China Makes High-Speed Laser Links in Orbit

A commercial Chinese firm has demonstrated ultrafast data laser transmission between two satellites, marking a step forward for the country’s communications megaconstellation plans.

Laser Starcom, a commercial aerospace firm established in Beijing in 2020, announced in March that it had achieved a 400-gigabit-per-second communications link between satellites. Its two satellites, Guangchuan 01 and 02, launched into low Earth orbit (LEO) in November last year on a commercial Zhuque-2 rocket developed by the Beijing-based Landspace company. The pair of Guangchuan spacecraft completed their optical transmission test on 18 March, according to a Laser Starcom statement, across a separation between satellites of 640 kilometers.

The company said the test established an enhanced intersatellite, or crosslink, communication system, supporting future high-bandwidth space networks. In addition to showcasing the growth of the nation’s satellite Internet development, the news further highlights the role of commercial companies in China’s growing space ambitions.

Laser Starcom did not release all the details of its test. However, its statement noted that the satellites transmitted 14.4 terabytes of “business data” over a total communication time of 6 minutes and 44 seconds. (Doing the math on these numbers yields less than 300 Gb/s of bandwidth. The quoted 400 Gb/s rate, however, includes other components, including service and protocol data.) The company also reported that the satellite links remained stable over time, with tracking errors of less than five microradians (0.0002865 degrees).

The tracking precision, the company says, was essential to the laser-link test. While terrestrial fiber optics require no pointing precision, laser optical communications in orbit require ultraprecise steerable telescopes to be able to send beams either to another satellite or, adding in atmospheric complications, to the ground.

The fact of being in orbit brings additional challenges. Satellites in LEO—a region above Earth at an orbital altitude of between 300 and 2,000 km—travel at approximately 28,000 kilometers per hour, or 7.8 kilometers per second.

“Those steerable telescope assemblies need to compensate for the high speed that satellites are traveling at in space,” Harald Hauschildt, head of the European Space Agency’s Optical and Quantum Communication Office, told IEEE Spectrum. So fast, steerable mirrors are needed to keep crosslink tracking during signal transmissions—because even tiny misalignments of the laser traffic can break the connection.

What Are the Challenges of Laser Satellite Communications?

The challenges Laser Starcom and other commercial and research efforts around the world face are worthwhile, because laser links operate at far higher frequencies than radio waves.

The European Space Agency is working with partners to develop its High-throughput Digital and Optical Network (HydRON) laser-based satellite system and aims to demonstrate optical communications networks in orbit at speeds up to 100 Gb/s and higher. This data rate, according to the ESA, is expected ultimately to be scalable to one terabit per second. And according to Hauschildt, HydRON could in that sense one day compete with terrestrial fiber-optic networks. The E.U. is in fact developing a satellite communications constellation, IRIS², in response to SpaceX’s Starlink as well as China’s Guowang and Qianfan constellations.

Jade Wang, of MIT’s Lincoln Laboratory, was part of the team that developed TBIRD, a compact payload on a small satellite that demonstrated a 200 Gb/s space-to-ground link in 2023 and tackled some of the added challenges of transmitting through the atmosphere—including atmospheric turbulence scattering or distorting the beam. The Lincoln Lab researchers achieved this high-data-rate optical link with a commercial off-the-shelf optical transceiver.

“We developed an automatic repeat request protocol, and we wrapped it around the existing transceiver, and we did it in a way that was efficient and that allowed us to get error-free performance space-to-ground,” says Wang.

That test followed a long line of experiments, including the 2013 NASA lunar laser communication demonstration (LLCD), that solved challenges such as achieving a high accuracy of pointing for the beams.

Regarding the Laser Starcom test, Wang says it appears to have achieved a higher data rate than has been achieved globally before. “But if they’re still using commercial technologies, it’s more of an incremental change than a technological shift,” Wang says.

What Are the Next Steps for Laser Satellite Communications?

The already operational Starlink uses laser crosslinks of around 100 Gb/s. But the speeds achievable, Wang says, represent trade-offs among a number of parameters including terminal size, power usage, and data rate.

“So if you want more data rate, you have to have more power and/or a larger class [of terminal],” Wang says. “It’s just about what the mission wants.”

Laser Starcom’s website says the company’s terminals support multiple communication rates of 10 Gb/s, 100 Gb/s, and 400 Gb/s. These scalable rates could suit different mission requirements.

While satellite crosslinks are now fundamental to the proliferated LEO satellite communications systems noted above, there are other uses. Space-to-ground laser communications can boost the amount of data sent back to Earth in one transmission.

Remote sensing satellites collect huge amounts of Earth observation data but have only about 5 minutes of visibility to a ground station as they whizz across the sky. Laser communications, with vastly higher transmission rates compared with those of radio frequencies, can significantly increase the volume of data returned to Earth during each ground pass. The same goes for data-heavy science missions. Laser comms could also support countries’ lunar plans, which in China’s case is the International Lunar Research Station.

“What I’m hoping to see is a robust industry, supporting free-space optical communications for multiple applications, and not just proliferated LEO,” says Wang. “I’m very excited with what’s happening in laser communication and for where this field is going next.”