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Space Science

Laser for Satellite to Satellite Communications 170

Posted by michael from the doubles-as-death-ray dept.
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!"
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Laser for Satellite to Satellite Communications More

Laser for Satellite to Satellite Communications

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  • "while Artemis is temporarily in a parking orbit at 31 000 km."

    I had to pay $13.00 to park at Navy Pier....I wonder how much parking costs at 310000 km?
  • Is relative - 7000m/s sounds impressive but when BOTH spacecraft are travelling at 7000m/s in the SAME relative direction they are actually travelling ( in relation to each other ) at 0m/s

    2c

    • Is relative - 7000m/s sounds impressive but when BOTH spacecraft are travelling at 7000m/s in the SAME relative direction they are actually travelling ( in relation to each other ) at 0m/s

      Yeah, but they're not both travelling in the same direction or the same speed.

      They're in different orbits, which means that their speed and direction is always different (except for instantaneously occasionally when they can have the same direction, but still different speeds)

    • Re:Speed (Score:1, Insightful)

      by Anonymous Coward
      this already is the relative speed between thos two.
      Artemis probably is at below 2000m/s while SPOT4 should be at around 9000m/s.
    • Common misconception. There isn't one velocity called orbital velocity that you have to reach to orbit. The velocity needed for a particular orbit is dependent on it's height: the lower you are the higher the speed. Also if the two satellites are traveling in opposite directions then it's more like 14km/s. The only time the relative velocity will be 0 is if they are both in exactly the same orbit.
      • "The only time the relative velocity will be 0 is if they are both in exactly the same orbit."

        The above is not true, the higher the orbit the greater the length of the orbit. The outer sat has to travel faster to keep up with the inner sat.

        So they can have a relative velocity of zero when they are not in the same orbit.
      • The only time the relative velocity will be 0 is if they are both in exactly the same orbit.

        ...and on top of each other. Velocity is a vector. Being in the same orbit does not mean that they do not move relative to each other (depending on what reference frame you are using). Even if the two spacecraft were in the same orbit, the tracking system would still have to continuously repoint the laser/reciever.
  • If they transmit data and later convert it to decimal, it's ust another example of a lame base being used yet again. Why not binary or hexadecimal? Why decimal? Why?
  • How long was the link up for... I'm wondering because one is in a fixed orbit over a single piece of land, while the other is flying around the earth once what? every 90-120 minutes?

    The sheer logistics of keeping that link up would be nearly mind boggleing.... So is anybody going to try this with a spot on the planet with a bird up in the sky (satellite)?

  • Yes! (Score:1, Funny)

    by Anonymous Coward
    We can finally stream porn 31000km above the Earth!

    I wonder what the latency of that link is?
    • Well the latency between the two sats would be distance/speed of light as there is just a vaccumm in between them. Its the time to downlink the info to the earth station which is the problem due to the atmosphere getting in the way which will increase the latency significantly.

      Of course this could just end up as a hub/router with sats taking it in turn to transfer info and then have it down loaded to a base station which would save the hassle of having to include gear to transmit to earth on each one.
  • Article submitter wrote:
    SPOT 4 is supposed to serve as a data communications relay between Artemis and the receiving station in Toulouse.

    Then, Tollhouse takes the data and uses it to make cookies [betterbaking.com].

    -nukebuddy
  • maybe this will help out in the future for satellite imaging to stream video back to earth instead of just pictures that take hours to transmit across space. i think it would be cool to get live video from a satellite orbiting jupiter at any given moment :)
    • maybe this will help out in the future for satellite imaging to stream video back to earth instead of just pictures that take hours to transmit across space. i think it would be cool to get live video from a satellite orbiting jupiter at any given moment :)

      Yeah, right, like it affects the transmission time in any way. The transmission time is still the same due to distance (doesn't change) and speed of light (doesn't change). Remember that radio waves travel just as fast as light?

      It mainly affects loss of communication between ground stations and satellites in low-earth orbit since they can't communicate when outside line-of-sight.
  • 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 (Score:4, Interesting)

      by Yazeran ( 313637 ) on Friday November 23, 2001 @05:28AM (#2603050)
      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.


      • 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"? (Score:3, Informative)

    by dusty123 ( 538507 ) on Friday November 23, 2001 @05:11AM (#2603004)
    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
  • I worked on this back when I was working at BAe Space Systems (since taken over by Matra Marconi).
    We had to high accuracy laser targeting systems for the Sat2Sat laser link working in the lab at BAe's Stevenage site 6 or more years ago...
  • 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.
  • I know it's minimal, but does anyone have any info on how much force the laser they use might exert?
    • According to the specs [esa.int], 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.
    • Compared to the exceedingly thin but still significant outer atmosphere they're flying in (well, the lower one anyway)? These would not be strong lasers by any stretch of the word. Hell, sunlight would be pushing on them much harder.

      I'd say undetectable. I could be wrong though. We seem to have a nack for measuring (or calculating) very small quantities. I'd like to hear an answer from someone who can work out a number though.

    • 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? (Score:3, Informative)

    by onion2k ( 203094 ) on Friday November 23, 2001 @05:28AM (#2603048) Homepage
    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.
    • 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).
      • 31,000 km is not geo-stationary, 35,785 km is. I made the same mistake at first because they are so close. Also, it didn't say that the orbit was above the equator, which is crucial. You are still correct that the period would be very close to 24 hours as it is about the same size and velocity as a geo-stationary orbit.

        So, yes i'm nit picking.

        Now the question is: Why isn't it in Geo-stationary orbit? Am I missing something?

    • 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.

      • Artemis is the outer satellite destined to go stationary (it's the relay/communications satellite), and SPOT is the inner, earth observing satellite (an optical multi-spectral remote sensing satellite, to be specific).

        Artemis should have been stationary already, but when it was launched earlier this year, the Arianne 5 rocket it was launched on failed in the upper stage and didn't get it out far enough to achieve it's intended geostationary orbit. They've been using the on board thrusters to move it out into the intended position since then.

        I'm surprised and impressed that they even attempted this given that Artemis is not in position, and that it worked. This bodes well for ESA's ENVISAT satellite, due to launch next year: Artemis will be relaying ENVISAT's data in a similar fashion.
    • All irrelevant. One satellite is in geostationary orbit, at about 38000km up. In other words, relative to the ground, it isn't moving at all. The 7000km/hr figure is for the low orbit satellite, which is orbiting quite fast. This ain't no record player... they are not orbiting at different speeds. Thus, the circumfrence of their orbit has little to do with the speed at which they orbit. I don't know the orbital equations, but I know that much.

      The 'stationary' satellite and the low orbit satellite only communicate for short periods, in which the low orbit satellite uploads its accumulated data to the stationary one, which then relays it to the ground.

      Communicating with low orbit satellites has always been a pain. You need to track them accurately as they zoom across the sky. This new technology lets the ground crew track the stationary satellite instead, saving them lots of effort, and probably reducing costs.

      Raven

  • Er.. (Score:1)

    by TA ( 14109 )
    I think you got that wrong. Artemis is the relay satellite, SPOT4 is not.
  • Now that's actually more scary than anything else. If a civil institution is capable of laser-linking satellites, I'd say it's pretty much a given that the military is perfectly capable and probably has been for years to deploy lasers for destructive purposes in earth orbit. Who knows what's meanwhile up there? And I thought Star Wars (SDI) has just been US-propaganda to speed up the USSR's economic meltdown...
    • It's a whole different ballgame. For comms purposes in space you need microwatts/milliwatts. For comms purposes to ground (the article mentions establishing a link ground-space) the system probably uses a few watts.

      To get a hard kill on an ICBM you'd probably need something on the order of: megawatts/gigawatts (???). You are talking about going from a semiconductor laser (not too different than a laser pointer) on a very sexy tracking platform to something that currently takes up 2 lab buildings not including the generators and capacitors and is probably good for a limited number of shots.

      Also, notice that there is a 2-way beacon mode at the start of communications. This makes it simpler (note: I didn't say simple!) to establish 2-way comms. I just don't see anyone placing beacons on ICBMs. The IR signature of the exhaust plume is a pretty good indication, but that's about all you'll see.
      • Actually the important parameter is not bare power (Watts) but fluence=power/area (Watts/cm^2).This is important in this case because over the large distances you have enormous widening of the beam.
        Second thing important is the temporal structure of your beam. It's a large difference whether you deposit 10 W continous wave or whether you have 10 W average power but concentrate that in very short pulses.
        Taking both points into account you can reach with todays femtosecond pulse Lasers a peak fluence of many Terawatts/cm^2 (of course only for short times on very small spots). Btw. you can cram such a laser setup in 1 or 2 m^3 if you really want. That doesn't mean you can use it for SDI, but you can definitely burn holes with it (in the lab).

  • Is it just me who wonders in amazement at the cynicism over the missile defence tests about whether it's possible to hit a missile travelling at great speed (GPS or no GPS) - and yet now everyone oohs and ahhs and has no problem with NASA aiming a laser at something moving at 7000 m/s... :)

    -- Pete.

    • Not NASA.

      ESA == European Space Agency.
      Yes, europeans can do stuff in space too!

      Besides, aiming at something that you know the size, speed and direction of can be nowhere near as hard as hitting something as small as an ICBM that is being tracked from space. I think
    • Well, light does have less momentum and is easier to redirect at a distance than a BB is. Knowing where the object is does you no good if you can't put holes in it.

      So, how much energy can you cram into a laser beam these days? Anyone lazed ionizing UV and soft Xrays yet? Imagine a nice sharp beam of gammas. Yikes, I'm vaporized.

    • 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.

  • 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...
    • It appears that things have evolved significantly. Do a search on adaptive optics and all sorts of incredible stuff will come up. I would bet that adaptive optics combined with the lastest in signal processing could do alot barring the weather. I don't know. Niche application perhaps?
    • Appently you can get satelistse that read your licence plate - optics are that good now!
  • Sure, you get the laser link up and running fine, transferring all your data, and suddenly a piece of space debris intercepts the path of the laser. So some amount of data is lost.
    What kind of redundencies are employed to work around this problem?
    Or isn't it a problem at all?
    • This is no problem:
      Every single low-level communication protocol for longer distances (i.e. more than 2m) I know of has a way to detect (and sometimes correct) nearly all kind of transmission errors. And every higher-level communication protocol (say, TCP/IP) knows how to deal with them (usually with retransmission).

      (BTW: It's extremely unlikely that it will ever happen that some kind of space debris, a meteorite or another sattelite will ever come into the laser beam and interrupt it...)
      • "BTW: It's extremely unlikely that it will ever happen that some kind of space debris, a meteorite or another sattelite will ever come into the laser beam and interrupt it..."

        What do you base this statement upon?
        Do you have any specific stats?
        AFAIK communications sats are high up above the atmosphere. So even if small fragments from other sats/meteorites come along, they won't burn up (as they do, fortunately, for us).
        And last I heard, there's quite a lot of debris up there to make the scenario I implied quite probable.
        • Very simple: The number of fragmets larger than a few centemeters is know, IIRC it's about 10k, the vast majority of them in the lower orbits (~400km, i.e. the orbits of space shuttle and second or third stage burn-outs).
          Considering the vast amount of space around the earth this is not much more than nothing.
          And now do some maths (if you want). Let's estimate a laser beam of 1cm area, satellites 50000km apart, which makes 5km of "space".
          the whole space between 800km and 31000 circular orbits is 4/3*pi*((31000+6000)-(800+6000))~2.1*10^14km of space. Let's assume 10000 fragmets are equally distributed in this space, wich means one fragment for 2.1*10^9km, which means for a given time the chance that a fragment is in the "beam array" is about 1:4200000000. (seing fragments as points here).

          And this is a over simplefied calculation - first the "effective" laser beam is no cylinder, it's more like a cone and the base is the sensor area of the receiver. For a fragment to have an effect on the communication it has to cover (say) at least 50% of the sensor area (no idea how big that is) for at least half the time of a bit at 50*10^6 Bit/s this is 10ns - given the size and speed of the average fragment and considering the calculations above it's extremly unlikely that this will ever happen in the up to 5 periods per day of communication for 4 to 20 minutes between the two satellites.
  • Now all they have to do is increase the intensity and they've beaten us to Strategic Defense Initiative.

  • Laser alignment (Score:3, Informative)

    by SomethingOrOther ( 521702 ) on Friday November 23, 2001 @06:58AM (#2603242) Homepage

    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!

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

    D.
    • 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!
  • Now 15 year-olds can launch DOS attacks with laser pointers.
  • Isn't NASA doing something to this effect, only using Cisco gear to help with the wireless link?

    My girlfriend works at NASA as a Tech, and something I hear all the time is about their funding, and how it's really hard to get anything real done around there without a whole lot of BS. PHB's trying to get Win2K on P5-75's; and some other obsurd stuff to just make you question why we've cut their budge a lot.


  • Anyone know anything more about this? Could this be a way to get a link to and from the ground?

    73,
    dit dit
  • We (the Europeans) did put lasers on satelites but we're not pointing them at one another to chat. It's our new tactical warfare rig. Very cool. It can kill someone from orbit with pin-point acuracy. Compared to this the American Missile Shield is a just a neat toy.

    So the sentence:

    The link is running at 50Mbps and the two satellites are currently orbiting at 832km and 31000km respectively.

    Is just code for:

    The system is running at Full Power and the two satellites are currently orbiting above Washington and Moscow respectively.

    Insert "Bad Guys Laughter" here...

  • It is called metempsychosis -- soul travel (of the psyche) from one place to another. If this astounding SlashDot report is true, then our lush, green planet Earth stands on the space-port doorstep of intelligent ethereal beings [sourceforge.net] flitting about from satellite to satellite on a beam of laser light.

    But what happens, Scottie, if you are beaming up an AI Laser-Mind and you miss the receiving satellite? Does the robot soul or consciousness [sourceforge.net] sail off eternally into the far reaches of the universe?

    And how will this satellite-to-satellite laser-beam technology be used more mundanely, before the arrival of Technological Singularity? [caltech.edu]

  • Laser's that accurate must have many other applications.


  • Now THAT is what I call P2P communication!

  • Who cares about linear speed... (Score:3, Insightful)

    by javatips ( 66293 ) on Friday November 23, 2001 @10:22AM (#2603676) Homepage
    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.

  • ...In science fiction. larry Niven wrote about using lasers to communicate with other ships out in the solar system via a series of sattelites that had "relatively fixed" position relative to the solar system. It was for his "Gil the ARM" series and others. And that was a while back.

    Ver precise, and even better, incapable of intercepting transmissions. Good to see this coming to light. "Weeeeery eeenteresting."

    Of course, when are we going to see this technology used to guide in laser guided bombs and missiles from sattelite? Or, do we already have it and we don't know about it?

  • Sat2Sat communications evolution (Score:3, Informative)

    by Markonen ( 56381 ) <marko@karppinPLANCKen.fi minus physicist> on Friday November 23, 2001 @01:59PM (#2603833)
    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...
    • 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.

      Hardly "just" an incremental advance, since beamwidth is effectively a function of wavelength, the beamwidth is going to be around 10000 times narrower for optical vs microwave communications. For an optical communications link, you have to use active tracking systems to keep the transmitter and reciever continuously aligned (with some sort of feedback). 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.
      • > 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
  • It's a bird!
    It's a plane!
    Ow, that thing blinded me!

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