What I actually wanted to posit in relation to that is that one could get sooner a c-cabable backbone sibling by marrying two ideas: the airborne concept ongoing as outlined plus what NASA is planning to bring about for the space backbone, e.g [1][2]. It's laser based instead of directed radio-wave only. Sure, both is in the speed range of c, apparantely, laser transmission has in addition a significantly higher bandwidth to offer. "10 to 100 times as much data at a time as radio-frequency systems"[3]. Attenuations to photons in clean atmospheric air are neglible (few mps - refractive index of about 1.0003), so actually a neglible slowdown - easily competing with top notch fibres (99.7% the vacuum speed of light). Sure, that's the ideal case, though, if cleverly done from the procurement of platforms and overall system steering perspective, might feasible.

Todays laser links are in the few km per hop range, with is easily at least one magnitude shorter than radio based equivalents.

Hold on! This is a severe oversimplifcation, isn' it. The devices you're probably referring to are in the low-end segment, dillentically and maybe terrestrially operated only - to mention a few limiting factor conceivable possibly being perceived.

Certainly, there are range limiting factors when fully submerged in the near-ground atmospheric ranges. E.g. in the darkest snow storm, one cannot expect optics to be reliablly working - admitting that. Nothwithstanding, recent research[1] showed astounding achievements of FSOs even in harsh atmospheric conditions - "up to 10 gigabits per second" while in vivid movement, in heavy fog ... for a single pathed laser.

90% mass of the atmosphere is below 16 km (52,000 ft), therefore also most of it's randomness[2]. Meaning, one only had to surpass this distance to more decently unfold the capabilities of an airborne backbone. Therefore, a hierarchy of airborne vessels might be necessary. Might smaller, more numerous ones gatewaying the optics out of the dense parts of the atmosphere to the actual backbone-net borne lasers, might by doing this relaying not laser beam based. Far more mitigation techniques are conceivable. From there on, the shortcomings appear controllable.

I don't know the physics behind it, but people who have better insight than I do tell me "it's hard" to run longer hops (if one wants any kind of high bitrate).

If one looks up what is achievable in space, where the conditions shouldn't be too different from earth atmosphere over 16 km. Thousands of kilometres for a single hop, single path. Now imagine a decent degree of multipathing.

Physical intricacies are certainly a headache in this topic, though shouldn't be decisive, I'd dare to categorize the largest complexity compartment of such a system into the algorithmics for steering, converging or stabilizing the airborne components, directing the optics problerly and in time. The overall automatic or even autonomic operations to abstract it.

Probably, me forming some papers wrapping this up would be worthwile.

[1]https://phys.org/news/2017-08-high-bandwidth-capability-ships.html

[2]https://arxiv.org/pdf/1705.10630.pdf

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Besten Gruß

Matthias Tafelmeier