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

Line of Sight

[ectoraige]

I wonder if they have to allow for signal loss caused by bits of space junk floating by...

Okay, the odds are probably pretty damn small, just a thought.

Besides, slashdot seems screwed, I'm curious to see if I can still post...

Re:Line of Sight

[Yazeran]

Well it would only be a problem if a large object (like another satelite) came to be in-between. Even laser light diverge at these distances, so you only point the beam at the other satelite. The beam-diameter would be more than 1 meter, and if a small dust-grain was to come in-between, it would be impossible to detect it. Remember, that light do bend arround corners (quantum mechanics; slit experiments) so even if the dust grain vas directly between the laser and the sensor on the other satelite, it would not 'turn off' the beam. You vould only notice (if at all) a small decrease in light intensity.

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.

Re:Line of Sight

[apsmith]

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.

Yeah, I was thinking of the same technique - once you have the communications link up also, you could use the same directional information to transmit power also, something that's been proposed as a way to make use of Solar Power Satellites. This is a very important accomplishment!

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 ... ;-)

Damn

[Graymalkin]

I was just wondering the other day how feasible it would be to use a laser as a communication device between two birds. You can get really nice range with little EM interference with only a wee bit or output power. The one obstacle I kept running into whilst pondering a laserlink was keeping the beam aimed at another bird in a different orbit. Well hot damn and way to go. I guess I was hit in the face with the same muse as the dudes at the ESA just a little bit late and without any satellites under my control to play with...so far.

Re:Damn

[onion2k]

Yes.. the Lesser Spotted Marsh Warbler is famous in ornithological circles for its advanced SatComms..

Re:Damn

[Graymalkin]

I was thinking more along the lines of African swallows. Weird. 20 damned seconds to reply. Has it been 20 seconds yet?

Side effects of lasers?

[billn]

I know it's minimal, but does anyone have any info on how much force the laser they use might exert?

Re:Side effects of lasers?

[abrett]

According to the specs, the link laser will operate at a mean power of 60mW. Using F = P/c, we get a force of roughly 200fN (200 femto Newtons!). Just to give you some idea of the effect that would have, Artemis' mass at launch is 3100kg, so this means that if it was to emit a 60mW beam in the same direction for 10 years, its speed would change by approximately .01 mm per sec.

Re:Side effects of lasers?

[Sentry21]

Almost none.

I'm assuming you mean kinetic force, and the kinetic force applied to the recieving end of the laser varies inversely (according to some formula) with the reflectivity of the surface the laser is hitting and the distance between the objects (maybe), and it's only really noticable when you get into the gigawatt ranges.

--Dan

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?

[onion2k]

Even factoring in the Earths radius in makes the two orbits 7532km and 38000km .. thats a difference of a factor of 5 and a bit. 7000m/s-1 surely wouldn't be enough to keep the relative distance the same? (And, rereading the article, it doesn't say the relative speed in 7000m/s.. it says, and I quote, the partner spacecraft flying at a speed of 7000 m/s.. there is no mention of the second crafts speed.

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

[imrdkl]

I wondered about this. But the article says that eventually the outer (SPOT) satellite will go stationary, and then initiate up to 5 ordinary uploads per day. I think this is the orbit dependency on the inner (Artemis) satellite? I also dont know what, exactly, the SPOT is taking pictures of, but at 50mb, it can probably get alot of them down to the people who are interested in them. I tried a 20min period using bc(1) and came up with about 10 CDROMS worth SPOT pics.

Wow. Now, if we could just get laser-pens to be this accurate, then they may actually someday actually replace wooden pointer sticks and fumbling fingers under the overhead lamp trying to make a point during a conference session. heh.

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...

Re:earth to satellite link

[isorox]

Appently you can get satelistse that read your licence plate - optics are that good 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!

Re:Laser alignment

[isorox]

Really? I'm looking at laser systems for broadcasting the campus TV to different buildings. http://www.canon-europa.com/products/network_produ cts/canobeam came up on my search, the network version operates up to 622Mbps, upto 1000m [depending on weather], or 156MBps upto 2000m. It works at 785nm (+-15).

Uninterceptible communications

[The Dodger]

Interesting side effect of this is that these communications can't be intercepted, unlike RF/microwave broadcasts.

D.

Re:Uninterceptible communications

[isorox]

Actually they can, just put a receiver in the beam :)

You'll notice it though, unless the receiver was small enough (a few cm's), and near the destination, to only get a small amount of the beam (which diverges by a lot at these distances). The intensity would be lowe rat the destination, but with a small enough bug it would be possible. The bug would have to be very sensitive though!

Re:But do they use decimal?

[tomknight]

Because all your bases.....

Tom.

Who cares about linear speed...

[javatips]

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.

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...

Re:Sat2Sat communications evolution

[blair1q]

> With RF systems, you would need some tracking for any long term communications, but you could base that purely on satellite ephemeris, a much simpler problem.

You'd start the beam search using ephemerides, but RF crosslinks do use signal-strength components for feedback-based antenna steering control.

You're dead right that getting it done with lasers is several orders of magnitude more cool.

This also has the potential to tack a few more zeroes onto the accuracy of orbital position determination. Interferometry could get you sub-nanometer resolution. I can't imagine why you'd want that, but I can imagine someone else can.

--Blair

Re:Just a thought!

[Dyolf Knip]

I thought that exact same thing. They spend billions on missile-based defense systems and as an afternoon exercise the ESA does the same job with a laser. Non-projectile weapons are obviously the way to go.