Slashdot Mirror
An Interplanetary Laser Communications System
caffiend666 writes "A news article at Yahoo states NASA is planning on testing the first laser-based interplanetary communications system on the Mars Telecommunications Orbiter to be launched in 2009. 'Unlike radio frequency signals that wash over the entire Earth, Fitzgerald and his colleagues will be shooting for a much smaller target - the southwestern corner of the United States.' Does this mean we will soon have telescopes outside of our homes soon to pick up high definition TV signals instead of our current 18 inch dishes?"
Radio, or electromagnetic radiation, is a fancy name for a special spectrum of invisible light. Yes, Virginia, your radio is replaying music broadcast over light!
Also, a laser is a special form of coherent light. It just means that all the wavelengths in the beam of light are the same wavelength. It also means that the beam of light doesn't disperse very much unlike incoherent light (which no one can make heads or tails of what it is trying to say).
Since the radio requires a specific band to tune in to, it makes sense that the broadcasting station not waste time generating unnecessary wavelengths and focus on only those wavelengths that correspond to our chosen band. This restricts us to AM (amplitude modulation) bands only, but since we're trying to get data signals and not Martian stereo there is no big loss.
So why deal with visible light lasers when it could be invisible and work just as well?
I only recently started taking chemistry courses though, somebody correct me if I am wrong.
Careful with names containing L slashdot.org/~AiphaWolf_HK slashdot.org/~AlphaWoif_HK slashdot.org/~AiphaWoif_HK
One of the limitations for geosynchronous satelites is that their proximity to each other is limited due to the unavoidable spread of the signal. Shorter wavelength means a tighter signal, which means more satelites.
Of course... cloud cover is a problem, but there are ways around that (like those robot blimps that loiter in a given area above the clouds).
I rarely criticize things I don't care about.
light is not restricted to the visible light we can see. radio waves are a form of light. so is infared. gamm radiation, microwaves, etc.
"goodbye and hello, as always" ~Prince Corwin, from Zelazny's Amber series
And supplementarily, lasers were originally called optical masers.
"I'm not impatient. I just hate waiting." - My Dad
That is why some are looking for lasers
One Man's Trash Is Another Man's Treasure.
All light is electromagnetic radiation, but not all electromagnetic radiation is light. Light is the small, visible portion of the elecromagnetic spectrum. So, Virginia's radio is *not* replaying music broadcast over light.
"I'm not impatient. I just hate waiting." - My Dad
What does that have to do with anything? Going between planets we will still be limited to sending messanges at the speed of light. aprox 16 minutes from here to mars. There most definitely *will* be a lengthy delay between messages.
Well.. maybe. Or Maybe not. But Definitely not sort of.
The ultimate system for communications would be using neutrinos. Only two downsides. One producing enough neutrinos to create a signal and coming up with a compact means of detecting them. With current technology it would probably take a detector the size of a football field to recieve a binary SOS and the only major source of them are events like super nova. The benefits would be enormous given a single signal from a single point could reach the entire planet passing directly through the earth with little or no loss. It's doubtful there will ever be a practicle way of doing it but it would be a way of sending a signal with little interference except for naturally occuring neutrinos.
Hams object, not because it's a good and valid method of delivering bits, but because it interferes with emergency communications.
8
There's lots of ways to get good Internet feeds to folks; just look at what Robert X. Cringely has done with 802.11b. Look in the archives of his columns at www.pbs.org and see there are untapped alternatives.
To understand why we're concerned, go switch your hi-fi to AM, tune to a vacant spot between stations, and turn up the volume about half way. Then, try to have a phone conversation over a bad cellular connection with your ear six inches from the speakers, and you will still have an easier time communicating than hams will when we experience the 16 db over S9 interference already demonstrated by BPL.
I will make a small wager with you, shaka999. If you live within North America, I'll wager your state's or province's emergency plan counts on hams. So does your county's emergency plan, and your city's.
You see, hams _practice_ at getting data through emergency conditions. We do it at our expense, with equipment we buy, build and maintain ourselves, without government funds.
There's even a subsection of every national ham organization dedicated to emergency services. Yeah, I belong to one, and was out in the last ice storm, two months ago, delivering nurses to the local hospital because the roads were otherwise impassible, and the locals had already overloaded the cellular network to the point where a fast busy tone or "All Circuits Busy" signal was as likely as dial tone.
BPL threatens the entire ability to function on the frequencies needed the most for long-range communications, the HF bands. If this interfered with TV (VHF and UHF), well, everyone would kvetch, but instead the power companies have designed these systems to use HF (aka shortwave) frequencies.
Long range radio relies on HF, because it takes those lower frequencies to effectively bounce off the inner layer(s) of the ionosphere. Higher frequencies (VHF, UHF, SHF, microwave) just zip right through the F, F1 & F2 layers, so we can't do bank shots to get a signal from Earthquakestan to Resourceland to let them know how many units of Type A to send.
Satellite? Well, gee, that presumes the ground stations survived that quake/tornado/hurricane/typhoon, that the power didn't fail, and the phone lines to the earth station still work. Oh, yeah, and IF there's a free satellite channel for us, which NASA's problems have not made any easier.
Now, America's three-quarters of a million hams are not alone here, as you make it seem. The NTIA (National Telecommunications and Information Administration), who you'd expect to be gung-ho over more bandwidth to previously underserved areas, and also FEMA (Federal Emergency Management Agency), have gone on record to object. They document that BPL was a complete disaster, interference-wise, when tried in Japan. The Austrian trials are on hold because the power companies there were not able to rein in the interference.
But, it's Politics with a Capital P; who is beholden to whom, and who bought whom.
Now, you might say, 'well, if there's a disater, the power's down, right'? Not necessarily. BPL can cause interference for miles and miles, but if a hospital needs to call for blood, what's the power company supposed to do, shut down the entire grid?
Besides, remember that hams buy their own gear to practice and learn with. If we can't use HF, well, no one will buy new HF gear, no one will learn the tricks of HF (which is _very_ different than the skills needed for the garden-variety, talk-around-town two meter and 70 cm band users), and no one will bother to keep the automated packet netowrks in service, the digital backbones of the ham world which move the vast majority of message traffic.
Sometimes, _nothing_ but Morse ("the original digital") will get through, but with BPL jamming the HF spectrum, morse will become a dead letter.
I mean, man, you can put a bra on Michael Powell, and yuk it up all you want (see URL) but, damnit, these changes will *kill* people.
http://www.wweek.com/story.php?story=485
But it does matter in practice.
Background noise. The electromagnetic background noise level varies enormously with frequency. Here optical communications is actually at a big disadvantage compared with microwave, mainly because stars are brightest in the visible and near infrared. (Fortunately, it's fairly easy to exclude stars from interplanetary links with narrow-field telescopes.) The microwave range between 1 and 10 GHz is pretty quiet, which is why it's so heavily used for satellite and deep space communications. Below that range you start to run into sources of noise other than thermal radiation, such as lightning and radiation from charged particles trapped in magnetic fields.
Bandwidth. Optical frequencies have much more room for broadband signals, but in practice microwave bandwidth is plentiful for deep space communications. Those links tend to be signal-to-noise ratio limited, not bandwidth limited.
Antenna gain. Although the inverse square law applies equally at all wavelengths, antennas are not equally effective at all wavelengths. A receiving antenna's performance depends primarily on its aperture, the area with which it collects radiation, and that's independent of wavelength. But a transmitting antenna is different. The beamwidth of an antenna depends on its diameter in wavelengths, so a given antenna will transmit a narrower, tighter beam at shorter wavelengths, so more of it will land on the receiving antenna (assuming it's pointed accurately). So if you use a given pair of antennas on a given point-to-point link and vary just the wavength, the end-to-end power transfer efficiency will improve with shorter wavelengths at a rate of 6 dB per octave.
Atmospheric absorption. Space is an empty vacuum, but the attenuation of the earth's atmosphere is a complex function of frequency. Below about 30 MHz, the ionosphere acts like a mirror; that's how "shortwave" broadcasts get worldwide coverage. There's a broad window from about 30 MHz up to about 10 GHz. Above that frequency, water vapor becomes increasingly important. There's a sharp absorption line at 60 GHz due to oxygen absorption, and above there it becomes increasingly opaque up until the infrared. There's another broad opening in the infrared and visible range, followed by more absorption bands in the ultraviolet (due, among other things, to the ozone layer).
This leaves two places for interplanetary communication links: the microwave range between 1-10 GHz, and the optical range. The advantage in going optical lies entirely in the increased transmitter antenna gain that would allow much more of the limited spacecraft transmitter power to be directed to the receiving antenna on or near earth.
Forgive me if I am skeptical. A flashlight bulb has a very slow response time; feed it a low-frequency square wave, you get a sine(ish) wave. Feed it a high-frequency square wave and you get a steady light. I have a hard time beleiving that a flashlight bulb could transmit a 10,000Hz audio signal -- those light bulbs in your house? They run on A/C, but they stay bright enough in between cycles that you don't see the 50 or 60Hz flicker.
Not that I would doubt a 3 digit UID, who also lives next door to the Beast, but maybe someone can explain this apparent non sequitur?
DRM 'manages access' in the same way that a prison 'manages freedom'
Well, yes, it kind of is, depending on the definition you use.
LK
"Hi. This is my friend, Jack Shit, and you don't know him." - Lord Kano
no way... how fast can an incandescent filament change brightness? Could you get audio frequencies as high as a few thousand Hertz?
I've seen kits to modulate lasers with audio (and even video) -- they specifically use a laser module with the proper (lack of) regulation so that it works cleanly. Similar circuits are used with simple IR LEDs for those "wireless" headphones that are line-of-sight.
With those solid state devices, i'd expect pretty "instant" response in brightness output. That's really neat that your dad got it working with a plain old flashlight.
INsigNIFICANT
Laser has at least two major problems that I can think of.
1. Lasers are pretty damn inneficient. At least compared to radio equipment that can be very efficient. When you're in the 2 percent range you're happy.
2. Lasers are very high frequency. This is bad. Higher frequencies are absorbed MUCH more readily and are blocked by interfering objects. They also lose power faster through general attenuation through free space much faster than lower frequencies.
And if you think the laser will make a small dot we can see, you're wrong. The laser light will probably cover half the other planet (this works out to look like attenuation)
Basically, I dont see the reason to use lasers over long distances when lower freq RF works a lot better.
Huh? Lasing has nothing to do with collimation! Most lasers aren't collimated! You can collimate any EM source (like a light bulb!) - a collimated beam is a beam with a fixed width down the direction of propagation. Perhaps you were confusing coherence with collimation?
Yes, it will work - I've done it myself.
You don't need much modulation of the light beam - just a percent or so will be enough to detect, and you won't see a percent modulation with your eye (unless you have a reference to compare against).
Yes, you aren't going to be pushing 20Hz-20kHz across this - between the thermal mass of the filament and the slow response of the CdS cell you're going to be lucky to get 3kHz, but that is good enough for voice.
www.eFax.com are spammers
There is an RFC that addresses this, and support for it seems fairly well deployed (Linux kernel 2.4 had it but it was disabled, kernel 2.6 used a 2**7=128 scaling factor). The new option allows 1 GByte windows. Even with this RFC in place, you'd only get a 25% utilization between Earth and Mars (Send a GB, wait for 3GB's worth of send time).
I became aware of it having been recently bitten by a window scaling bug in a router between my PC and where I work. I found the RFC quite interesting.
Tiller's Rule: Never use a word in written form that you've only heard and never read. You will end up looking foolish.
If you don't know anything about Nikola Tesla pick up a book called "Man Out of TIme", it's a good primer to look deeper.
People have little idea of what this man *continues* to give to our societies, the military certainly does.
http://www.teslascience.org/pages/tesla.htm
"In December of 1900, after wrapping up his preliminary testing he returned to New York to begin work on the full sized prototype worldwide broadcasting station.
The main structure built to house equipment for this station and known as the Wardenclyffe Laboratory Building, is still standing near the Long Island community of Shoreham, New York.  Not a great amount has been learned about the station's specific design details.  It is quite certain that there would have been major similarities between it and the large 1899 apparatus in Colorado.  Tesla's investigations at Wardenclyffe were brought to an end due to a lack of research funding.  The building was abandoned and Tesla's tower was eventually demolished during the early years of World War I.
One interesting feature of Tesla's world system for global communications, had it gone into full operation, would have been its capacity to demonstrate on a limited scale the wireless transmission of electrical power. If the prototype communications station at Wardenclyffe had shown the feasibility of wireless power transmission, then Tesla intended to build a full scale power transmitter at Niagara Falls, site of the first commercial three phase AC power plant mentioned earlier."
http://www.teslascience.org/index.html
http://www.teslasociety.com/dream.htm
http://www.teslasociety.com/picture6.jpg
http://www.teslasociety.com/wirelesssystem.gif
http://www.teslasociety.com/signaltomars.htm
http://www.teslasociety.com/mars.html
~hylas