Producer: Popular Science China
Production: Kawato Space
Producer: Computer Network Information Center, Chinese Academy of Sciences
When you look up at the vastness of the universe, have you ever wondered how astronauts and vehicles on missions in space connect with the ground? Do long distances make data transmission difficult?
Traditional deep space communications: If you want a good signal, the antenna cannot be short
Before introducing deep space communications, we must first understand the "radio frequency transmission system" used in deep space communications. The so-called RF transmission system refers to a device that can carry out RF communication, and the radio spectrum used for space communication is mainly concentrated in 30MHz to 30GHz. This part of the spectrum covers some microwave communications and shortwave communications, mainly used for satellite signals, television signal transmission, etc.
Spacecraft performing deep space missions typically use the 2.02 to 2.30 GHz frequency band for communication. With radio frequency transmission systems, deep space probes can communicate with The Earth, including transmitting data.
The problem with this mode of communication is obvious: the data is received slowly. Taking the New Horizons Pluto probe as an example, data is transmitted back from 7.3 billion kilometers away from Earth, and the downward speed is only 1-4KB per second, and it takes about 20 months for all the data to be transmitted.
Knowing the "radio frequency transmission system", we return to deep space communication itself. In principle, deep space communication has two core elements: a transmitter and a receiver.
The transmitter encodes modulated information onto electromagnetic waves, changes the characteristics of the waves, and "embeds" the relevant data. Electromagnetic waves travel through space to reach the receiver, which demodulates and decodes the electromagnetic waves to obtain the sender's information.
With radio frequency transmission systems, larger aperture antennas are required to improve the signal-to-noise ratio. But improving the signal-to-noise ratio also means that the transmission rate per second will be limited by compressing the bandwidth.
ILLUMA-T laser demodulation device installed on the International Space Station (Image: NASA)
We can't blindly expand the antenna aperture, NASA's deep space network uses a 70-meter antenna, the weight has reached more than 7,000 tons, and the servo mechanism is also very large. Therefore, to increase the bandwidth so that it can carry more data per second, you need to change the way you communicate.
Canberra Deep Space Communications Centre radio antenna in Canberra, Australia, changes the way communications can carry more data (Image: NASA)
Want to watch a 4K HD moonshot video? Laser communication to help you
Recently, NASA launched a revolutionary way of data communication from space to the ground, the Laser Communication Relay Demonstration Project (LCRD). On December 7, 2021, the project entered the engineering verification phase.
The experiment was conducted in a geosynchronous orbit of 35,000 kilometers. The specific embodiment is: the STPSat-6 probe into orbit, the use of air-to-ground lasers to connect Hawaii and California two ground stations, the use of optical terminals on the International Space Station to receive and transmit data, to verify the improved laser communication technology.
Optical module of the LCRD demo project to emit infrared lasers (Image: NASA)
If high-orbit laser communication is successful, laser communication can also be performed at positions below the synchronised orbit.
The LCRD demonstration project is NASA's first two-way optical communication relay satellite, its predecessor is the Lunar Laser Communication Demonstration (LLD), which was launched in 2013, which verified the feasibility of space laser transmission, while the former increased the transmission speed of space lasers from hundreds of millions of bits to 1 billion bits, improving bandwidth.
Lunar laser communication demonstration project launched in 2013 to verify the feasibility of Earth-Moon laser communication (Image: NASA)
NASA's laser communication relay demonstration can be said to open the door to future space communications, which means that deep space communication technology will undergo significant changes.
For example, during the Apollo moon landings, the lunar spacecraft used an RF system to transmit the lunar landing status in real time, and the ground could only receive granular black-and-white video. If laser transmission is used, then the 4K HD lunar landing video can be uploaded back from the lunar orbit. In terms of transmission rate, the data transmission rate of sending and receiving data through invisible infrared lasers is 10 to 100 times that of traditional spacecraft RF communication systems.
A demonstration of lunar laser communications in 2013 has proven that we can transmit data from the moon to Earth at a rate of hundreds of millions of bits per second, the equivalent of transmitting more than 100 high-definition TV channels at the same time.
The aim of this laser communication relay demonstration project is to achieve a rate of 1.2 gigabits per second, at which time we can upload a movie back from a synchronous orbit in under a minute.
Laser communication systems are smaller, lighter, and consume less power than RF systems, and these advantages, combined with higher bandwidth, can greatly help unmanned probes and astronauts to explore various celestial bodies in the solar system.
There is hope for the live moon landing, but there is still a long way to go
The most realistic implication of laser communication is that we have the opportunity to see the process of returning to the moon. The Moon is about 380,000 kilometers away from us, and for the speed of light approaching 300,000 kilometers per second, it can be said that there is no noticeable delay. In this way, the live broadcast of the moon landing will become clearer, equivalent to watching a high-definition live broadcast.
Of course, space laser communication also has disadvantages. Unlike traditional spacecraft RF communications, optical signals cannot penetrate clouds. This means that if space-based laser-to-ground communication is to be carried out, multiple relay stations need to be established to avoid communication interruptions due to weather reasons. NASA chose two ground stations, Pingdingshan in California and Akala in Hawaii, because it was characterized by the lack of clouds here.
Laser communication ground receiving device in Hawaii (Image: NASA)
Moreover, the development of laser communication is only in its infancy, and RF communication does not have to worry about being replaced. Laser communication can supplement the lack of radio communication, making the communication capabilities of deep space missions more powerful. Strictly speaking, deep space communications are more than 2 million kilometers, and NASA's tests in synchronous orbit and lunar orbit are only the initial stages of laser communication, and only by establishing a perfect connection mechanism within the Earth-Moon system can it be extended to other deep space missions.
NASA Laser Communications Demonstration Satellite (Image: NASA)
When extending communication distances, the problem of delay is difficult to avoid. For example, if an astronaut is sent to Land on Mars, the signal delay is about 4 minutes when Mars is closest to Earth, and about 24 minutes when it is farthest from Earth. Obviously, we have to first ensure the stability of the laser communication in transmission before we can deal with the other effects of the delay.
Laser communication has a great future, and real-time control is not a dream
In terms of later applications, laser communication can also be extended to exploration missions for asteroids, solar systems and other distant objects. In addition to the basic transmission data, 3D high-definition video signals can be transmitted to Earth, so that scientists on the ground can control landing devices on distant objects in real time, essentially remote monitoring and control. This application scenario is suitable for unmanned lander survey of the surface of extraterrestrial objects, and even the combination of radio frequency and laser can be used to explore the underground caverns of Mars in real time.
With the approaching trend of celestial integrated information network, the traditional microwave communication method is limited by bandwidth and rate, and it is becoming more and more difficult to meet the current multimedia service needs, and laser communication is becoming one of the best solutions to replace microwave communication. Especially in large-scale aerospace projects, such as air transport, space launch and other major applications, laser communication technology can also build high-dynamic satellite laser networking and expand the scope of space-based information services.
Laser communication technology can also build a high-dynamic satellite laser network to expand the scope of space-based information services (Image source: NASA)
It is worth mentioning that China's Beidou satellite and ground stations also use laser signal transmission, and the related high-speed communication experiments have been completed, and the speed is faster than the 5G network.
In the near future, China will have the world's first laser communication network covering the world, and the large-scale application of satellite Internet is also coming. Recently, the news that a new generation of heavy rockets are being developed has also been disclosed by CCTV and other media, which means that China may achieve manned moon landing in the near future. With the continuous maturity of laser communication technology, it is believed that it will also be applied to the field of manned moon landing to achieve real-time live broadcast of China's moon landing.
bibliography:
[1] Laser Communication Relay Demo Project Homepage:
https://www.nasa.gov/mission_pages/tdm/lcrd/index.html
[2] NASA'S NEXT STEP IN OPTICAL COMMUNICATIONS
https://www.nasa.gov/sites/default/files/atoms/files/lcrd_fact_sheet.pdf
[3] Zheng Yunqiang, Liu Huan, Meng Jiacheng, Wang Yufei, Nie Wenchao, Wu Junxia, Wei Shuzu, Wei Sentao, Yuan Zhanchao, Wang Wei, Xie Xiaoping. Research progress and trend of space-based laser communication and key technologies[J/OL].Infrared and Laser Engineering:1-15[2021-12-17].