LUCAS system to open up new possibilities in space with inter-satellite optical communication
What is the Laser Utilizing Communication System: LUCAS?
The Laser Utilizing Communication System (LUCAS) is set to enable the collection and high-speed transmission of large volume data by establishing an optical link between the Earth observation satellites and the data relay satellites. We spoke with Hiroaki Miyoshi, leading the LUCAS project within NEC talking about the difference between optical communication and conventional radio wave communication, the technologies used in optical communication and the value this system can bring to our society.
-- Could you give us an overview of the JAXA-led project involving the Laser Utilizing Communication System（LUCAS）?
Miyoshi: The Internet that we use every day is provided over an optical fiber network. LUCAS attempts to use these optical communications technologies to relay between satellites, which until now has been carried out using radio waves. Optical communication has three major advantages over radio communication. The first is that optical communication is capable of transmitting a large volume of data. Radio frequencies are limited in bandwidth and the amount of data that can be transmitted is roughly proportional to the frequency. The operating frequency of the 5G communication system that is currently in widespread use is about 28 GHz, but optical communication has a frequency that is 100 to 1,000 times higher than 28 GHz. This makes it possible to transmit large volumes of data.
The second advantage is that optical communication is license-free. The use of radio waves requires a license because of the limited number of available frequency bands that must be shared by many users. In contrast, because the frequency band of optical communication is largely immune to signal interference, it is license-free and offers a virtually unlimited bandwidth.
The third advantage is that optical communication ensures a high level of confidentiality. Data is transmitted through optical communication between satellites that are separated by a distance of about 40,000 km. When radio waves are sent over this distance, the diameter of the beam spreads to about 60 km. On the other hand, the diameter of optical communication beams are narrow—about 600 meters—making interception of the beam extremely difficult.
-- LUCAS is built to leverage these advantages, is that correct?
Miyoshi: LUCAS is a system name which connects the Earth observation satellites located in low earth orbit which is around several hundred kilometers in altitude, with the optical data relay satellite located about 36,000 km above the Earth's surface both controlled from ground control. The optical data relay satellite was launched in November 2020 and is in geostationary orbit positioned over Japan. With future launches of the Earth observation satellites equipped with NEC laser communication modules, LUCAS will finally be entering the operational phase.
What does it mean to relay data in space?
-- Please explain the development of the Laser Utilizing Communication System to date.
Miyoshi: The world's first inter-satellite optical communication experiment was conducted in 2005 as a concerted effort between JAXA and NEC from Japan, the European Space Agency (ESA), and a private company from Germany. The experiment used laser light with a wavelength of 1 μm (micrometer).
Subsequently, Europe has been advancing efforts, mainly led by the public sector, to use inter-satellite optical communication in the areas of security and climate change. The wavelength used for communication is 1 μm, the same as in the experiment. While beams at this wavelength are powerful and can travel long distances, they are only used in space projects, which presents the disadvantage of making equipment development more expensive.
In contrast, in order to reuse optical communications technologies, parts and components produced in fiber communications, Japan and U.S. is developing inter-satellite optical communication using a 1.5-μm wavelength, which is widely used in terrestrial optical fiber communications. However, beams at this wavelength have less power than beams at 1μm, so it is the development of optical communication equipment is needed to increase the power.
LUCAS uses a 1.5-μm beam and NEC has been working to develop devices for successful communication at this wavelength. If successful, LUCAS will enable standardized inter-satellite optical communication and pave the way for commercialization in this field.
--What possibilities will the success of LUCAS bring about?
Miyoshi: The concept for the operation of LUCAS is to use optical communication to relay data collected by Earth observation satellites to optical data relay satellites and from there to Earth. This operation has two major advantages.
The first advantage is the dramatic increase in the amount of data that can be observed by Earth observation satellites. The amount of data obtained from a satellite is strongly restricted by downlink bandwidth from a satellite to the ground, which is influenced by the communication time with a satellite. Until now, an antenna deployed in the Arctic has mainly been used to communicate with observation satellites. This means that communication is possible only when a satellite passes over the North Pole, which is typically a duration of 5 to 10 minutes with each pass. In other words, communication with a satellite is possible only for a maximum of about 10 minutes for every 100-minute orbit. By using a relay satellite, communication time can be extended to a maximum of 40 minutes on each orbit. This makes it possible to increase the amount of data that can be collected.
The second advantage is increased observation speed. Observation satellites circle the earth about 15 times a day with each orbit taking about 100 minutes. When communicating directly with an observation satellite from the ground, the satellite is instructed to acquire data from an antenna on the ground, and the data is received on the ground after the satellite has circled the earth. This results in a time loss of 100 minutes. On the other hand, with a relay satellite, data acquisition instructions and observation data can be immediately accessed by an Earth observation satellite. Since the relay satellite is geostationary over Japan, it is always available for communication.
LUCAS greatly outperforms conventional technology in terms of data transmission speed and antenna size. Compared with JAXA's "Kodama" Data Relay Test Satellite, which transmitted data at a speed of 240 Mbps and had an antenna with a diameter of 3.6 meters, LUCAS can transmit data seven times faster at a speed of 1.8 Gbps and has an antenna with a diameter of 14 cm, which is about 1/30th the diameter of Kodama’s antenna. This means that LUCAS is capable of transmitting more data despite its smaller size.
Three challenges to achieving inter-satellite optical communication
--What is NEC's roles in the LUCAS project?
Miyoshi: While JAXA is responsible for managing the LUCAS project, NEC has contributed to the project as a leader in optical communication system design, equipment development, and component technology verification.
Although it is often thought that a system can be completed by simply combining components, because we have ventured into a completely new field with inter-satellite optical communication, we had to start development without knowing what components to use and how to combine them to create a system. The entire system had to be designed from scratch, component specifications had to be defined, and verification had to be performed after the components were assembled. This was the series of tasks for which NEC was responsible.
-- What were the most difficult aspects of the project?
Miyoshi: Three main points come to mind. The first challenge was to develop technologies for stably amplifying low-output laser light with a wavelength of 1.5 μm to a relay satellite 40,000 km away. On Earth, optical fiber submarine cables traverse across the ocean between repeaters spaced roughly every 100 km to amplify the light traveling down the fiber. In space, however, it is not possible to set up such relay points between satellites. Therefore, it is necessary to amplify the optical power inside the satellite by a factor of 30 to 40. Amplifying the power through a fiber as thin as 8μ in diameter increases the energy density and temperature, making it extremely difficult to ensure stable operation of the equipment. NEC's technological capabilities is demonstrated through the achievement of optical communication under such conditions.
The other major challenge was to develop an optical acquisition and tracking technology. As I mentioned earlier, the diameter of the beams is about 600 meters, and both the emitter and receiver of the beams are moving at a considerable speed through space. The optical acquisition and tracking technology is an extremely advanced and required repeated simulations using digital twin technology, which virtually replicates the system on a computer.
The third challenge was to ensure ability to function in the harsh environment of space. The temperature variation in outer space is extreme with the temperature rising to over 100℃ in direct sunlight and dropping to -120℃ in the shade. Although the thermal control function ensures that the temperature fluctuations inside the satellite is not as severe, temperatures still fluctuate by several tens of degrees. The semiconductors in devices that emit lasers are very sensitive to temperature changes, and a large change in temperature will cause the wavelength of the laser to shift. Therefore, temperature control is essential to stabilize the wavelength. Wavelength also shift due to the Doppler effect which is the change in frequency due to the movement of a satellite in its orbit. This is the same phenomenon that occurs when the high pitch of the siren of an approaching ambulance drops in pitch as it passes by. Technology to subtly control and compensate for such shifts in wavelength is also very important and developing this technology was an enormous challenge.
-- What are the factors that helped you overcome the challenges faced in developing these technologies?
Miyoshi: NEC has been engaged in technology development in various fields under the banner of deploying technologies from the ocean floor to outer space. Even in the field of optical communication technology, NEC has supported communication networks by developing new technologies that are suitable for each environment, such as terrestrial and submarine environments, and we are leveraging our extensive experience to deploying optical communication in space. There are many companies that can quickly develop superior component technologies, and we have collaborated with them on various occasions. However, I believe that there are not many companies that can match NEC in terms of technological capabilities and human resources to provide comprehensive support in completely new fields such as space.
Inter-satellite optical communication enhances accuracy in disaster risk management
--What are some of the social issues that can be solved by using LUCAS?
Miyoshi: There are many possibilities, but we believe that a particularly major one is in the area of disaster risk management. The acquisition and transmission of large volumes of data through inter-satellite optical communication will enable the collection and analysis of data, which will improve the accuracy of disaster prediction and management. By collecting a large amount of data during ordinary times and conducting analysis and simulations, we can identify trends in disasters and minimize damage when they occur. These are some of the ways we expect the LUCAS system to be used. In addition, the use of inter-satellite optical communication systems will enable rapid data transmission, making it possible to swiftly collect information in the event of a disaster.
-- Finally, what are your thoughts on the future of space development and the space utilization business?
Miyoshi: Japan’s style of space development used to be one that focused on building on and creating new value from the Western model of space development. However, Japan's space development technology has already caught up with, and in some fields surpassed, that of the West. Becoming a leader in technology development also means that we have a responsibility to think about how we can improve society through the use of such technology. I believe NEC must implement measures to fulfill this responsibility going forward.
At the same time, I feel that the Japanese mindset is not yet completely prepared to take challenges head on. Challenges are always accompanied by failures, but where possible, we should forge ahead without fear of failure. Of course, we should not run head first into every opportunity recklessly. As new technology emerges, we should have open discussions on how they can can be used to benefit everyone and then invest boldly to drive innovation. Space has great potential for such investment. While promoting collaboration and co-creation with people from various fields, I would like to help build a Japanese mindset and culture that inspires people to take on challenges in the new field of space. This is a goal I hope to achieve.