Space-ground optical (laser) communication

 

Inter-satellite laser links are in use now, but the technology for optical links to the ground is still being developed and tested. 

Optical frequency laser communication links have many advantages over radio frequency (RF) links:

• Optical transmission is much faster than RF communication.
• Optical terminals are smaller and cheaper than RF terminals and use less power.
•  It's harder to intercept or jam optical signals so they are more secure and, conversely, better for clandestine use.
• Multiple optical beams can be transmitted simultaneously to multiply speed.
• Optical transmission is license-free. (There isn't enough RF spectrum to accommodate all of the currently proposed satellites).

SpaceX is equipping its new satellites with inter-satellite laser links (ISLLs). They now have over 8,000 optical terminals in orbit (3 per satellite) and they communicate at up to 100 Gbps. The other low-Earth orbit Internet service providers will follow SpaceX's lead.

Optical communication works well between satellites in the vacuum of space, but optical signals are weakened and distorted by clouds, rain, water vapor, dust, heat gradients, pollen, etc. in the atmosphere so today SpaceX and others use RF frequencies for communication between space and the ground.

Optical space-ground projects

Given the long list of optical advantages, many organizations are working on technology to adjust for atmospheric interference and use optical communication between space and the ground. The following are a few examples of optical communication research and development by NASA, universities, the military, private industry, and the Chinese. 

Ten years ago, NASA demonstrated optical communication between a satellite orbiting the moon and Earth, and they are updating that now. They have a data relay satellite in geosynchronous orbit for relaying data from other satellites to the ground and they are working on transmission from deep space beyond the Moon so we will be able to see video from Mars when we land there. They have also transmitted data between a cubesat with a 2.3 kg payload and the ground at a rate of 200 Gbps. 

ETH Zurich test site

Researchers at ETH Zürich have transmitted data from a mountaintop to their lab 53 kilometers away at up to 0.94 Tbit/s/channel. (Note that the top of the stratosphere is only 50 km from the ground). They adjust for atmospheric variance using sophisticated algorithms and terminals with adaptive optics that can correct the wave phase 1,500 times per second. Their technology can scale up to 40 channels and they are working on more efficient modulation schemes.

The ETH Zürich transmission was from a fixed point on top of a mountain to their lab – how about from a moving satellite? LEO satellites move across the sky at an angular tracking rate of ∼1 deg/s and researchers at the University of Western Australia have demonstrated that they can maintain contact with a drone moving back and forth at that rate. 

In Ukraine, SpaceX Starlink has demonstrated both the military value of satellite Internet and the drawback of being dependent on a private company.

SDA Tracking Layer constellation (source)

The Space Development Agency (SDA) of the Space Force is developing two constellations, a Tracking Layer constellation for warning of, tracking, and targeting advanced missile threats and a Transport Layer constellation providing connectivity to the full range of warfighter platforms. 

There will be ISLLs, using SDA standard optical communication terminals, within and between the early constellations. The early satellites will use RF links to the ground, but optical links are planned.

Tracking plus transport to intercept missiles with planned
optical links to the ground (source)

Space Force policy is to scale up its use of commercial capabilities and Mynaric has been awarded Tracking and Transport Layer contracts for their CONDOR Mk3 optical terminal. CACI International and Tesat terminals have also been certified and will be used -- standards enable competition.

Mynaric has also been selected to participate in a demonstration of links between various space-based optical terminals and an optical ground station they will design. 

The SDA is also working with Aalyria, a startup with two products, Tightbeam and Spacetime, that are based on intellectual property acquired from Google. 

Tightbeam is an optical communication technology that sounds similar to that of ETH Zürich. Using adaptive mirrors and proprietary algorithms, they have transferred data to and from a local mountain at 400 Gbps per channel, (They can use four channels simultaneously). They recently signed a maritime contract for connectivity "starting at" 100 Gbps. 

Tightbeam is only available through Spacetime, an extremely ambitious network operating system for controlling fixed and mobile assets and the links between them on Earth, in the air, and in space. Spacetime runs a simulation of the network and if an upcoming problem is predicted -- for example a weather event or an airplane banking -- Spacetime will reconfigure the network to route around it in 200 ms. 

(Spacetime is open source with open APIs and Spacetime networks can “federate,” accessing each other’s assets to create a “network of networks.” Sound familiar? APIs are open and they hope to establish standards -- reminiscent of Ethernet vs early proprietary LAN technology. I recommend watching this Spcetime presentation).

Intelsat has provided geostationary satellite communication since the 1960s and is also working with Aalyria on multi-orbit service and space-to-ground optical communication. (They are also considering a medium Earth orbit constellation -- could federating with SES's mPower constellation be an alternative to creating their own)?

I searched for and found two Chinese optical space-ground experiments, one by Beidou in 2021 and a recent test by The Chinese Academy of Sciences with a 10 Gbps transmission rate. I checked with Blaine Curcio, an expert on Chinese space, and he does not know of other tests.

Ground infrastructure

If projects like the above succeed in developing cost-effective space-ground optical communication technology, we will need significant investment in well-designed ground infrastructure. Optical antennas can be added to existing RF ground stations or new optical ground stations can be built.

World cloud cover map (source)

Augmenting existing ground stations makes sense if they are in suitable locations because they already have real estate, power, and Internet connectivity. For example, SpaceX has 75 gateways in North America, several of which are in arid regions of northern Mexico and the US southwest.

New ground stations with optical gateways will also be needed. They should be in relatively cloud-free places and, if possible, near centers of demand and locations with access to high-speed terrestrial Internet connections and power. The current locations of astronomical observatories might be considered.

African gateways

One of the United Nations Sustainable Development Goals is "to build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation" -- to reduce the digital divide. The needs of underserved areas should also be considered in locating ground stations. Today, SpaceX has only two RF ground stations in Africa, and there are arid regions in the north and south that might be suitable locations for optical ground stations.

ISLL path between arid areas in Mexico and Africa (source).
Such opportunities will increase as ISLLs proliferate.

Even if locations are carefully selected, routing around unfavorable weather or other atmospheric problems will occur at times. That will be facilitated by the proliferation of ISLLs. Furthermore, the addition of ISLLs to sharply inclined orbits will facilitate routing around winter in the northern and southern hemispheres.

Addendum

This post is based on a presentation at a recent UN Internet Governance Forum panel but it has been significantly revised and extended. You can get a copy of the revised PowerPoint presentation here.

The presentation includes a Frequency terminology cheat sheet.

For an excellent tutorial on the properties of laser light, click here.

Thanks to Brian Barrit of Aalyria and Shane Walsh of The University of Western Australia for their input.

Update 11/3/2023

NASA's ILLUMA-T optical terminal will be delivered to the International Space Station in a SpaceX Cargo Dragon launch scheduled for no earlier than November 5. Once installed on the exterior of the space station it will enable two-way communication through the data relay satellite mentioned above to and from optical ground stations in Hawaii and California at 1.2 Gbps.