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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?"
Earth - 'Hey' ...' ;)
Mars - 'Hey'
Earth -
Mars - '...'
Earth - 'a/s/l?'
-Teiresias
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?"
.
No.
Because for television broadcast to the general population you want to wash the signal over the whole earth, rather than trying to target each receiver. And if you think your reception sucks when it's raining out now. .
KFG
I always wondered why they would want to use the visible spectrum...
We *CAN* make Laser-Radio waves! They go through atmosphere and trees and buildings....
-Bill
will this be implemented with sharks with frikin lasers on their heads?
"goodbye and hello, as always" ~Prince Corwin, from Zelazny's Amber series
It's unlikely you'd use lasers for wide scale signal distribution. A laser must be aimed, and to provide a signal to a thousand receivers you would need to fire a thousand beams, or have some intricate device that actively retargets thousands of times per second, squirting packets off to each receiver. Moving parts, complicated, no clear advantage.
Lasers for interplanetary communication is another thing. It's one sender to one receiver, and then you can go radio for inside planetary systems. Eg, you could set up a Mars Relay Station that takes low power local radio transmissions and beams the info back to Earth via laser, and vice versa. You get the advantage of cheap, small radio technology plus the range and bandwidth of laser.
Some serious lag in UT2004
If you could reason with religious people, there would be no religious people
a little math...
344 million km / (0.3 million km/sec) = 1147 seconds travel time
1147 seconds * 30 megabits/sec peak rate = 4.3 Gigabytes in transit at any instant.
HIV Crosses Species Barrier... into Muppets
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?
Here's a story about an ambitious plan to build a laser-based interplanetary communications network and the only thing the story submitter is concerned with is how this will influence his TV reception.
This, my friends, is why the human race is doomed. Here on slashdot, where we care more about science than most people, all some people can think about is how a new technological advancement can facilitate the transmission of market-research-constructed-SitComs or advertisements for the latest yuppie gizmo to their home.
I'd be happy if I could communicate with women. Why don't they work on that first?
The Internet is full. Go Away!!!
Suck on this, inverse-square law!
English is easier said than done.
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
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
When you add lasers to anything, the net benefit is multiplied by %5555. Interstella 5555 is a prime example.
Ninjas also benefit from lasers ovbiously.
The advantage is that lasers are collimated, which means that the light doesn't spread out in a cone. Since you're concentrating the energy on a few hundred square miles rather than a few million square miles, you can broadcast with a lot less power. You can also make much more reliable communications, which means your bandwidth is higher.
In theory you can do this with any wavelength of light; if you do it with microwaves it's called a maser rather than a laser. Higher frequencies mean more bits, which is a good reason to choose light over microwaves, but the light is absorbed by clouds. I'm not sure about microwave frequencies, and I'm not sure if anybody's ever built a laser-type thing for radio frequencies (raser? I find people joking about it on the Internet but it doesn't seem unreasonable to me).
Eventually you might want a relay system: Mars to earth-orbiting satellite via laser, which then amplifies it and relays it to the earth on a frequency which cuts through coulds better, or just saves it up for a time when it can get through. But the first step is to see if you can get light accurately aimed at the Earth.
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.