Comment by nomel

1 year ago

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?

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nomel

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sephamorr  1 year ago

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.

  • _0ffh  1 year ago

    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.

londons_explore  1 year ago

A laser that transmits data at 100Gbps can also transmit at 1 bit per second with an additional path loss of 110 decibels.

You'd normally achieve this by transmitting a well-known pseudorandom sequence. You also need clock stability into the ppb range.

A path loss of 110 decibels is huge. It can easily account for your lenses being hugely off axis.