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Comment by pifm_guy | Hacker News Reader

Don't optical links to space suffer terribly from atmospheric distortion?

Imagine looking at a shell on the bottom of a swimming pool while there are ripples in the water....

Usually the shell is a bit distorted. But at some points in time, you see two shells... And other points in time, none.

If the water represents the atmospheres shimmering due to changing density, and the shell represents the satellite you're trying to receive data from, then at some points in time, you won't be able to receive any data at all, because the receiver cannot see the satellite.

Network links that are up and down every few milliseconds aren't very useful for much apart from bulk science data download. Perhaps that's why this is marketed for science missions rather than space internet?

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Yes weather is still a challenge for space to ground communication. However there are numerous techniques being developped to handle this. One is to use adaptive optics similar to what is used by telescopes, a similar approach can be done digitally (like MIMO antennas in wireless). Now if the sky is overcast all these techniques don't help, however you only need relatively few ground stations to essentially get to above 99% availability.

That said the first rollout of optical comms is to intersatellite links. There the only issue is loss due to diffraction (and pointing etc., but that's a different discussion). Optics has a big advantage because diffraction is proportional to the inverse of wavelength, and the wavelength of light is several orders of magnitude smaller. This determines the size of the transmit and receive antennas/apertures needed, or the loss for a given size. For realistic parameters optics would have 30-50dB less loss which is translates directly to SNR and thus allows the much higher rates.

Source: Doing research in this area

Can confirm this from my own experience. Just elaborating on the availability requirements, the idea is that clouds aren't everywhere at the same time, so if you have enough in-space bandwidth, you can hop your traffic around the constellation until you reach a satellite that is above a cloud-free area.

Can orbital angular momentum modulation help here? I'm a deepnoob here but iirc some forms of polarisation are more or less affected by 'water in the sky' and masks.

I'm not an expert in this area, but if I remember correctly a lot of satellites use radio links in the neighborhood of 10 Ghz for ground communication because they've found that band isn't affected much by atmospheric conditions.

10-100 Ghz has very few bands where water is an absorber. It tends to fall between the molecular and atomic modes of many molecules, so you only suffer free space loss (which is still significant over long range at high frequency).

"traditional" geostationary satellites use various c, ku, ka and high ka bands. Anywhere from 6 to 30 GHz. Depends a lot on how new it is, what its intended use is and transponder/antenna configuration. and also depending on the intended size/type of earth stations and remote terminals.

in addition to tracking/telemetry/control low data rate radios in the L/S-bands.

A large reason why these are aimed for scientific missions is because the single photon detectors utilized for receiving these transmissions are still incredibly expensive. They are basically liquid-helium cooled arrays of nanowires (SNSPDs) which act like micro balometers - pretty sweet tech, but not commercial grade just yet.

Is there a reason you can’t use an avalanche photo diode biased in Geiger mode to achieve this?

If biased properly an APD can be used for single photon counting and they only cost $70-250, I’m just not sure if they hold up in space without modification.

The nanowire localizes the photons in space, time, and energy. APDs can't do energy.

The cryogenics allow using cryogenic-level bandgaps/carrier-energies, so each optical photon turns into many carriers which then flow out the wire in both directions, the delay between the pulse on both ends localizes the impact along the wire.

SNSPDs have lower dark count rate, and especially lower timing jitter in the near-infrared than APDs.

The deep space optical demonstrations I know of all work in the near infrared (e.g. 1550 nm), just like most of terrestrial fiber optics used for communication. Probably because there's lots of infrastructure at that wavelength already, and it's a good passband through earth's atmosphere.

In network terms the result is packet loss, which is unavoidable. You could shoot through atmosphere with a lower frequency light like infrared but that suffers a reduction of bandwidth and is still subject to packet loss, just less.

I have had to live off a personal satellite in the middle of uncivilized nowhere for a year (literally). Packet loss is the true regulator of speed irrespective of bandwidth numbers.

there's a big section of the article that talks about that (at a high level). Essentially all in the second half of the article with the title "From radio waves to laser light"

There's an entire field of Adaptive Optics for Free-Space Communications (AO-FSOC) where the turbulence of the atmosphere is actively monitored in real-time, and the laser wavefront is pre-compensated to minimise the effects of atmospheric distortion.

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