zlacker

>>alden5+(OP)
My understanding of the state of the art of inter-satellite optical links is that they have only been used between satellites that are basically in the same orbital plane and in more or less the same orbit. That is, the angle from one satellite to the other changes very very slowly, so that the optics don't have to do much tracking -- and consequently satellites can only form an optical link with other satellites that are ahead or behind themselves in ~ the same orbit.

Cross-plane optical links would have a trickier tracking problem.

While there's no explicit mention of same-plane vs cross-plane optical links, I assume that the first time people have a public cross-plane optical link, they will make a big deal out of it. :)

The article also mentions that SpaceX would need to do further study before using laser links between satellites and ground stations-- this kind of optical link would require both more angular tracking and probably atmospheric correction as well.

>>ble+c55
Take a look at the slides from the presentation, I think the geometry clearly shows cross-plane links in the mesh. Having worked on these types of systems, I've had more difficulty with the lookahead angles (rx from where the target was, tx to where it will be due to speed of light) than the tracking -- fine tracking performance was required for all modes, and it largely became a GNC and acquisition time issue (since they're ephemeral) for the cross-plane links.

>>sepham+Nq5
In general, how is the initial alignment performed?

Is there rough pointing, followed by some rastering, until the sensor gets a hit? Maybe with some slight beam widening first? My assumption is that you would want exactly one laser, one sensor module, and probably a fixed lens on each? Is the sensor something like a 2x2 array, or pie with three pieces, to allow alignment? Or is it one big sensor that uses perturb and observe type approach to find the middle?

Also, is there anything special about the wavelengths selected? Are the lasers fit to one of the Fraunhofer lines? 760nm seems like a good choice?

>>nomel+MQ5
Alas there is no 'in general'. Acquisition is often the secret sauce due to, among other challenges, the extremely tight alignment requirement -- thermal shifts, satellite wobbling, etc, are all critical to manage.

On wavelengths, if you're trying to hit 100gbit+, you're probably having to use coherent optics, and there aren't many technology options or wavelengths on the market.

>>sepham+WU5
You got it exactly right! I worked on a simulation model of the complete optical setup of a laser terminal with movable mirrors and all including the fricking servo motors and a simple orbital model for the relative satellite positions. Plus an interface to drop in the actual acquisition and tracking code used on the embedded control system. All of that just to be able to do reasonably realistic simulations for verification and tuning of the secret sauce.