# Will future deep space optical communications “ground stations” actually be in space, or on the ground?

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)

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

DSS-43 from here from NASA.

Summary of DSN diameters and transmit powers in this answer

• 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 5:00
• 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
• @uhoh That sounds really interesting. Can you expand on what you mean by intermittent sampling and give an example? – ChrisR Nov 18 '20 at 4:34
• 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
• @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