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.
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.
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
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.
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.
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.
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.
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).
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.
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.
Solution for the lag will be a special category "bot play". The bot will not be programed but will behave like a black box, computer ai will record you every time you play in a local game and will slowly build the bot around. Then the bots will compete on a server, maybe separated by categories determined by previous recording time.
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.
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.
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!
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.
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.
One speed interferes with multi-player twitch FPS here, and it's not the bandwidth - it's the speed of light. Try playing an FPS with 2+ seconds of lag and it'll be very, very unpleasant.
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.
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.
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...
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.
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
Which would be a nearly 500km wide for a 1mm beam on earth from a red laser at 630nmThe shortest wavelength laser I can find online is experimental violet at 400nm which would make a 300km beam on the moon.