Future Deep Space Communications using optical and radio

[Don2]

This seems like a good time for a new thread. I will start by replying to some posts from June 2022 from this thread: https://forum.nasaspaceflight.com/index.php?topic=52493.0

For those 5-10 meter receiving scopes, how far a range can that include? 

"Future Outer Planet Missions can achieve 10 Mb/s from Saturn and 250 kb/s from Neptune by scaling Flight Transceiver to 40cm and 20W"
"Challenge for distances beyond Saturn : acquire and track link without a laser beacon from Earth"
Quotes from here:
https://www.lpi.usra.edu/opag/july2014/posters/9-DSOC_OPAG_Poster.pdf

How big would the earth side have to be?

They are using the 5m Mount Palomar telescope to receive and a 1m telescope to uplink. They would like to use 10m class telescopes, but those would probably cost $150 million each. They think they need about eight of those to ensure cloud free reception.

They are using a 5kw laser for uplink. I wonder if they will need restricted airspace around that transmitter.

Would it need to be that expensive? For $150 million you get an astronomical-grade 10 m telescope, but is sub-wavelength surface accuracy really needed for receiving? Lower quality light buckets like they use in Cherenkov arrays would be much cheaper.

Maybe not. There is a proposal to put some mirrors on one of the old DSN dishes to convert it to an optical receiver. Perhaps a low quality light bucket is what they had in mind.

On the other hand, the advantage of an astronomical grade mirror is that it can also be used for astronomy. The demand for optical communications will ebb and flow, depending on what missions are flying. Astronomy observations could be fitted in around the communications mission. A network of 8m telescopes would be very good for ground based astronomy. Astronomers could use them when the planetary missions weren't busy.

If they were willing to cut the data rate a little, they might be able to put an network together with legacy assets. Australia has the 3.9m Anglo-Australian telescope. Europe has the 4.2m William Herschel telescope. The Magellan telescope in Chile is 6.5m.

https://en.wikipedia.org/wiki/Anglo-Australian_Telescope
https://en.wikipedia.org/wiki/Magellan_Telescopes
https://en.wikipedia.org/wiki/William_Herschel_Telescope

Also a global telescope network would be useful for tracking inbound asteroids, so it would help planetary defense. And it would be useful for following astronomical transients discovered by the Vera Rubin.


I don't think an orbiting receiver makes any sense. They studied a 1-2 m telescope for that. That would probably cost $1-3 billion. A 10m telescope on the ground is $150 million. The ground telescope can also work during daylight at a reduced rate. You can find sites that are clear 70% of the time. Space isn't competitive.

Here is a link to the discussion of optical in the Advanced concepts section
https://forum.nasaspaceflight.com/index.php?topic=55833.0

Here is a link to a discussion of the cost of Deep Space Network expansion
https://forum.nasaspaceflight.com/index.php?topic=55869.0

NASA presentation on optical comms
https://www.nasa.gov/sites/default/files/atoms/files/tglavich_dsoc.pdf

[redliox]

Now for satellites positioned to relay for counteracting solar conjunctions, what would be ideal sizes of their receivers for either optical or radio communication?  Let's presume a gigabit for optical and 10 kilobits for radio from a source at Jupiter.  Would a 1 meter scope and a 5 meter dish suffice?...

[Don2]

Space to space communications is probably the best place to use optical.

At 0.4 AU, they plan to uplink 292kbits/s from a 1m telescope on Table Mountain to a 22cm receiver on the spacecraft. That gives some idea of what a relay at L4/5 might be able to do. Even 1m might be too expensive for a relay.

[Don2]

Here is a potential solution to the Mars Communications relay issue.

It is a smallsat concept using rideshare on a launch to geosynchronous. It is an areostationary communications weather and communications relay satellite. It is an ESPA class spacecraft with 220kg wet mass.

"https://www.hou.usra.edu/meetings/lowcostmars2022/pdf/5048.pdf"

"We selected a Mars synchronous orbit at 0°N, 254° W, that views the Curiosity, InSight, and Perseverance landers at about 11° emission angle. MSO can communicate at maximum data rates with all three landers simultaneously, and at any time of day (on Earth or Mars), enabling 24/7 operations. "

"Expected peak downlink from Mars is 815kb/s (Ka-Band January 2025), X-band uplink 30kb/s. UHF Prox-1 Forward link 39kb/s, UHF Prox-1 Return link 88kb/s from areostationary orbit."

"A four S/C network costs ~$75M, or $100M with 33% reserve."

[Don2]

One of the problems with optical communications to Earth is that large receiving mirrors are required, and large astronomical grade mirrors are quite expensive. Multiple sites would be needed to provide insurance against any one site being clouded over.

There is a much cheaper design of telescope which doesn't create an image but just acts as a big light collector. These are used to detect the Cherenkov radiation emitted when high energy gamma rays hit the upper atmosphere.
https://en.wikipedia.org/wiki/IACT
(Attached image of H.E.S.S. 2 array by Klepser at English Wikipedia, CC BY-SA 3.0)

These are amazingly cheap. A collector with capability equivalent to a 24m diameter mirror can be built for about $8 million and a 12m diameter costs less than $2 million. Compare that to about $60 million for a new 34m DSN antenna or about $150 million for an 8m astronomical telescope!

The study I link to below concludes that this would work as an optical communications receiver, and gives the cost.
https://arxiv.org/abs/1512.00002
I think NASA should buy one of these and test it against the Deep space Optical Communications terminal on Psyche, the Laser Communications Relay Demonstration and the optical terminal on Artemis.
https://en.wikipedia.org/wiki/Deep_Space_Optical_Communications
https://en.wikipedia.org/wiki/Laser_Communications_Relay_Demonstration