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

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.

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

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.

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.

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

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.

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.

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.

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.

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

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.

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

> interplanetary internet

Winternet!

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

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...

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

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

oh boy.

Speed of light, not sound.

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

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.

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.

Or the cold.

LAZERS RULE!