The laser beam didn’t exactly descend in pristine condition. After traversing 36,000 kilometers through the tumultuous skies above southwestern China, the atmosphere worked its usual magic on light. The signal ended up dispersed and, by the time it reached Lijiang Observatory, it had transformed from a focused stream of data into a feeble, indistinct glow spanning hundreds of meters of chilly mountain air.
Most receivers would have picked up mere static. But this one somehow detected a one gigabit-per-second data stream hidden within the chaos. Interestingly, the laser transmitting this data operated on just 2 watts—less energy than a small LED bulb. Remarkably, when measured at a distance that equals the Earth’s total circumference, this setup transferred data five times quicker than standard Starlink speeds.
It’s essential to note that these two systems are designed for distinct environments. **Starlink** satellites travel just a few hundred kilometers above Earth, communicating with consumer terminals through radio frequencies. In contrast, the Chinese experiment utilized a **geostationary satellite** stationed much farther out—60 times the distance. What’s more, the receiver used wasn’t a simple rooftop dish. It was, in fact, a 1.8-meter telescope paired with advanced signal processing equipment. The significant takeaway was that a laser downlink from such a high orbit could achieve gigabit-per-second speeds while still allowing for a meaningful payload.
The Atmosphere Scrambled the Beam Almost Instantly
On the night of this test, the air above Lijiang was anything but calm. The atmosphere over the high peaks of Yunnan is a turbulent mix of layers, each differing in temperature, density, and refractive index. As a laser beam traveled through this disarray, it became bent, scattered, and distorted in real time. These disruptions shifted every few milliseconds, transforming what started as a clear beam of light into an unpredictable jumble by the time it reached the telescope’s mirror.
Engineers have had to grapple with this issue using two main strategies. **Adaptive optics** employs a deformable mirror comprised of hundreds of small segments that flex independently. This setup senses how the incoming wavefront has been altered and applies a corresponding correction. However, it’s always a step behind, especially when things get really turbulent; when that happens, the mirror simply cannot keep pace.
On the flip side, **mode diversity reception** embraces the disruptions and seeks out any intact fragments. The jumbled beam gets divided into multiple spatial channels, providing different views of the damaged signal. Some channels manage to isolate clearer pieces than others. By merging the strongest signals, the receiver can partially reconstruct the initial transmission. This method tends to perform better under severe turbulence than adaptive optics can alone, although it unfortunately still leaves behind any usable signal.
So far, neither approach had successfully achieved a geostationary optical link exceeding gigabit speed with such a low-power transmitter. Yet, the team, led by Wu Jian of Peking University of Posts and Telecommunications and Liu Chao from the Chinese Academy of Sciences, combined both methods.
The Receiver Selected the Three Strongest Channels from Eight
The incoming beam first struck the telescope and passed through a correction phase fitted with 357 micro-mirrors that adjusted in real-time, responding to atmospheric distortions detected on-the-spot. The aim here wasn’t to create a flawless beam; rather, it was to stabilize it enough for the next stages to be effective.
Following that, the light moved into a multi-plane light converter that divided the signal into eight distinct spatial channels. A digital processor assessed all eight and pinpointed the three strongest channels to combine, while the noisier five were disregarded, channeling the refined result into the decoder.
Previously, the combined system rendered the signal around 72 percent usable. After processing, that figure improved to 91.1 percent. This boost catapulted the data rate to 1Gbps on merely 2 watts of transmitter power. The laser was weak, the orbit was challenging, and the atmosphere was not on their side. Still, the receiver succeeded, accepting the reality that the beam would arrive fragmented and focused on recovering whatever pieces were intact.
The South China Morning Post reported that this speed means a high-definition movie could be sent from Shanghai to Los Angeles in less than five seconds. Yet, this was just a single demonstration under specific conditions. The reported figures stem from real measurements—not simulations. Moreover, they represent just one data point and not an ongoing service.
A Fixed Point in the Sky Holds Its Own Value
There’s a clear advantage for low Earth orbit satellites: their proximity. A geostationary satellite, positioned at 36,000 kilometers, has to penetrate through the thick turbulence of the lower atmosphere. The beam inevitably diminishes significantly just due to distance. Therefore, the engineering hurdles become much steeper.
However, the advantage of a geostationary satellite is its permanence. It doesn’t zip across the sky; it remains fixed, maintaining a continuous connection with a single ground station indefinitely. For situations that can’t afford interruptions—like disaster response networks or secure military communications—this stable position is worth the harsh distance.
Laser wavelengths can transmit much more data than radio frequencies and are generally more challenging to intercept or disrupt. Yet, the atmospheric and distance obstacles have kept geostationary optical links limited to modest demonstrations. The experiment in Lijiang illustrates that a practical receiver design can bridge the gap without requiring an excessively powerful transmitter for orbit.
The Significant Advancement Occurred on the Ground
The satellite transmitter itself wasn’t anything groundbreaking. Just two watts is minuscule in the context of space. The real innovation lay in the receiver’s capacity to salvage a signal that had already endured atmospheric interference. This flips the narrative usually associated with space communications, which primarily focuses on what’s launched into orbit. The Chinese team managed to shift the bulk of the work to the ground.
The setup in Lijiang isn’t a consumer-ready product. The entire system, including the telescope, deformable mirror, multi-plane light converter, and real-time processor, occupies a research facility. This infrastructure serves a vital role: acting as a limited number of high-capacity ground stations that feed satellite data into terrestrial fiber networks.
