[ble]

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.

[ortusdux]

> “Another really fun fact is that we held a link all the way down to 122 kilometers while we were de-orbiting a satellite,” he said. “And we were able to downstream the video.”

> For the future, SpaceX plans on expanding its laser system so that it can be ported and installed on third-party satellites. The company has also explored beaming the satellite lasers directly to terminals on the Earth’s surface to deliver data.

[FpUser]

So what happens to the laser beam when there are clouds?

[sephamorr]

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.

[nomel]

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?

[ble]

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.

[stcredzero]

GNC?

[sandworm101]

Tracking is an issue, but doppler can also be a thing. At orbital speed (actually up to 2x orbital speeds) the doppler effect between two satellites can change the frequency enough to cause interference. Moving a scope to track a moving target is one problem, allowing the algorithms to adapt at the frequency shifts on the fly another.

[Scoundreller]

Indeed Iridium had to deal with the same thing (or I guess, didn’t):

“ Cross-seam inter-satellite link hand-offs would have to happen very rapidly and cope with large Doppler shifts; therefore, Iridium supports inter-satellite links only between satellites orbiting in the same direction.”

https://en.m.wikipedia.org/wiki/Iridium_satellite_constellat...

[dreamcompiler]

Doppler is not a big problem with lasers because the carrier frequency is so much higher than RF that it doesn't matter; it's bang-bang AM modulated.

I'm assuming two things: That something like Manchester coding is being used so that some clock skew is tolerable, and that the laser carrier is not in fact being frequency or phase modulated. Last I checked FM and PM of optical frequencies was not yet practical outside of laboratories, but I'm happy to be corrected.

[ble]

Links between satellites closing range near 2x orbital speed have two problems: - bigger doppler - the lifetime of the link is much shorter

[_0ffh]

Nah, I once did a job for a guy and they did LEO-GEO distances alright iirc and LEO-Earth in the mid-end 2000s, which has to deal with some pretty high angular velocities, if not as potentially high as LEO-LEO when they don't happen to be relatively nicely aligned. (In case that sounds strange, the guy was one of the two owners of a small, very specialized company that in turn was subcontracted by a rather bigger company. These laser terminals were quite the beasts and not really cheap.)

[londons_explore]

With 266,000 acquisitions per day, I suggest they are regularly doing links across orbital planes.

If they could only do in-plane links, they would have barely any acquisitions per day, because most links would stay up for long periods of time.

[Symmetry]

Right. The Iridium network had communication between satellites in different orbital planes passing each other but that was a pretty unusual capability.

[akira2501]

They do have counter rotating planes though, so there are places where two satellite tracks next to each other moving in opposite directions, and these pairs of satellites cannot use the cross plane communication mode.

Additionally, their inter satellite links use regular Ka band radio.

[Scoundreller]

I wonder how iridium actually handles the tracking (or if it’s just slow enough and lack of attenuation in free space just lets them blast it).

And if they have zones where they don’t go to adjacent orbits, but instead go up or down within their orbit for the handover between orbits.

[calrain]

I'll assume there is a lot of double/triple (or higher) accounting going on here as data is sent through multiple relay hops to get the intended target.