Comment by sephamorr

1 year ago

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.

13 comments

sephamorr

Reply
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?

  • 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.

ble  1 year ago

The "routing in the mesh" slide? Definitely given where the satellites are in that picture some of the links would have to be cross-plane, it's just the whole thing looked so messy (even with it being geo-referenced on a globe) that I didn't know whether to consider it a "real routing example" vs a "notional routing example that we overlaid on the globe".

Sounds very cool that cross-plane links are doable, even if they have predictable complications compared to in-plane.

I would have thought that someone would make a big deal (have a press release, e.g.) out of successfully establishing cross-plane links, but maybe it just doesn't seem that impressive to people who already have good enough precise predictive ephemerides or satellite states to make those links in the first place.