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Deep space communications ground stations are on the ground because their transmit and receive electronics alone is bulky and heavy, not to mention their 34 and 70 meter dish antennas!

But those constraints change when it can be a 1 meter or smaller telescope with a laser diode or photodiode (or superconducting thingy) at the focal plane, exploiting the much smaller theoretical diffraction limit. For example right now in DSN Now I see DSS-14 a 70 meter dish receiving signals from Juno 8.4 GHz. That's a wavelength of 3.6 centimeters, so $(D/\lambda)^2 \approx \text{3.8} \times \text{10}^6$.

If instead a 0.85 meter telescope was used at 850 nm (a random, typical AlGaAs laser wavelength) we'd have $(D/\lambda)^2 \approx \text{10}^{12}$

Remember that the far-higher gain is also available at the other end of the link as well, so this analysis is a gross underestimate of the total improvement in link budget going optical, but that's okay because I haven't included some of the challenges.

A few watt laser is comparable to a few watt deep space probe transmitter, and only four orders of magnitude lower than a DSN transmitter (except for things lke Has DSS-43 ever been used in high power mode (>>20 kW) for an emergency situation?) so our circa-one-meter telescope "ground station" beats a 70 meter dish if it can be pointed steadily and around any intervening clouds

Hubble can be pointed steadily and around any intervening clouds, and there are several high altitude locations around the world where optical telescopes can be pointed steadily and usually around intervening clouds, often using adaptive optics. (see also the surprising answers to Why aren't ground-based observatories using adaptive optics for visible wavelengths?)

Question: Optical deep space transceiving stations of circa one meter diameter could be Earth orbit or on the ground in several places, with adaptive optics if necessary. What are the most important technical tradeoffs that will determine where they will most likely end up being deployed?

Below this answer I'd summarized the following collection of Q&A on optical communications for deep space:

"interplanetary radio communication" will go away and be replaced by optical in the not-so-distant future because a 20 kW transmitter or liquid helium-cooled front-end receiver at the end of 34 meter dish on Earth can be replaced by a few watt laser diode and an avalanche photodiode or one of those superconducting thingies at the end of a 20 or 50 cm diameter telescope. (see also)

See also Quantitatively, why will optical communication be better than X-band for deep-space communications? and How is long-distance optical communications coming along in space? and Are there plans or a program for an optical relay pathfinder for deep space? and What optical design is used by the GEDI's receiving telescope and how is the secondary held in place? (optical com will look similar)

and When will STP-3 launch with the new optical space coms test and why is it late? and What GEO relay satellite will the ISS use for end-to-end optical communication with a ground station?

DSN Now screenshot

A very heavy ground station, very likely to remain on the ground for the foreseeable future:

enter image description here

DSS-43 from here from NASA.

Summary of DSN diameters and transmit powers in this answer

uhoh
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  • companion question: "...extending the Starlink network using longer-range laser comms and relay satellites to reach Mars and Starships in-transit" (Musk & Shotwell) – uhoh Nov 17 '20 at 02:45
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    It's notable that as far as has been publicly discussed, Starlink will have optical interlinks only for satellite-to-satellite communications. For talking to the ground, it uses radio. – Christopher James Huff Nov 17 '20 at 05:00
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    If the optical deep space transceiving station is placed in an area with (almost) no clouds like the Atacama desert, it should work. – Uwe Nov 17 '20 at 10:19
  • @Uwe I guess there will be need for a few of them for 24 hour coverage, like the way DSN is set up. Look for millimeter-wave radiotelescope sites or infrared observatories; those are sensitive to water content and so are located in high altitude areas with little water and therefore not many clouds. – uhoh Nov 17 '20 at 10:43
  • My understanding of the industry is that laser comms is still seen as only possible for decently large spacecraft and only for communications in space. Space to Earth leads to lots of distortions due to the atmosphere (DSN provides ion- and tropo-spheric correction info, and weather info). Only decent large spacecraft usually have good enough attitude control to sustain a laser link: pointing requirements are pretty stringent. – ChrisR Nov 18 '20 at 00:36
  • @ChrisR the reason that I discuss adaptive optics in the question is that it can correct much of the distortions in the atmosphere, especially when the target is small as opposed to a wide field, and the aperture modest (circa 1 meter). It's contingent on being at fairly high altitude (both for reduced turbulence and for lower water vapor and so fewer clouds) which means the current DSN sites are not optimal for optical. – uhoh Nov 18 '20 at 01:11
  • @ChrisR I also disagree that the size of the spacecraft is important; the only thing that's needed is a fast actuator at the focal plane, the same way that active image stabilization works in cameras and even some binoculars, and some modest articulation with a lower frequency response, so the footprint on a spacecraft will mostly be just the size of the telescope itself. A 20 or 30 cm Cassegrain on the spacecraft (with a tiny MEMS-actuated optical coupler at the focal plane) and a circa 1 meter telescope on the ground at several astronomical observatory sites is all that's needed. – uhoh Nov 18 '20 at 01:15
  • I've done some casual numbers and the link budget, even with an uncorrected 1 arc second beam for the ground station due to atmospherics is way better than a dish in space and a dish on the ground at 8 GHz. The only problem I see is that it only works at night and in good weather, which makes astronomical sites around the Earth prime candidates. – uhoh Nov 18 '20 at 01:21
  • I'll trust your calculations as I haven't done them, but I'll say that given how busy the DSN is, night comms only is not great. Concerning the fast actuator, it won't matter if your spacecraft can't maintain good pointing to the ground. Think about a cubesat with a magneto-rod as its attitude control in LEO, it probably won't be able to continuously point to the ground station for more than a few seconds. The "larger spacecraft" statement was a shortcut for "spacecraft with good ADCS." – ChrisR Nov 18 '20 at 03:40
  • The other thing to consider for cislunar and deep space spacecraft is that the DSN is also used for ranging, and that requires continuous tracking of the spacecraft for a few hours (2 to 8 hours usually). That will generate range and range-rate data using the same transmission freq than for data comms. Finally, generating a powerful laser onboard requires a good amount of energy, and that shouldn't be neglected either. – ChrisR Nov 18 '20 at 03:42
  • @ChrisR the question's title begins "Will future deep space optical communications..." so it doesn't ask about replacing all aspects of DSN and similar networks. Instead, it only asks about extending capabilities of communications to deep space spacecraft via optical signals. But this brings up an intriguing question! Closest distance at which cubesat has communicated with the Deep Space Network? Cis-lunar ever? – uhoh Nov 18 '20 at 03:52
  • @ChrisR your comment about ranging is really important and thought provoking. Since a 1 meter aperture telescope can slew way faster than a 34 or 70 meter dish antenna, you can now think about intermittent sampling. Even though an extended timespan of data may be required, you don't necessarily need to monitor continuously as long as the phase noise of the encoded signal is sufficiently small. The bandwidth of the ranging signals must be tens of MHz at most, you need to sample a long time for accuracy, BW of an optical link can be tens of GHz so modulated Gold code can can come 1000x faster! – uhoh Nov 18 '20 at 04:04
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    @uhoh That sounds really interesting. Can you expand on what you mean by intermittent sampling and give an example? – ChrisR Nov 18 '20 at 04:34
  • @ChrisR thanks, I'll do it in the form of a question within the next 24 hours. – uhoh Nov 18 '20 at 04:45
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    SpaceX is already deploying laser interlinks on Starlink satellites, which certainly qualify as small spacecraft. They need only equip a few satellites in one or two planes with some additional links for interplanetary relay satellites, and they'll have massively redundant downlink capability through the phased array antennas used to provide internet service. – Christopher James Huff Nov 21 '20 at 15:59
  • @ChristopherJamesHuff does "...equip... with some additional links..." mean that they could add a few spacecraft with larger aperture (higher gain) optical links with better pointing capabilities necessary "...for interplanetary relay satellites..."? – uhoh Nov 21 '20 at 23:01
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    @uhoh the standard Starlink sats are equipped with enough laser links to connect to neighboring Starlinks (likely previous and next in the same plane, and some more for crossing planes), and I was mainly thinking that they'd need additional links to handle duties outside of that...though they might just redirect one of the normal links instead. I wouldn't expect the low-altitude Starlinks to directly handle interplanetary comms, but I can see the standard inter-Starlink link hardware being sufficient for reaching a relay satellite in a higher orbit. – Christopher James Huff Nov 21 '20 at 23:17
  • @ChristopherJamesHuff me too, thanks! – uhoh Nov 22 '20 at 00:32
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    @ChrisR just posted: Long delay-doppler measurements on deep space craft; couldn't optical measurements be made intermittently, mixed with contacts to other spacecraft? – uhoh Nov 22 '20 at 01:32

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One report of a study on the topic funded by NASA JPL in 2005 is "Deep-space to ground laser communications in a cloudy world". Based on global cloud statistics, they worked out the probability of success as a function of number of receiving stations and optimized their location, and found that the number needed to achieve the desired availability was well over budget. One ground station in the Atacama is good, but multiple ones all in the same desert are not, because the autocorrelation length of the cloud/no-cloud signal they found is around 600 km, so if one station in Chile was clouded out, so would the other ones probably be at the same time. You want them all at high altitude mountain observatories, but scattered over every continent, including Haleakala, Kilimanjaro if you could get permission, and other widely separated high and dry places. Another problem is turbulence, which since it is caused largely by solar heating, is worst exactly where clouds are least frequent, so you need astronomy-grade adaptive optics and probably multi-meter deformable mirrors to be able to correct the signal as it arrives. Aerosols are another consideration, which makes coastal regions less good due to sea spray, even separate from the humidity that produces clouds. My own conclusion from following this group's work for several years was that the right way to do it is laser from deep space to earth orbit, with the space-to-ground links in radio for affordable availability.

Ryan C
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  • Thank you for the both thoughtful and thorough answer! After reading it I'm completely convinced. I do realize however that my question offered a false binary choice; there is of course a third option; telescopes hanging from high altitude balloons :-) I'm imagining something like a mashup of Project Loon + ASTHROS somehow. It could be overseen by surplus managers from JWST and the F-35, and SpinLaunch can help raise early funding. – uhoh Mar 02 '21 at 08:40