China's BeiDou in race with Nasa over laser communications in space

China has conducted a pioneering high-speed communication experiment using lasers, rather than the usual radio signals, between satellites in its BeiDou navigation system and ground stations on Earth.

The method could allow a satellite to beam data to the ground at several gigabytes per second, rather than kilobytes at present, according to researchers involved. BeiDou's fastest communication performance in the experiment remains classified.

China and the United States are in an intense race to establish laser communication networks in space. Nasa announced on Monday that, after a two-year delay, it would next month launch an experimental satellite to conduct similar experiments, testing data transmission via laser beam at 2.8GB per second.

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BeiDou satellites normally connect to ground users by radio signal, which can transmit only short text messages because of limited bandwidth. With the help of a laser, the network could transmit data a million times faster to almost any location at any time.

Laser communication offers wider bandwidth, is less vulnerable to being tapped by spies or jammed during electronic warfare, and uses lighter, more compact technology.

BeiDou is the world's largest global satellite navigation network, with more satellites in orbit than GPS. It provides both positioning and communication services, a unique ability with a wide range of civilian and military applications.

The Chinese Academy of Sciences said last Friday that in the latest experiment scientists proved that BeiDou's laser signals could be received stably in challenging environments such as cities, where long-range light-based communication would be difficult to establish and maintain, given disturbances in the atmosphere. It did not say when the experiment took place.

A ground station for laser communication is usually a fixed facility with sophisticated devices including a large telescope, a beam tracking and locking unit, and equipment for signal processing.

However, the Chinese scientists and engineers managed to pack everything into a car to allow mobile deployment, said the project team, from the academy's Changchun Institute of Optics, Fine Mechanics and Physics in the northeastern province of Jilin.

Their findings suggest that China could become the first nation to own a laser communication network with global coverage, according to some space scientists.

"This is a revolution," a Beijing-based space communication researcher said.

The bandwidth of laser beams can reach one terabyte (1,000GB) per second. They are also less susceptible to being tapped by a third party, and more difficult to jam during electronic warfare than microwaves, according to the researcher, who requested not to be named, citing the technology's sensitive application in the military.

A traditional communication satellite is usually bulky, because it requires huge antennas and lots of power to generate and transmit large numbers of radio signals. Laser devices are smaller and lighter, potentially allowing satellites built mainly for other purposes to establish high-speed communication with one another or to the ground, according to the researcher.

The idea of a laser communication satellite has been around since the 1960s. The US, Japan and some European countries have launched numerous research programmes to develop the technology, but failed to solve some of the practical problems involved.

One major challenge was the atmosphere: air molecules could absorb or reflect light, resulting in too few light particles reaching the ground station.

Turbulence could also distort or shake the laser beams so much that the light signals became too blurry to read, especially in urban areas with intense human activity.

China joined the race late in the 1990s, but joint efforts by research institutes have offered solutions.

Researchers in Shenyang say they have developed a telescope mirror that can change shape with an electric charge, to reduce the blurring effect caused by air turbulence.

Some quantum breakthroughs have found immediate applications in laser communications, so intense Chinese investment in quantum programmes has helped.

Mozi, the world's first quantum satellite, reached a staggering download speed of 5.1GB per second in 2016, thanks to sensitive laser equipment developed by Chinese quantum scientists.

Soon after the Mozi experiment, the Chinese space laboratory Tiangong 2 conducted the world's first space-ground laser communication in daytime, overcoming another major obstacle in practical application.

In 2019, a laser ground station in southwestern China downloaded data from the experimental satellite Shijian 20 at a record speed of 10GB per second.

In recent years, Chinese space authorities have unveiled several plans to launch a large number of small communication satellites into lower-Earth orbit.

Whereas the Starlink by SpaceX - a satellite constellation providing internet access to most of the planet - was limited to a bandwidth of about 200MB, the Chinese networks would use laser technology to increase the speed at lower cost, according to some researchers involved in the programmes.

China's Earth orbit, moon and Mars space assets are growing rapidly, and linking them using laser beams can improve their performance.

The overall positioning accuracy of the BeiDou network, for instance, could be increased by six to 40 times by synchronising the satellites' atomic clocks with laser beams, according to a Chinese military study last year.

The Chinese government and military have also considered drafting an international standard for laser communication to strengthen China's leading position in the field, according to another military study published in September.

This article originally appeared on the South China Morning Post (SCMP).

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