Successful International Joint Experiment Between Japan and Germany in the Field of Space Optical Communication Using a Small Satellite | National Institute of Information and Communications Technology

Points

Successful experiment to receive downlink light at NICT optical ground station from DLR optical terminal mounted on Univ. of Stuttgart’s Flying Laptop satellite
Succeeded in the initial experiment of the newly developed atmospheric turbulence measuring device and simple optical ground station for future optical ground station technology enhancement
Acquired valuable data to contribute to the research and development of future space optical communication technology

The National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.) has conducted an international joint experiment with the German Aerospace Center (DLR) between the optical terminal (OSIRISv1) onboard the University of Stuttgart’s Flying Laptop satellite and NICT's optical ground station equipped with newly developed optical bench with fine-pointing system. In February 2021, success in receiving the downlink light from OSIRISv1 at NICT's optical ground station was achieved.

At the same time, an initial experiment of the newly developed atmospheric turbulence measuring device was performed successfully. Furthermore, a successful demonstration experiment using a simple optical ground station composed of low-cost commercial parts was conducted by receiving laser light from the satellite.

The OSIRISv1 optical terminal, mounted on the University of Stuttgart’s Flying Laptop satellite uses body pointing for tracking and this is the first time a successful experiment with such implementation is performed in Japan. As a result, valuable experimental data that will contribute to the research and development of future space optical communication technology has been acquired.

Background

NICT is conducting research and development of space optical communications for the advancement of future satellite communication. Between 2014 and 2016 multiple experiments were performed between the small optical communication transponder (SOTA) and not only optical ground stations in Japan, but also in Europe (German Space Agency (DLR), French National Centre for Space Studies (CNES), and European Space Agency (ESA)), and Canada (Canadian Space Agency (CSA)), and valuable space optical communication experiment data was acquired. NICT has signed a joint research agreement with DLR, that has been developing several optical-communication payloads (OSIRIS). As a result, NICT has carried out an international joint experiment using the onboard optical terminal(OSIRISv1).

Results of this experiment campaign

During this experimental phase that took place between the end of January and the beginning of February 2021, NICT planned experiments to receive the laser light from OSIRISv1 at the optical ground station in Tokyo, which is equipped with a 1-m telescope (Fig. 1). In this experiment, NICT used the newly developed optical bench with fine-pointing system that has been developed for the future High Speed Communication with Advanced Laser Instrument (HICALI) experiments (Fig. 2), which performance was confirmed in advance. It is the first time an experiment with body-pointing implementation was conducted successfully in Japan.

In addition, in this experiment, NICT succeeded in the initial test of the newly-developed atmospheric-turbulence measuring device (Fig. 3), which contribute for the estimation and mitigation of the atmospheric-turbulence effects on the space optical-communication links.

Furthermore, aiming for global spread of space optical communications, it is necessary to develop a small low-cost ground station. A small 20-cm-order off-the-shelf telescope was installed in parallel of the 1-meter optical ground station and successful experiments in receiving the downlink light (first light) were performed (Fig. 4).

The collected valuable data during these successful experiments is important for the modelling of the atmospheric turbulence and tracking errors, and is expected to contribute to the further development of the space optical communication technology.

Fig. 1. Optical ground station with 1-meter telescope.

Fig. 2. Newly-developed optical bench with fine pointing system (Rx/TX single-mode fiber coupling, fine-pointing system consisting of fast-steering mirror and quad cell photodiode).

Fig. 3. Newly-developed optical atmosphere fluctuation measuring device (left), and infrared-camera image of the light received from OSIRIS (right).

Fig. 4. NICT optical ground station with a 1-meter-telescope, and small ground station based on a 20-centimeter telescope.

Future prospects

By proceeding with the analysis of the gathered experimental data, research and development of a complete easy-to-use receiving system with single-mode fiber coupling technology, low-noise optical amplification technology, and high-sensitivity receiving technology on the receiving side is planned. This is expected to contribute to the development and popularization of space optical communication systems in future. Furthermore, this successful demonstration of international system interoperability with OSIRISv1/Flying Laptop (Fig. 5) is an important contribution for The Consultative Committee for Space Data Systems (CCSDS), where standardization of space optical-communication technology is currently taking place.

Role sharing of each institution

NICT: Development of measuring instruments for optical ground stations and optical reception experiments, and preparation for the experiments in Japan
DLR: Development and operation of OSIRISv1
University of Stuttgart: Development, integration and operation of the Flying Laptop satellite

Supplementary material

Current NICT activities in the field of space optical communications

NICT is developing the HICALI payload to be mounted on the ETS-9 to demonstrate 10 Gbps-class optical satellite communication between geostationary orbit and the ground. The HICALI experiments require an optical ground station (Fig. 1) with optical bench with fine-pointing system that has not been available so far. Such system has been newly developed (Fig. 2) and includes single-mode fiber coupling for both transmit and receive sides and a fine-pointing system consisting of fast-steering mirror and a quad cell photodiode.

The optical communication link is strongly depending on the atmospheric turbulence effects. To mitigate these effects, modelling of the laser propagation through the atmosphere and other effects, such as vibrations and tracking errors must be considered. In this experiment, a newly developed atmospheric turbulence measuring device has been installed. It includes not only the recommended by standardization DIMM [1], but also capabilities to estimate the propagation path profile. Also, for tracking error modelling, it is important to acquire experimental data from different system implementations.

Tracking system implementation in this experiment [2]

Due to the long distance in the space optical communication links, the laser beam is very narrow and often with Gaussian profile, which leads to significant power drop with even minor optical axis misalignment. Due to the satellite movement, atmospheric turbulence, vibrations, etc., there is always optical axis misalignment that must be compensated in real time using pointing system. In order to receive enough light, the receiving telescope often has big aperture and the pointing mechanism is often called coarse pointing system and is based on 2-axis gimbal, moved by motors. To be able to track Low-Earth-Orbit satellites, the coarse pointing system should be able to move the big-sized telescope with very high speed which affects its pointing accuracy.

On the other hand, for high-data-rate links, the optical detector has very small size (e.g. systems with single mode fiber used as receiver with 10-micrometer core) and high pointing accuracy is necessary. In order to cover such requirements, a supplementary fine-pointing system is added to the coarse pointing one. Generally, such system is based on imaging sensor (quad cell photodiode) and fast-steering mirror.

Until now the NICT optical ground stations were used for low data rates and operated only with coarse tracking system, but for the HICALI experiments a fine-pointing system has been developed and its performance was evaluated during this successful experiment[3].

Pointing systems on the satellite side

For satellite orbit control, there is often an attitude determination & control subsystem on the satellite side, and the direction in which the satellite is facing can be changed. In order to reduce the size of the optical communication-equipped device on the satellite side, there is a configuration called body pointing, in which the supplementary tracking system of the 2-axis gimbal is removed and the attitude determination / control system is used for tracking. Since the tracking accuracy changes depending on the attitude determination / control system, additional fine pointing system can be added if necessary. In the case of OSIRIS v1 installed in this experiment, only body pointing tracking is available.

References:

[1] "Real-Time Weather and Atmospheric Characterization Data," CCSDS Informational Report, CCSDS 140.1-G-1, 2017.

[2] K. Shiratama, et al., "Development status on High-Speed Laser Communication Terminal “HICALI” onboard ETS-IX," ICSOS 2019

[3] コレフ　ディミタル, “光衛星通信技術，”電気計算、5号、2020

Glossary

Body pointing

In order to decrease the overall dimensions of the optical payload, the typically-used for coarse tracking 2-axis gimbal is removed, and instead the satellite attitude determination/control system is used to move the whole satellite to point towards the target on the ground.

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Small optical communication device (SOTA)

SOTA (Small Optical TrAnsponder) is an ultra-compact optical communication device with a telescope diameter of about 5 cm, mass of about 6 kg, length of 17.8 cm, width of 11.4 cm, and height of 26.8 cm.

Optical communication experiments using light with a wavelength of 1.5 microns, which is the same as the one in the optical fiber network on the ground, were conducted between a low-orbit satellite and the ground to obtain basic knowledge necessary for future space optical communication technology development and in-orbit demonstrations.

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OSIRIS series

The German Aerospace Center (DLR) is developing the optical communication terminal OSIRIS series. OSIRISv1 was launched in 2017 on board the Flying Laptop satellite developed and operated by the University of Stuttgart. It is configured to track by body pointing, has a maximum communication speed of 80 Mbps, and uses light with a wavelength of 1.5 microns, which is the same as the optical fiber network on the ground.

Fig. 7 Roadmap of OSIRIS series
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High Speed Communication with Advanced Laser Instrument (HICALI)

Engineering Test Satellite-9 (ETS-9) is an experiment test satellite under development by Japan Aerospace Exploration Agency (JAXA), Ministry of Internal Affairs and Communications, National Institute of Information and Communications Technology (NICT), and Mitsubishi Electric. It will be launched by an H3 rocket during 2023. NICT is aiming to mount the developed HICALI (High Speed Communication with Advanced Laser Instrument) on the ETS-9 satellite to demonstrate 10 Gbps-class optical satellite communication between geostationary orbit and the ground [2].

Fig. 8. HICALI components overview.
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Atmospheric turbulence

The atmosphere consists of volumes with different temperatures and refractive indices respectively. The wind further mixes these volumes creating laser propagation paths with different refractive indices, which leads to several atmospheric turbulence effects. For example, wavefront distortions, received power fluctuations, beam wandering, etc. (Fig. 9).

The communication link is strongly depending on the atmospheric turbulence effects. To mitigate these effects, modelling of the laser propagation through the atmosphere and other effects, such as vibrations and tracking errors must be considered. In this experiment, a differential image motion monitor (DIMM) device has been installed to measure the atmospheric turbulence strength using the Fried parameter (Fig. 3) [1].

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