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Laser for Satellite to Satellite Communications
heby writes: "Last night ESA successfully tested the first laser link between two satellites (SPOT 4 and Artemis). SPOT 4 is supposed to serve as a data communications relay between Artemis and the receiving station in Toulouse. The link is running at 50Mbps and the two satellites are currently orbiting at 832km and 31000km respectively.
According to ESA "The main challenge in establishing an optical link between satellites is to point a very narrow beam with extreme accuracy to illuminate the partner spacecraft flying at a speed of 7000 m/s." Way to go, ESA!"
According to ESA "The main challenge in establishing an optical link between satellites is to point a very narrow beam with extreme accuracy to illuminate the partner spacecraft flying at a speed of 7000 m/s." Way to go, ESA!"
Well, this is indeed a real challenge. Some time ago we also tried to build a laserlink and succeeded with 128kbit (IrDA). The link was very stable, there was no problem accomplishing a link at around 2km. Next we tried to "upgrade" to 10baseT but sadly never found time to finish this. If anyone is interested, have a look at: http://strike.wu-wien.ac.at/~dusty/projekte/laserl ink/index.shtml
Considering the vastly different orbit heights (832km and 31000km) surely the two satellittes must be going at very different velocities. A little basic mathes show us :
2 * PI * 832 = 5,227,610m
2 * PI * 31000 = 194,778,744m
So the total linear distance travelled in each orbit is very different (assuming that the two heights are taken from the centre of the Earth. Which they aren't. Can't be bothered to factor in Earth's radius). So, at 7000m/s-1, the outer satellitte would take about 8 hours longer per orbit, evidently showing the relative distance would be changing, and making the targetting process much more of a challenge.
So.. presumably 7000m/s-1 is the speed of one of the satellittes (I'm guessing inner)..
PS. I think my mathes is screwy. Its early. I have no coffee.
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Besides, i think they would have included some error-recovery system in their data link, this is standard for all data-transmission links (even home networks on ne2000 compliant netcards).
The real feat here is that they could point a narro beam at a mowing target and keep it there (autonomously that is).
The trick could be to use the gradual decrease in beam intensity as you move to the outer portions of the beam, and send this information back to the other satelite to re-adjust. If you used several sensors spaced some distance apart, you could determine the direction the beam has to be moved (Theoretically that is). I do not know if this could be done in real life as i'm no laser specialist or space engineer.
Yours Yazeran
Plan: To go to Mars one day with a hammer.
Yes.. the Lesser Spotted Marsh Warbler is famous in ornithological circles for its advanced SatComms..
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I work with free space lasers as part of my PhD and I can assure you they can be an absolute b*stard to align properley, even accross a small lab bench into a detector. Hats off to em!
I'd be interested to know what wavelength these devices operate on. (I'm assuming they are semiconductor devices as nothing else would be light enough to launch into space) Blue semicondutor lasers (with nitrogen doping) are becomming cheaper and cheaper and can carry more data (because of the shoter wavelength) per sec but may not be as reliable as "traditional" longer wavelengths.
A few months ago we tried rigging up a "laser ethernet" conection from our physics dept to our house (its line of sight). Only by making teh beam very divergent did we manage to get any sort of alingment, and that was on a clear day! It was nowhere near good enough for us to be able to use the universitys fat pipes from home!
Anyone quoted by a reporter knows how little they understand
Don't believe what you read is the truth.
What amaze me with this kind of PR is that they always use large number to impress people.
The fact that the linear speed difference between the two satellites (from previous post, I assume that the 7000m/s is the speed difference between the two satellites) is not very important. What is important is the angular speed.
It is a lot easier to target an object moving at 100Km/h at a distance of 100 meters than to target the same object at a distance of 10 meters.
It seems to me that this is just an incremental advance from older satellite-to-satellite communications systems.
Military satellite networks, for example MILSTAR have already implemented very narrow beam communications between satellites. This has been necessary to prevent interception or jamming of the signal.
The advances here probably relate mostly to greater-precision mechanics and more powerful CPUs. I don't know if the data rate mentioned is a big leap or not, but considering the fact that the MILSTAR network carries all the photographic and video intelligence gathered by NRO's Improved CRYSTAL satellites the MILSTAR bandwidth must be pretty impressive too...
Marko Karppinen