A Laser Message from Space(science.nasa.gov)
science.nasa.gov
A Laser Message from Space
http://science.nasa.gov/science-news/science-at-nasa/2014/18jun_opals/
104 comments
It was referring to the violence of apartheid. You left out the opening lines mentioning the shattering of shock windows, the bomb in the baby carriage, and the soldiers on the side of the road, which change the entire meaning of the lines you posted.
My guess is that those lines didn't remind the author of lasers in space.
This is a pretty big deal. At the moment there are two main methods to communicate with the ISS, TDRSS and UHF.
UHF can only be used when the ISS is over the US.
TDRSS is a shared satellite network between several government agencies. NASA receives "allotments" of time with the network. So if the ISS is outside of UHF range, and another agency has the TDRSS the ISS goes black - we lose all comm with it.
A network of laser receivers across the globe could potentially eliminate the current "black outs" NASA has when communicating with the ISS.
UHF can only be used when the ISS is over the US.
TDRSS is a shared satellite network between several government agencies. NASA receives "allotments" of time with the network. So if the ISS is outside of UHF range, and another agency has the TDRSS the ISS goes black - we lose all comm with it.
A network of laser receivers across the globe could potentially eliminate the current "black outs" NASA has when communicating with the ISS.
I find this kind of surprising. First off, I'd figure that NASA would set up a few UHF stations around the world in locations with decent internet access (friendly countries or military bases), and route comms via that. Maybe you don't have total coverage, but at least you don't have to wait an hour before it's back over the US.
Secondly, there many more options available than TDRSS or UHF. Amateur radio enthusiasts frequently make contact with astronauts (2 meter and 70cm bands, IIRC). While that doesn't really solve the blackout problem, I find it hard to imagine that there isn't a lower-frequency radio which could maintain voice contact with the ISS.
Secondly, there many more options available than TDRSS or UHF. Amateur radio enthusiasts frequently make contact with astronauts (2 meter and 70cm bands, IIRC). While that doesn't really solve the blackout problem, I find it hard to imagine that there isn't a lower-frequency radio which could maintain voice contact with the ISS.
I believe NASA does have a few UHF stations elsewhere, or at least used to. As of a 1998 document about the shuttle program [1], they listed 6 UHF air-to-ground stations, 3 of them in other countries. The U.S.-based ones were in Guam, Hawaii, and Florida; and the others were in Ascension Island (a British dependency halfway between Africa and South America), Bermuda (a British dependency off the east coast of Canada), and Dakar, Senegal.
[1] http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/s...
[1] http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/s...
Just a clarification: Bermuda is not really close to any major coastline, and certainly not the Canadian one.
https://www.google.com/maps/place/Bermuda/@31.2546129,-41.52...
https://www.google.com/maps/place/Bermuda/@31.2546129,-41.52...
850 miles (closest point is in North Carolina) isn't necessarily far.
When someone says "off the coast", I would not expect 850 miles of distance. And, even more, when they say "off the east coast", I would not expect it to be almost directly south of the east coast. Those were the points I was attempting to relay.
Other more obscure methods of communicating with the ISS include the Russian Lira and Regul systems as well as the Japanese Ka-band dish on the external lab.
Losing all communications with the ISS doesn't happen frequently though does it? Almost never from what I can tell.
http://www.nasa.gov/mission_pages/station/expeditions/expedi... Seems like the last time it happened and it was big news. I can't imagine how scary it would be to be in space without any way to communicate with earth. I guess the crew return vehicle would still be usable and the station would still operate as normal for a good while, but it would still be very scary.
Here are some links to the live status of those systems:
http://spacestationlive.nasa.gov/displays/cronusDisplay2.htm... UHF
http://spacestationlive.nasa.gov/displays/cronusDisplay3.htm... S-band
http://spacestationlive.nasa.gov/displays/cronusDisplay4.htm... Ku-band
Click "Show Table" in the bottom right corner of that page to see the raw data for each display.
It would be nice to see some data-rate stats on those connections. Also a better dashboard would be fun to build for the ISS, some interesting data points are available.
Also apparently the high-speed Ku-band is available about 50% of the time. http://www.reddit.com/r/IAmA/comments/18pik4/i_am_astronaut_...
http://www.nasa.gov/mission_pages/station/expeditions/expedi... Seems like the last time it happened and it was big news. I can't imagine how scary it would be to be in space without any way to communicate with earth. I guess the crew return vehicle would still be usable and the station would still operate as normal for a good while, but it would still be very scary.
Here are some links to the live status of those systems:
http://spacestationlive.nasa.gov/displays/cronusDisplay2.htm... UHF
http://spacestationlive.nasa.gov/displays/cronusDisplay3.htm... S-band
http://spacestationlive.nasa.gov/displays/cronusDisplay4.htm... Ku-band
Click "Show Table" in the bottom right corner of that page to see the raw data for each display.
It would be nice to see some data-rate stats on those connections. Also a better dashboard would be fun to build for the ISS, some interesting data points are available.
Also apparently the high-speed Ku-band is available about 50% of the time. http://www.reddit.com/r/IAmA/comments/18pik4/i_am_astronaut_...
Can't speak for S-band, but Ku-band data rates are 25 Mbps Earth-to-ISS and 300 Mbps ISS-to-Earth, which is the maximum supported by TDRSS Ku-band links. Data rates may drop a bit depending on the individual satellite involved, configuration, etc.
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You can see when the ISS is in range by watching the live video stream. From what I have seen it's rarely out of contact and they obviously have bandwidth to spare for that.
Elaborating on TDRSS a little bit - the ISS has allotments on both S- and Ku-band transponders (there are several per satellite) and hands off between satellites (TDRSS is geostationary) as it orbits, it's rare that another TDRSS customer has something important enough going on that the ISS gets no allocation at all for more than an orbit.
IMHO, the real application for laser communications is for high bandwidth inter-satellite links.
IMHO, the real application for laser communications is for high bandwidth inter-satellite links.
> IMHO, the real application for laser communications is for high bandwidth inter-satellite links.
Iridium-esq constellations.
Iridium-esq constellations.
One of the side effects of the laser is that only the receiving station it is pointed to can receive it. That is both good and bad, if you're partners running the ISS don't trust you, they might worry about what you're sending on the communication channel that they cannot hear.
Interesting. If a 2.5 watt laser was used for communication from the ISS to Earth what wattage would be needed to send a high-bandwidth message back from Mars (or Titan)? How does that compare to traditional radio-based communication?
This point-to-point communication is also interesting in relation to the Fermi Paradox. It seems likely that a sufficiently-advanced race would beam their messages directly to their recipients rather than wasting energy (and privacy) transmitting an omnidirectional signal.
This point-to-point communication is also interesting in relation to the Fermi Paradox. It seems likely that a sufficiently-advanced race would beam their messages directly to their recipients rather than wasting energy (and privacy) transmitting an omnidirectional signal.
Well, if the ISS is around 420km up, and Mars is on average 225 million km away, and all other parameters of the laser are identical, then the laser would need to be around 700GW.
However, you could probably get away with a slightly less powerful laser if you decrease the bit-rate a little, improve the sensitivity of the receiver, and tighten the beam (which wouldn't be a problem if the laser is bigger). I got the impression that the 2.5W laser was way more powerful than necessary for basic communication.
Let's face it - Mars Explorer has a total power availability of around 450W, and it aims its dish at the earth with an accuracy of 0.04 degrees, and it manages to communicate. Replacing the radio with a laser may improve matters, but it may also require too much power.
However, you could probably get away with a slightly less powerful laser if you decrease the bit-rate a little, improve the sensitivity of the receiver, and tighten the beam (which wouldn't be a problem if the laser is bigger). I got the impression that the 2.5W laser was way more powerful than necessary for basic communication.
Let's face it - Mars Explorer has a total power availability of around 450W, and it aims its dish at the earth with an accuracy of 0.04 degrees, and it manages to communicate. Replacing the radio with a laser may improve matters, but it may also require too much power.
Here's a paper from NASA's (budget-axed) Mars Laser Communications Demonstration. They estimate 10-100 Mbps bandwidth, on a 5 W laser system (300 W pulsed, but only drawing 5 W from the PSU).
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38254/1/04... and also
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38024/1/04...
context: https://en.wikipedia.org/wiki/Mars_Telecommunications_Orbite...
There's a much more interesting paper somewhere that designs a system for interstellar lasers..
edit: I can't find what I'm thinking of, but this optical SETI experiment is relevant. It looked at 577 nearby stars (<50 parsecs = 163 light years), and claims it could detect a 50 kW (!!) optical laser pointed from one of them (presumably at a negligible bitrate).
http://arxiv.org/abs/astro-ph/0112479
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38254/1/04... and also
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38024/1/04...
context: https://en.wikipedia.org/wiki/Mars_Telecommunications_Orbite...
There's a much more interesting paper somewhere that designs a system for interstellar lasers..
edit: I can't find what I'm thinking of, but this optical SETI experiment is relevant. It looked at 577 nearby stars (<50 parsecs = 163 light years), and claims it could detect a 50 kW (!!) optical laser pointed from one of them (presumably at a negligible bitrate).
http://arxiv.org/abs/astro-ph/0112479
That's cool, thanks. Basically, the ISS laser experiment was not stretching the bounds of the method at all.
Put a proper telescope at the receiving end, and it changes the game completely. It's simply amazing how little light modern telescopes are capable of detecting and measuring.
Note that your arxiv paper describes detecting a 50kW laser that is diffraction limited for a 10m aperture. That in itself would be a quite interesting piece of equipment to build.
Edit: Interesting - the abstract seems to have been mis-copy&pasted, with 60kW instead of 50kW.
Put a proper telescope at the receiving end, and it changes the game completely. It's simply amazing how little light modern telescopes are capable of detecting and measuring.
Note that your arxiv paper describes detecting a 50kW laser that is diffraction limited for a 10m aperture. That in itself would be a quite interesting piece of equipment to build.
Edit: Interesting - the abstract seems to have been mis-copy&pasted, with 60kW instead of 50kW.
I found what I was thinking of:
"The Technical Case for Optical and Infrared SETI"
http://seti.harvard.edu/oseti/tech.pdf
http://seti.harvard.edu/oseti/
"The Technical Case for Optical and Infrared SETI"
http://seti.harvard.edu/oseti/tech.pdf
http://seti.harvard.edu/oseti/
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> Well, if the ISS is around 420km up, and Mars is on average 225 million km away, and all other parameters of the laser are identical, then the laser would need to be around 700GW.
What if we tried more power?
http://what-if.xkcd.com/13/
(I am a bad person. I am sorry.)
What if we tried more power?
http://what-if.xkcd.com/13/
(I am a bad person. I am sorry.)
Well, presumably since laser light is coherent, you only need to worry about the Earth's atmosphere. In this case, it seems that a 2.5 watt laser would be sufficient, assuming you can point it at Earth precisely and ignoring Mars' atmosphere.
There is still a problem with regards to the Fermi paradox: sure you'd want to beam directly, but where do you point your lasers? You only know by observing the not-so-coherent light leaving Earth and arriving at your planet, which may be very far away. You haven't really solved anything because you have to know where to point your laser in the first place.
There is still a problem with regards to the Fermi paradox: sure you'd want to beam directly, but where do you point your lasers? You only know by observing the not-so-coherent light leaving Earth and arriving at your planet, which may be very far away. You haven't really solved anything because you have to know where to point your laser in the first place.
>you only need to worry about the Earth's atmosphere
I assume you're meaning this is the main source of attenuation (signal loss) if we assume the space between Earth and Mars is an empty vacuum. This (technically called scattering) is not the main source of loss for such a system.
Coherent just means the light is all in phase, it doesn't mean it doesn't spread out as it travels. If it didn't spread out at all, your receiver on earth would be just as wide as the laser transmitter (about the size of a coin), and you'd have to be locked on the entire time... not very practical. It looks like NASA has intentionally focussed their LASER into a wide cone for precisely this reason.
The distance from Mars would cause the beam (cone) to spread out too much, thereby reducing the power received on earth. You would have to increase your laser power on Mars to compensate for this or make the beam more directional with some focusing optics.
I assume you're meaning this is the main source of attenuation (signal loss) if we assume the space between Earth and Mars is an empty vacuum. This (technically called scattering) is not the main source of loss for such a system.
Coherent just means the light is all in phase, it doesn't mean it doesn't spread out as it travels. If it didn't spread out at all, your receiver on earth would be just as wide as the laser transmitter (about the size of a coin), and you'd have to be locked on the entire time... not very practical. It looks like NASA has intentionally focussed their LASER into a wide cone for precisely this reason.
The distance from Mars would cause the beam (cone) to spread out too much, thereby reducing the power received on earth. You would have to increase your laser power on Mars to compensate for this or make the beam more directional with some focusing optics.
Larger than the power issue, it seems like latency would be a big challenge that would need to be solved to achieve Mars <=> Earth communication. There's already a very high latency (about a day) with this proof-of-concept while waiting for the ISS to pass over the same station in California again.
The latency would be significantly longer than a day for two single transceivers, one on Earth/ISS, and one on Mars, to align again once they've broken alignment—Earth-Mars oppositions happen about every two years.
A solution to both problems would be more transceivers: the Earth to ISS station problem could be addressed by setting up more transceivers on earth underneath the ISS's ground tracks, enabling more frequent transmissions. The Mars solution would probably require a lot more cost/effort/creativity, but a mesh of transceivers, all in concentric orbit around the Sun between the Earth and Mars, should be able to relay data between the two points with a best-case 4- to 30-minute latency (bounded by speed of light, distance varies between Mars and Earth on a two-year period)
The latency would be significantly longer than a day for two single transceivers, one on Earth/ISS, and one on Mars, to align again once they've broken alignment—Earth-Mars oppositions happen about every two years.
A solution to both problems would be more transceivers: the Earth to ISS station problem could be addressed by setting up more transceivers on earth underneath the ISS's ground tracks, enabling more frequent transmissions. The Mars solution would probably require a lot more cost/effort/creativity, but a mesh of transceivers, all in concentric orbit around the Sun between the Earth and Mars, should be able to relay data between the two points with a best-case 4- to 30-minute latency (bounded by speed of light, distance varies between Mars and Earth on a two-year period)
This is mentioned in the comments in the TCP implementation of the Linux kernel:
373 * ... Note that 120 sec is
374 * defined in the protocol as the maximum possible RTT. I guess
375 * we'll have to use something other than TCP to talk to the
376 * University of Mars.
Why would you need to wait for opposition? As long as you have direct line of sight to Mars, that is good enough. Granted, the latency is only going to increase when not in opposition, but they already have ways to deal with that, such as IPN. [1]
Conjunction, of course, would be a much tougher feat without some sort of relay system. I'm sure that wouldn't be insurmountable though, and I think they built that into IPN as well, just like how the internet can route around problem areas.
[1] http://en.wikipedia.org/wiki/Interplanetary_Internet
Conjunction, of course, would be a much tougher feat without some sort of relay system. I'm sure that wouldn't be insurmountable though, and I think they built that into IPN as well, just like how the internet can route around problem areas.
[1] http://en.wikipedia.org/wiki/Interplanetary_Internet
This is one of the things SETI struggles with. Their entire system is designed around the assumption that radio waves are going to be the way these advanced civilizations communicate. It might just be likely they use lasers or other methods and maybe treat radio as an antiquated backup.
>Their entire system is designed around the assumption that radio waves are going to be the way these advanced civilizations communicate.
Yes, but don't think of it like Sweet 98's greatest hits. SETI specifically looks in the hydrogen frequencies because they're less noisy. This goes with the assumption that if they're at least as smart as us, they'll have similar astrophysics knowledge, and therefore know that a radio signal there would be easier to pick out against background.
Yes, but don't think of it like Sweet 98's greatest hits. SETI specifically looks in the hydrogen frequencies because they're less noisy. This goes with the assumption that if they're at least as smart as us, they'll have similar astrophysics knowledge, and therefore know that a radio signal there would be easier to pick out against background.
Using line of site communication such as laser would probably not be the best way for interstellar communications. Largly because as empty as the universe is. It's hard to shoot a straight line in any direction and not hit something, and also the high energy requirements.
Lasers are only being used in this case for bandwidth issues not latency issues. As both lasers, and radio travel at the speed of light.
SETI is using the most likely method of detecting communications as we don't know of any faster means to communicate than EMR.
Lasers are only being used in this case for bandwidth issues not latency issues. As both lasers, and radio travel at the speed of light.
SETI is using the most likely method of detecting communications as we don't know of any faster means to communicate than EMR.
Complete tangent of course, but I wonder if aliens using laser communication methods might be why we aren't being bathed in radio waves.
That's one of the proposed solutions to the Fermi Paradox. Human TV signals are already dying out as they get supplanted by cable/fiber, and what's left like cell phones is constantly being engineered to use less power.
That seems unlikely to me. Why wouldn't at least a few alien civilisation keep broadcasting radio signals just to let others know they are there too ?
Not for very long. http://xkcd.com/1377/
Maybe there a much better ways of doing so that we discover in another couple decades? Who knows.
Maybe the falloff in output has convinced them our civilization is dying and not worth contacting.
Radio waves go in multiple directions, lasers can only work in short range, unless you find a superb trick to keep the alignment perfect at great distances.
Actually, I think this is less of a problem than you think. Lasers have significant divergence, see for example the Marcy paper an astro-ph that was referenced elsewhere in this thread. Telescopes can already track targets at the diffraction limit, so it's no different to send a laser.
What is different is that you have to know the relative proper motion of the target, since you have to "lead" the target just like you do when skeet shooting. I don't know whether the lead angle is significant, but proper motions of nearby stars have been measured (e.g. by Hipparcos http://en.wikipedia.org/wiki/Hipparcos) and calculating the resulting lead angle isn't difficult.
What is different is that you have to know the relative proper motion of the target, since you have to "lead" the target just like you do when skeet shooting. I don't know whether the lead angle is significant, but proper motions of nearby stars have been measured (e.g. by Hipparcos http://en.wikipedia.org/wiki/Hipparcos) and calculating the resulting lead angle isn't difficult.
I don't know if you need a "trick"... Just several orders of magnitude more precision?
Yeah, and just like for telescopes, when you need several orders of magnitude more precision, you need something huge to do so. Unless we find a way to build huge spaceships in space, it's not coming anytime soon.
Simple, you just keep tapping it with a rubber hammer until you get alignment.
Link to the JPL project page with more detail
<http://phaeton.jpl.nasa.gov/external/projects/optical.cfm>
Interestingly this is a Phaeton project. Phaeton projects are designed for early career engineers to jumpstart their experience. Very cool.
<http://phaeton.jpl.nasa.gov/external/ProgramOverview/home.cf...
<http://phaeton.jpl.nasa.gov/external/projects/optical.cfm>
Interestingly this is a Phaeton project. Phaeton projects are designed for early career engineers to jumpstart their experience. Very cool.
<http://phaeton.jpl.nasa.gov/external/ProgramOverview/home.cf...
And it got to the ISS via a Space-X Dragon! Lasers, privately funded spaceships, hello world...too much fun :)
finally, the future!
It will never be the future until I have a flying car, sorry.
We can't have flying cars till we can figure out autopiloted cars. I don't want drunk flying :(
Give Elon some time, damn.
How close are we to space-based laser ISPs? I imagine that you squeeze terabits of optical bandwidth into a single mirror "dish"..
The big problem is getting through the Earth's Atmosphere. Space observatories are nearly exclusively located on mountains or areas with little atmospheric 'pollution'. I'd imagine even the OPALS system can only be used when it's not too cloudy/rainy.
This technology does exist on land however. Free Space Optics[1] has been around for a while but hasn't taken off in a big way because it's less reliable than sending light down a fibre cable, even though it is cheaper.
[1] http://en.wikipedia.org/wiki/Free-space_optical_communicatio...
This technology does exist on land however. Free Space Optics[1] has been around for a while but hasn't taken off in a big way because it's less reliable than sending light down a fibre cable, even though it is cheaper.
[1] http://en.wikipedia.org/wiki/Free-space_optical_communicatio...
imagine a world where this was the norm. You wouldn't need to worry about outages due to falling trees or utility poles exploding or people screwing with your wires.
Then you step outside to look up at the sky and you lose your eyesight
Then you step outside to look up at the sky and you lose your eyesight
Day of triffids scenario aside, no you won't lose your sight from a 2.5W laser pointing downwards from 260 miles up. The power level is too small.
People think that lasers produce a parallel beam. They don't - they are diffraction limited by the width of the beam. The narrower the beam, the more it spreads out. For example, a 1cm wide beam produced by a perfect laser would spread out to around 20m wide when it hits the ground from 260 miles away. That's assuming visible green light - if it's infra-red (or even just red) then the spread will be wider.
People think that lasers produce a parallel beam. They don't - they are diffraction limited by the width of the beam. The narrower the beam, the more it spreads out. For example, a 1cm wide beam produced by a perfect laser would spread out to around 20m wide when it hits the ground from 260 miles away. That's assuming visible green light - if it's infra-red (or even just red) then the spread will be wider.
But it wouldn't work on a cloudy day, presumably.
[deleted]
Latency is a real problem. Distances to satellites in orbit are very long. Also, there is a scalability problem. How many ground targets can a satellite simultaneously provide service to? My guess is, not a lot.
This could be an incremental bandwidth (but not latency) upgrade over existing satellite internet service to remote areas (by transmitting to a single ground receiver that serves a local area), but that's about it.
This could be an incremental bandwidth (but not latency) upgrade over existing satellite internet service to remote areas (by transmitting to a single ground receiver that serves a local area), but that's about it.
"Distances to satellites in orbit are very long."
That's not a fundamental limit. Existing satellites have high latency, because they're sited at insanely high altitude -- ~36,000 km (6 earth radii; 120 light-milliseconds (-> 240 ms minimum round trip)). This is for engineering and economic reasons which aren't solid: one, because geostationary [0] orbits allow dumb dishes that can't track moving objects; and two, because it allows small satellite networks -- i.e. one satellite covering a whole continent -- commensurate with the small size of the market.
If instead you had a network of satellites at say 500-1,000 km (unjustified guess), the latencies could be no worse than a direct optical fiber.
edit: Here's a sophisticated diagram, https://i.imgur.com/t1SOVpZ.png
[0] https://en.wikipedia.org/wiki/Geosynchronous_orbit#Geostatio...
That's not a fundamental limit. Existing satellites have high latency, because they're sited at insanely high altitude -- ~36,000 km (6 earth radii; 120 light-milliseconds (-> 240 ms minimum round trip)). This is for engineering and economic reasons which aren't solid: one, because geostationary [0] orbits allow dumb dishes that can't track moving objects; and two, because it allows small satellite networks -- i.e. one satellite covering a whole continent -- commensurate with the small size of the market.
If instead you had a network of satellites at say 500-1,000 km (unjustified guess), the latencies could be no worse than a direct optical fiber.
edit: Here's a sophisticated diagram, https://i.imgur.com/t1SOVpZ.png
[0] https://en.wikipedia.org/wiki/Geosynchronous_orbit#Geostatio...
What you're describing is essentially a data version what the Iridium network provides. Iridium is a constellation of 66 satellites at 450 miles up. Unfortunately, when Iridium was launched, forward thinking wasn't part of the plan -- data is stuck at around ancient dialup modem speeds.
There were plans for similar services, such as Teledesic, which went nowhere. I guess that enough land-based internet covers the majority of the target market, so there isn't enough market left over to justify the cost of a high speed satellite data provider. Remember, in LEO orbit, the satellites have to be replaced after about 5 years or so (atmospheric drag, and they run out of booster fuel).
Lower cost to launch via Space-x reusable rockets may change the cost equations though.
There were plans for similar services, such as Teledesic, which went nowhere. I guess that enough land-based internet covers the majority of the target market, so there isn't enough market left over to justify the cost of a high speed satellite data provider. Remember, in LEO orbit, the satellites have to be replaced after about 5 years or so (atmospheric drag, and they run out of booster fuel).
Lower cost to launch via Space-x reusable rockets may change the cost equations though.
Solar powered boosters and spaceodynamic lifting bodies.
If you're talking lasers though, don't forget to factor in the time needed to reacquire a new satellite in once the existing one goes over the horizon. The shorter the orbit, the more often you'd have to do this.
I actually have zero idea how such a thing could work, which is why I'm asking here. :)
If they had a way to have almost unlimited ground targets (maybe using a rotating mirror, or something like https://www.kickstarter.com/projects/117421627/the-peachy-pr...), I could deal with the not so bad ~240ms lag for most of my communications (if you aren't gaming), especially if it's faster then my slow cable connection....
Speed of light: ~299,792 km / s Geostationary orbit distance from earth: ~35,786 km
Speed of light: ~299,792 km / s Geostationary orbit distance from earth: ~35,786 km
A grid of sats designing for blanketing the earth in comms is going to be at most 400 miles up, most of my packets go way further than 800 miles, so no, latency won't be an issue.
Yes, but how do they keep the sharks alive up there?
Space-to-ground laser communication is a neat trick, but you still have the problem that you need to have a line of sight to the receiver in order to use it. The good news is that you are not quite as limited by bandwidth during the time that you can see the receiver. It would be nice if we could get the sort of international cooperation that would allow for continuous contact, but unfortunately, the politicians in charge usually don't care that much about space research. We'd probably have better luck with one or more reflector satellites that could bounce the laser signal back to the receiver.
Space-to-ground laser communication is a neat trick, but you still have the problem that you need to have a line of sight to the receiver in order to use it. The good news is that you are not quite as limited by bandwidth during the time that you can see the receiver. It would be nice if we could get the sort of international cooperation that would allow for continuous contact, but unfortunately, the politicians in charge usually don't care that much about space research. We'd probably have better luck with one or more reflector satellites that could bounce the laser signal back to the receiver.
I'm guessing that this is way harder than the article makes it sound. The satellite is moving very fast and has to hit an extremely precise target, but adding on another layer of complexity, the atmosphere and temperature differences effect the transmission.
Affect. The laser effects the transmission, but the atmosphere affects it. But not much. The laser was aimed with an accuracy of 0.01 degrees. Astronomical seeing (the distortion due to atmospheric disturbance) is on the order of an arcsecond, or 36 times smaller than this.
Indeed, but the mount the projector and receiver on must be constantly calibrating movements and adjusting smoothly.
In achievement terms this is almost all software (once you take putting stuff in space for granted).
Think of proving it to work in theory, ground based prototypes etc.
Error detection is also interesting (though mostly solved but with many options available). High bandwidth laser comms have been around for a while. Originally developed for the financial industry for intra city comms.
In achievement terms this is almost all software (once you take putting stuff in space for granted).
Think of proving it to work in theory, ground based prototypes etc.
Error detection is also interesting (though mostly solved but with many options available). High bandwidth laser comms have been around for a while. Originally developed for the financial industry for intra city comms.
To be honest, it's probably less difficult than the tracking that a compact disc player has to achieve to play an off-centre bent wobbly CD. The error correction would be fairly similar too. How long have CDs been around now?
Kind of related fun fact, when astronauts take images of Earth they have to rotate the camera while taking the photo to compensate for the ISS moving so quickly. I think there are now computer controlled gimbals that account for that automatically.
Same as a sports photographer tracking a race car.
Could we please stop with these headlines that hint at alien life?
It's the title of the target article.
They don't mention this anywhere, but for light based communication with more distant targets where the laser would need multiple seconds to reach its target, would they need to "lead the target" with the lasers pointing in both directions?
Also, what about the complications due to interspersed massive bodies creating gravitational lensing?! I guess this can all be accounted for in the aiming software... but it's not very simple!
Also, what about the complications due to interspersed massive bodies creating gravitational lensing?! I guess this can all be accounted for in the aiming software... but it's not very simple!
At the multiple seconds travel time level, gravitational lensing is completely insignificant. Come back when you have a round trip time of a million years past a massive galactic supercluster.
If you are a fan of videos I definitely recommend their YouTube channel for brief news updates.
The first thing I thought when I saw this video was definitely about the tech Li-Fi, there was a new quite long video about it recently here: https://www.youtube.com/watch?v=WRG9iXZbuAc
The first thing I thought when I saw this video was definitely about the tech Li-Fi, there was a new quite long video about it recently here: https://www.youtube.com/watch?v=WRG9iXZbuAc
Why is radio communication slower than visible light based ones? 0s and 1s should be the same speed no matter what the wavelength.
It's not about latency, its about bandwidth. Your data transfer rate is directly proportional to the frequency of the carrier wave. Visible light has a shorter wavelength (higher frequency) than RF, therefore can carry more information in any period of time.
Think about it, wavelength (or frequency, they're effectively the same measurement here) determines how fast you can transmit those 1's and 0's. It's not slower, it's just that you can't transmit as much information in the same time period.
Also, laser light for Telecoms is not normally Visible, it's usually Infrared which has a wavelength shorter than Microwave radio but longer than visible light.
Another bonus fact; in a vacuum, Radio and Light waves travel at the same speed. However, when light travels down an optical fibre it is slowed down by the glass so Fibre Internet is actually slightly slower than radio.
Also, laser light for Telecoms is not normally Visible, it's usually Infrared which has a wavelength shorter than Microwave radio but longer than visible light.
Another bonus fact; in a vacuum, Radio and Light waves travel at the same speed. However, when light travels down an optical fibre it is slowed down by the glass so Fibre Internet is actually slightly slower than radio.
So the speed of all fiber and radio is basically the same and it's the capacity that's different?
Does that mean that saying "internet connection speed" is a faulty term? We should all be saying "capacity" or data-rate?
Does that mean that saying "internet connection speed" is a faulty term? We should all be saying "capacity" or data-rate?
Ah for a second I thought this is it, we're all doomed. Still a pretty cool demonstration though.
Okay, how big is the video and what exactly was the bandwidth? Can't understand why they won't put the basics in the article.
So I illegally downloaded the video from YouTube; it appears to be 2657084 bytes, so 2657084/3.5 is 759166 bytes per second or 6073328 bits or 5.8 megabit per second. Reporting that as a maximum rate of 50 megabit seems ... a little "Comcastic". Or am I missing something?
Update: I see another article calling it a 175 megabit transmission so if that took 148 seconds that's an average rate of 1.2 megabit. It's a mystery to me why average throughput is just not interesting to the author of the original article. And presumably the downvoting I'm getting is from a similar sense of vapid apathy about intriguing detail.
So I illegally downloaded the video from YouTube; it appears to be 2657084 bytes, so 2657084/3.5 is 759166 bytes per second or 6073328 bits or 5.8 megabit per second. Reporting that as a maximum rate of 50 megabit seems ... a little "Comcastic". Or am I missing something?
Update: I see another article calling it a 175 megabit transmission so if that took 148 seconds that's an average rate of 1.2 megabit. It's a mystery to me why average throughput is just not interesting to the author of the original article. And presumably the downvoting I'm getting is from a similar sense of vapid apathy about intriguing detail.
I wouldn't expect the bits coming down from YouTube to be a meaningful representation of what they transmitted.
This page describes the mission goal as 10 megabits-per-second or higher, I guess they aren't making that up:
http://www.nasa.gov/mission_pages/station/research/experimen...
This page describes the mission goal as 10 megabits-per-second or higher, I guess they aren't making that up:
http://www.nasa.gov/mission_pages/station/research/experimen...
Why wouldn't YouTube be meaningful? It would make sense for the video to be stored and transmitted as h264 and then uploaded as is to YouTube. I doubt YouTube recompresses h264.
Because the version they put on Youtube is probably not the exact version they received from the ISS. The article repeatedly described the video as HD, and the Youtube version isn't. And if you aren't willing to take NASA at their word, it's not that hard to imagine how multiple versions of a video end up getting passed around an organization, and the one that gets uploaded is compressed for whatever reason.
It's certainly possible that it is of similar size. For it to be meaningful, I would want it to be overwhelmingly likely that it was the original.
They said that the video was transmitted repeatedly over the course of the test.
conceivable that the 3.5 seconds includes some overhead related to getting the thing powered up and pointed in the right direction. Also conceivable that it peaked at 50mbps for just a moment. I'll also throw in that whatever protocol they're using...I'm guessing it's not bit torrent over HTTP over TCP :)...has overhead as well to account for errors and OUTER-SPACE related nuances! anyways I'm willing to give them the benefit of the doubt.
>Unlike normal data transmissions, which are encoded in radio waves, this one came to Earth on a beam of light.
>radio waves
>beam of light
Radio waves are light...
Congratulations to rwitoff and team at JPL for pulling it off!
Reminds me of Paul Simon's song "The Boy in The Bubble" [1]
[1] http://en.wikipedia.org/wiki/Graceland_(album)
EDIT: Added link, not bad prediction considering it was released in 1986