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I'm currently diving into this topic and as I understand from the brief search there are two main types of inter-satellite link: radio and optical. As I can see, one of the main distinction of optical link is that we need to configure the direction (as opposed to radio, where we emit the signal broadly). First inter-satellite radio link was established in 1975, first inter-satellite laser link — in 2001. What I don't yet understand is that whether optical inter-satellite link and laser inter-satellite link are the same thing?

If it really is the same thing, then my questions are:

  • What are the basic characteristics of such communication channel? Bit rate? angles (which allow to establish connection)? distance?
  • Is the atmosphere an obstacle for optical communication, for satellite-Earth link or for two satellites with a big distance so the beam must go through atmosphere?
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    $\begingroup$ I'll leave a quick comment. If you had a 10 watt radio transmitter and a 10 watt laser that could be modulated, then there's nothing that requires you to make the optical beam any narrower than the radio beam. You could make them both say 3° wide and - very roughly speaking - a 20 meter dish antenna and a 20 meter optical telescope could pick up the signals equally well. But you can make the transmitted beam far far narrower if you wanted to and that would allow you to use a lower power laser and a smaller diameter receiver, and ya maybe you'd like the receiver outside of Earth's atmosphere. $\endgroup$ – uhoh Sep 29 '16 at 15:17
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Yes, the optical link being discussed is a laser--here's an article (behind a paywall, but the abstract mentions that the technology is laser technology) that discusses the exact satellite link you're talking about: http://ieeexplore.ieee.org/document/5475229/

Lasers do not have to be optical light. They can be anything from infra-red to X-rays, though X-ray lasers take a very large power source. But this particular link is an optical-light laser.

Laser-light does not remain perfectly coherent from transmission to reception, but it remains coherent enough that what starts out as a small beam at transmission (at low-Earth orbit) only expands to a few meters in width at reception (at geosynchronous orbit). A regular light would radiate in all directions, dissipating the beam far too rapidly, and would need to be far more powerful than a laser transmitter.

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  • $\begingroup$ Re your last paragraph, the diffraction limit still applies to lasers. For a beam sent from LEO to only be "a few" meters wide at GEO, the transmitter would need to have an aperture several meters in diameter, which is probably impractical (never mind the difficulty of pointing something that tightly collimated). Nonetheless, if you allow the beam to spread to a km or more at GEO, the transmitter can be quite small and you still end up with a much better link budget than radio. $\endgroup$ – pericynthion Oct 1 '16 at 23:57
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A bit of sparse data are available for the EDRS system from ESA: EDRS And a bit from Airbus EDRS

At least some basic data rates and distances.

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