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!"

Comments

Intersting stuff, want to try this "at home"?

[dusty123]

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

Re:Wow.... but how long was the link up for?

[ghoti]

You know, reading the article really helps ... it says "4 to 20 minutes", which is quite impressive. Amazing stuff, next thing you know they will be shooting down starting ICBMs with lasers ... ;-)

Both going at 7000m/s-1?

[onion2k]

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.

Re:Both going at 7000m/s-1?

[Yazeran]

Well you are a bit wrong about this, yes tha outer orbit is much longer than the inner orbit. An other thing you have to include is that it is gravity that keep satelites in orbit, and this scales with the square of the radius or the orbit. This boils down to Keplers third law:

the square of the orbits period are proportional to the cube of the orbits radius (for a circular orbit).

This results in that the outer orbit (which is a geo-stationary orbit) takes 24 hours whereas the inner orbit only takes some 100 minutes (give or takte).

earth to satellite link

[dario_moreno]

however, it was demonstrated in the sixties
by concurrent US and Soviet teams (Tatarskii) that
a laser link (although very secure and
promising in terms of bw ) between an earth station and a satellite was not feasible
due to atmospheric turbulence. Maybe
things have evolved now...

Laser alignment

[SomethingOrOther]

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!

Sat2Sat communications evolution

[Markonen]

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...