weekend.

7952• 13 years ago

The latency is sufficient that you could actually use space as a data storage medium. At 622Mbps there is 1,596Mb in transit at any given time.

ck23• 13 years ago

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...

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.

colanderman• 13 years ago

Depends on the focal length.

femto• 13 years ago

The width of the beam is determined by diffraction, caused by the finite aperture of the laser:

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/cirapp.htm...

wyager• 13 years ago

Even if the laser is extremely low divergence (maybe NASA is using some huge gas laser or something), atmospheric effects will still cause significant widening of the beam.

DanBC• 13 years ago

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

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.

ck23• 13 years ago

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.

noselasd2• 13 years ago

Somewhere on a cover event when they launched LADEE, someone asked that, which they answered the receiver had to be within a spot with a (radius or diameter, I can't remember) of 3 km on earth

Kabacaru• 13 years ago

So fine for torrenting crappy for gaming?

VLM• 13 years ago

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• 13 years ago

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

bdegman• 13 years ago

Also once this is a real problem, hopefully there'll be enough people up there to fill up their own Battlefield servers with just moon players.

aryastark• 13 years ago

"damn lagkilling moon noobs!"

peeters• 13 years ago

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.

throwaway91013• 13 years ago

Or FEC, and optionally reliable re-transmission. TCP won't work well due to the huge latencies involved.

kordless• 13 years ago

TCP can work decently with high latency, it just needs bigger buffers and to be tuned for the delay. FEC is awesome.

pilom• 13 years ago

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.

zhouyunqing• 13 years ago

I think it's the problems of the receiver. On the earth, we can build more elegant noise reduction systems, larger amplifiers and larger antennas.

trentmb• 13 years ago

More, fancier equipment on Earth to receive and perform signal analysis?

It's easier to get a new sensor online on Earth than on the moon.

tobylane• 13 years ago

Possibly it is due to the much wider dishes here. You can speak quietly to someone using a gramophone-like hearing aid.

anologwintermut• 13 years ago

It's probably easier to receive with better, larger, and heavier equipment and more power for processing. Couple that with the fact that control data is not high bandwidth, and you can see why it ended up that way.

commdude• 13 years ago

Two likely reasons are the effect of Earth atmosphere at points where the laser beam is narrow, i.e. near the transmitting end, and a more sensitive receiver at the Earth ground station than on the spacecraft.

beloch• 13 years ago

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...

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.

InclinedPlane• 13 years ago

Firstly, the spacecraft is not a big data consumer so it makes sense to concentrate on downlink bandwidth. Secondly, the telescopes on Earth can be much larger and use much better equipment so it's easier to pick up weaker signals.

moocowduckquack• 13 years ago

The uplink is going to be used for control data, which might be why they made the point of it being tested to be error free.

jbarrec• 13 years ago

That's some lag!

joshdance• 13 years ago

There is an xkcd that seems awfully appropriate here - http://what-if.xkcd.com/13/ - shooting lasers at the moon.

s800• 13 years ago

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

jpmattia• 13 years ago

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.

dghughes• 13 years ago

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• 13 years ago

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

freifunk_berlin• 13 years ago

Background information: http://www.spaceflight101.com/ladee-lunar-laser-communicatio...

devx• 13 years ago

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

cdurr• 13 years ago

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

zaroth• 13 years ago

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...

NASA Laser Communication System Sets Record with Transmissions to and from Moon

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