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In recent years, the necessity of free-space optical (FSO) communications has increased as a method for realizing high-speed communications between satellites and the ground. However, one disadvantage of FSO communications is the significant influence of the atmosphere. Specifically, FSO communications cannot be utilized under certain atmospheric conditions, especially in the presence of clouds. One of the solutions to this problem is the site diversity technique, which makes it possible to select a given ground station with better atmospheric conditions among a number of fixed ground stations. The other solution is to prepare a ground station that can be moved to a place with better atmospheric conditions. We applied the latter method and developed a transportable optical ground station in NICT. We utilize a realistic telescope diameter, which is about 30 cm at the maximum, capable of being set up quickly, and with a pointing accuracy of about 100 µrad. In addition, it is necessary to prepare a fine-pointing optical system that performs tracking with about 1/10 of the pointing accuracy of the telescope. In this paper, we report the results of the first performance test of the transportable optical ground station in NICT.

Compared to conventional large satellites in the past, small satellite classes (less than 150 kg) show their advantages for mass production, such as short time and low cost for development and launch, to cope with the demand for emerging missions that require a sufficient number of satellites in orbit. However, the traditional communication method, in which a low earth orbit (LEO) small satellite sends data to a ground station using radio frequency, has several disadvantages. Firstly, the limitation of radio-frequency bandwidth leads to a low data rate and difficulty in getting a frequency license. Secondly, there is a significant delay during which data cannot be sent to the ground due to lacking a line of sight between the LEO satellite and the ground station. Additionally, the duration time for the small satellite to communicate with the ground station is just less than 10 minutes approximately. To resolve the above issues, we investigate the case that a less-than-150-kg satellite carries out a laser communication link from LEO to a satellite in geostationary orbit (GEO). Due to the constraints of size, weight, and power (SWaP), traditional bulky LEO-GEO relay systems cannot be applied for the small satellite. However, using the combination of the satellite body pointing and a piezo Fast-Steering Mirror (FSM), which reduces the SWaP considerably, makes it feasible that the LEO-to-GEO communication can be implemented in a small satellite for the first time. While utilizing laser communication can increase the data rate, the relay communication via the GEO satellite helps the small satellite to extend the communication duration significantly. Moreover, since there is a line of sight between the two terminals in any of about 15 orbits per day of the LEO satellite, data taken by the small satellite can be downloaded to the ground via the GEO one in almost real time. This research aims at investigating and proving the feasibility of a small satellite to transmit a laser communication link to its GEO counterpart. In this paper, we describe the LEO-to-GEO laser communication of the small satellite with a study of pointing-budget and link-budget analysis. Furthermore, a hardware-based simulation of the fine control mechanism is conducted. The hysteresis that affects severely to the piezo mechanism, and hence, the final control accuracy, is modeled accurately and its effect is shown.

Recently, 100 kg-class satellites have been emerging to cope with demand of advanced missions that require a sufficient number of satellites in orbits. Short time and low cost for development and launch bring potential of mass production and feasibility for this satellite class to accomplish the above missions. However, the traditional communication method, in which a 100 kg-class satellite sends data directly to a ground station, is limited in terms of speed and duration. Moreover, the constraint of having a line of sight between the small satellite and the ground station makes it impossible to transfer data taken by the satellite to ground in real time. Therefore, a laser communication relay from the 100 kg-class satellite in low earth orbit (LEO, ≤2000 km) through a satellite in geostationary orbit (GEO, ~36,000 km) is potential to improve communication performance significantly. In this paper, we show a feasibility study of a LEO-to-GEO communication for a 100 kg-class satellite by conducting a parametric study of the communication model. We then discuss a scheme to optimize the laser beam waist dynamically to achieve the minimal average bit error probability (ABEP) under the pointing error caused by the LEO satellite's vibration. Using an asymptotic form of a modified Bessel function, we also introduce an approximation of the dynamic optimal beam waist. A comparison with the traditional fixed-beam optimization approach is also given to show the merits of the dynamic-beam controlling approach.

LEO-to-GEO intersatellite links using laser communications bring important benefits to greatly enhance applications such as downloading big amounts of data from LEO satellites by using the GEO satellite as a relay. By using this strategy, the total availability of the LEO satellite increases from less than 1% if the data is downloaded directly to the ground up to about 60% if the data is relayed through GEO. The main drawback of using a GEO relay is that link budget is much more difficult to close due to the much larger distance. However, this can be partially compensated by transmitting at a lower data rate, and still benefiting from the much-higher link availability when compared to LEO-to-ground downlinks, which additionally are more limited by the clouds than the relay option. After carrying out a feasibility study, NICT and the University of Tokyo started preparing a mission to demonstrate the technologies needed to perform these challenging lasercom links. Furthermore, to demonstrate the feasibility of this technique, an extremely-small satellite, i.e. a 6U CubeSat, will be used to achieve data rates as high as 10 Gbit/s between LEO and GEO. Some of the biggest challenges of this mission are the extremely low size, weight and power available in the CubeSat, the accurate pointing precision required for the lasercom link, and the difficulties of closing the link at such a high speed as 10 Gbit/s., Comment: 5 pages, 10 figures, 3 tables

In recent years, the necessity of satellite-to-ground optical communication has increased as a method for realizing higher-speed communications between satellites and the ground. However, one disadvantage of free-space optical (FSO) communication is the significant influence of the atmosphere. FSO communications cannot be utilized under certain atmospheric conditions, such as cloudy skies. One of the solutions to this problem is site diversity, which makes it possible to select a given ground station with better atmospheric conditions among a number of fixed ground stations. The other solution is to prepare a ground station that can be moved to a place with better atmospheric conditions. In this paper, we present the development of a transportable optical ground station currently being researched in NICT. In order to be transportable, it is necessary to build a system capable of travelling on public roads, installable in every place, and ready to be loaded on relatively-light trucks. For this purpose, a realistic telescope diameter is about 30 cm at the maximum, capable of being set up quickly, and with a pointing accuracy of about 100 μrad. In addition, it is necessary to prepare a fine-pointing optical system that performs tracking with about 1/10 of the pointing accuracy of the telescope. In this research, we will develop the base of the transportable optical ground station using the knowledge of mobile astronomical telescopes. With respect to tracking, we will develop a smaller and lighter fine-tracking system based on NICT’s previous experience. If necessary, we plan to develop an adaptive-optics system for correcting atmospheric disturbances to improve the fiber-coupling efficiency of the communication laser beam.

This paper explores the secrecy performance of the recently proposed intensity modulation/direct detection (IM/DD) terrestrial free-space quantum key distribution (QKD) system, by using a Gaussian-beam propagation model and considering the combined effects of atmospheric turbulence and legitimate transceivers’ misalignment. Secrecy performance metrics including quantum bit error rate (QBER) and ergodic secret-key rate are newly derived in closedform expressions, taking into account all combined effects of turbulence- and misalignment-induced fading channels, the eavesdropper’s location relative to the legitimate receiver, and receiver noises. To satisfy security constraints, the system designs based on the intensity modulation depth and beam waist of the Gaussian beam at the transmitter, and dualthreshold (D-T) selection at the receiver, are comprehensively discussed under turbulence and misalignment conditions as well as different eavesdropper’s locations. Monte-Carlo (M-C) simulations are also implemented to verify the analytical results. Remarkably, this paper also offers the first framework in the literature to evaluate the secrecy performance of free-space optical (FSO) systems considering the eavesdropper’s location under the effect of misalignment between legitimate transceivers.

Recently, satellite broadband communication services using Ka-band are emerging all over the world, some requiring capacities in excess of 100 Gbps. With the radio bandwidth resources becoming exhausted, high-speed optical communications can be used instead to achieve ultra-broadband communications. The National Institute of Information and Communications Technology (NICT) in Japan has over 20 years of experience in R&D of space laser communications with missions such as the Engineering Test Satellite VI (ETS-VI), OICETS, and SOCRATES/SOTA. We are currently developing a laser communication terminal named “HICALI”, aiming to achieve 10 Gbps-class space communications with a 1.5 μm-band laser beam between optical ground stations (OGSs) and the next generation high throughout satellite called ETS-IX with a hybrid communication system using radio and optical frequencies, which will be launched into the geostationary orbit in 2021. Moreover, we have studied laser communication terminals for terrestrial networks, as an alternative wireless system to radio frequency (RF) band. In 2014, we developed a terrestrial free-space optical communications network facility, named INNOVA (IN-orbit and Networked Optical ground stations experimental Verification Advanced testbed). Many demonstrations have been conducted to verify the feasibility of sophisticated optical communications equipment in orbit. We have conducted a feasibility study of a laser communication terminal for next-generation space networks following the above R&D trends in space communication networks, which is a high-speed, secure, small, and scalable laser communication terminal for optical ground stations (OGSs) and satellites or airborne terminals. In this paper, we describe the plan of NICT to develop a scalable laser communication terminal for next-generation space networks.

Research and development of a novel method for a secure free-space optical communication system has been done in NICT since 2018, and demonstration experiments between an aircraft and a transportable optical ground station are planned in near future. In order to establish a stable and highly accurate optical communication link, the system must have a fine pointing mechanism in both the aircraft and the ground station. A compact and light-weight tracking system is required to be mounted on the aircraft, and there will be needed to have an adjustment function of the beam divergence control to allow stable communication under various altitude and atmospheric conditions. The transportable optical ground station should maintain vibration resistance when moving, and it must be easily deployed on each site where is the appropriate optical ground station site with respect to atmospheric turbulence condition.

We study a dynamic scheme for optimizing the beam waist in low earth orbit-to-geostationary orbit (LEO-to-GEO) laser communication for small satellites under the influence of the pointing error on the transmitting side. Using the Marcum Q-function, we express the channel gain of the link in a closed form. Based on this expression, we obtain an accurate approximation of the optimal value of the laser beam waist via both numerical and analytical approaches to achieve the minimal average bit error probability (ABEP) of the LEO-to-GEO communication. The analytical approximation has a simple form and can be straightforwardly calculated and employed in practice. We numerically compute the ABEP with our analytical solution and the sub-optimal solution proposed in the literature. The results show a strong agreement between our solution and previous work.

National Institute of Information and Communications Technology (NICT)has initiated several R&D projects related to satellite laser communications. In this paper we present some of the recent activities in the field, including the Small Optical TrAnsponder (SOTA)for LEO satellites, High-speed Communication with Advanced Laser Instrument (HICALI)for GEO feederlinks, and the Observation system of the patch of Blue Sky for optical Communication (OBSOC)for site diversity and handover decisions. The main purpose for these projects is to increase the knowledge in the area and to promote the technology through successful demonstrations.

The transmission and reception of polarized quantum-limited signals from space is of capital interest for a variety of fundamental-physics experiments and quantum-communication protocols. Specifically, Quantum Key Distribution (QKD) deals with the problem of distributing unconditionally-secure cryptographic keys between two parties. Enabling this technology from space is a critical step for developing a truly-secure global communication network. The National Institute of Information and Communications Technology (NICT, Japan) performed the first successful measurement on the ground of a quantum-limited signal from a satellite in experiments carried out on early August in 2016. The SOTA (Small Optical TrAnsponder) lasercom terminal onboard the LEO satellite SOCRATES (Space Optical Communications Research Advanced Technology Satellite) was utilized for this purpose. Two non-orthogonally polarized signals in the ~800-nm band and modulated at 10 MHz were transmitted by SOTA and received in the single-photon regime by using a 1-m Cassegrain telescope on a ground station located in an urban area of Tokyo (Japan). In these experiments, after compensating the Doppler effect induced by the fast motion of the satellite, a QKD-enabling QBER (Quantum Bit Error Rate) below 5% was measured with estimated key rates in the order of several Kbit/s, proving the feasibility of quantum communications in a real scenario from space for the first time., 10 pages, 14 figures

Recently, satellite broadband communication services using Ka-band are emerging all over the world, some of them with capacities in excess of 100 Gbps. However, as the radio bandwidth resources become exhausted, high-speed optical communication can be used instead to achieve ultra-broadband communications. The National Institute of Information and Communications Technology (NICT) in Japan has more than 20 years of experience in R&D of space laser communications, with important milestones like ETS-VI (Engineering Test Satellite VI), OICETS, and SOTA. We are currently developing a laser-communication terminal called “HICALI”, which goal is to achieve 10 Gbps-class space communications in the 1.5-μm band between Optical Ground Stations (OGSs) and a next generation high-throughput satellite (called ETS-IX) with a hybrid communication system using radio and optical frequencies, which will be launched into a geostationary orbit in 2021. The development of test and a breadboard model for HICALI has been conducted for several years and we are now carrying out an engineering model as well as designing the OGSs segment. In this paper, we describe concepts and current design status of the HICALI system.

Broadband communication using satellites requires use of the Ka band. Services of this type are now emerging all over the world, some with capacities in excess of 100 Gbps. However, as radio bandwidth resources become exhausted, high-speed optical communication can be used instead to achieve ultra-broadband communications. The National Institute of Information and Communications Technology (NICT) in Japan researches and develops the next generation of space communication technology. A hybrid satellite communication system using radio and optical frequencies is being studied. Here, the research and development activities of NICT are introduced, along with the concept of the hybrid high-throughput satellite communication system with an optical feeder link. Examples of link budget analyses for 10-Gbps laser communication links are given for satellite links between the ground and geostationary Earth orbit.

Site diversity technology is critical for satellite-to-ground laser links availability improvement. In order to develop handover decision algorithm and to choose uncorrelated sites, the National Institute of Information and Communications Technology has developed a testbed with environmental-data collection systems, deployed in ten different sites within Japan. Three of these locations already have 1-meter telescopes installed for future satellite lasercom missions. In this paper we introduce the testbed and show some of the statistics of the collected data concerning site correlation and link availability.

Quantum communication, and more specifically Quantum Key Distribution (QKD), enables the transmission of information in a theoretically secure way, guaranteed by the laws of quantum physics. Although fiber-based QKD has been readily available since several years ago, a global quantum communication network will require the development of space links, which remains to be demonstrated. NICT launched a LEO satellite in 2014 carrying a lasercom terminal (SOTA), designed for in-orbit technological demonstrations. In this paper, we present the results of the campaign to measure the polarization characteristics of the SOTA laser sources after propagating from LEO to ground. The most-widely used property for encoding information in free-space QKD is the polarization, and especially the linear polarization. Therefore, studying its behavior in a realistic link is a fundamental step for proving the feasibility of space quantum communications. The results of the polarization preservation of two highly-polarized lasers are presented here, including the first-time measurement of a linearly-polarized source at λ = 976 nm and a circularly-polarized source at λ = 1549 nm from space using a realistic QKD-like receiver, installed in the Optical Ground Station at the NICT Headquarters, in Tokyo, Japan.

Research and development of space optical communications is conducted in the National Institute of Information and Communications Technology (NICT). The NICT developed the Small Optical TrAnsponder (SOTA), which was embarked on a 50kg-class satellite and launched into a low earth orbit (LEO). The space-to-ground laser communication experiments have been conducted with the SOTA. Atmospheric turbulence causes signal fadings and becomes an issue to be solved in satellite-to-ground laser communication links. Therefore, as error-correcting functions, a Reed-Solomon (RS) code and a Low-Density Generator Matrix (LDGM) code are implemented in the communication system onboard the SOTA. In this paper, we present the in-orbit verification results of SOTA including the characteristic of the functions, the communication performance with the LDGM code via satellite-to-ground atmospheric paths, and the link budget analysis and the comparison between theoretical and experimental results.

The amount of scientific data to be transmitted from deep-space probes is very limited due to RF-communications constraints. Free-space optical communication can alleviate this bottleneck, increasing data rate while reducing weight, mass and power of communication onboard equipment. Nevertheless, optimizing the power delivery from spacecraft to Earth is needed. In RF communications, the strategy has been to increase the aperture of ground terminals. Free-space optical communications can also follow it, as they share the limitation of low power received on Earth. As the cost of big telescopes increases exponentially with aperture, new ideas are required to maximize the aperture-to-cost ratio. This work explores the feasibility of using telescopes of the future Cherenkov Telescope Array as optical-communication ground stations. Ground-based gamma-ray astronomy has the same power limitation, hence Cherenkov telescopes are designed to maximize receiver's aperture with minimum cost and some relaxed requirements. Both critical issues of the reutilization and possible adaptations of the telescopes to optimize them for communications, and telescopes simulations and numerical computations of several link budgets applied to worst-case scenarios are discussed, concluding that the proposal is technically feasible and would bring important cost reductions and performance improvements compared to current designs for deepspace optical ground stations., 9 pages, 10 figures, IEEE Photonics Journal, Vol. 7, Issue 6 (2015)

A free-space laser communication system has been designed and partially developed as an alternative to standard RF links from UAV to ground stations. This project belongs to the SINTONIA program (acronym in Spanish for low environmental-impact unmanned systems), led by BR&TE (Boeing Research and Technology Europe) with the purpose of boosting Spanish UAV technology. A MEMS-based modulating retroreflector has been proposed as a communication terminal onboard the UAV, allowing both the laser transmitter and the acquisition, tracking and pointing subsystems to be eliminated. This results in an important reduction of power, size and weight, moving the burden to the ground station. In the ground station, the ATP subsystem is based on a GPS-aided two-axis gimbal for tracking and coarse pointing, and a fast steering mirror for fine pointing. A beacon-based system has been designed, taking advantage of the retroreflector optical principle, in order to determine the position of the UAV in real-time. The system manages the laser power in an optimal way, based on a distance-dependent beam-divergence control and by creating two different optical paths within the same physical path using different states of polarization., 9 pages, 12 figures. Appears in Proceedings of the SPIE Security+Defence (Prague, Czech Republic, 2011)

An air-to-ground free-space optical communication system has been designed and partially developed. The design covers both the communications between the airborne and the ground station, and the acquisition, tracking and pointing. A strong effort has been made in order to achieve the minimum payload power, size and weight, for which a MEMS modulating retroreflector has been chosen. In the ground station, a new technique for fine pointing, based on a liquid crystal device, is proposed and will be demonstrated, as well as other improvements with the aim of optimizing the ground station performance., Comment: 4 pages, 9 figures, IEEE 2011 International Conference on Space Optical Systems and Applications (ICSOS). arXiv admin note: substantial text overlap with arXiv:1506.08854

A new optical configuration is proposed for ground stations designed to receive information from modulating-retroreflector-based terminals in a free-space optical communication link. This configuration is based on the polarization discrimination of the ground station laser sent to the remote terminal. It achieves the optimal usage of the laser power, which is a key parameter in modulating-retroreflector-based links. A simulation has been made in order to model the system in terms of power losses and states of polarization and an experimental setup has been implemented to verify the validity of the simulation. A gain of over 6 dB has been measured using the new setup compared with a simple trajectory discrimination setup, achieving a very good agreement with the prediction. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2520–2522, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27116

The mass and the power consumption are the most constrictive restrictions in an Aerostatic Platform. We present in this work a bidirectional link based on a liquid crystal retro-modulator (LCRM). The use of this device displaces to the terrestrial station the tasks of powering the laser, and the pointing mechanism. Also, due to the non-conductive characteristic of the LCRM, the power requirements are low. The beam aperture of the upward link can be tailored with the compromise between the aperture losses, and the tracking mechanics. The range of the distances for the system presented is from 2 to 20 km, being the lower limit for the tests and checking of the system, and the higher the final system for a stratospheric survey Aerostat. The system will be modified to fulfill the requirements associated to the increasing distances, although the main characteristics (modulation schemes, modulator, etc.) will be maintained. The system uses two modulation schemes for each direction: in the uplink a carrier constant envelope FSK modulation is used. For the down link, part of the optical signal is polarization modulated and reflected to the earth. For increasing the data rate, a multi-polarization modulation is used. Actually, 32 polarization levels are used, increasing the data rate in a 5-fold factor. The use of differential scheme, i.e. the State of Polarization (SOP) change defines the data, not the SOP, avoids the necessity of SOP maintenance in the transmitter.

The signals that will be received on Earth from deep-space probes in future implementations of free-space optical communication will be extremely weak, and new ground stations will have to be developed in order to support these links. This paper addresses the feasibility of using the technology developed in the gamma-ray telescopes that will make up the Cherenkov Telescope Array (CTA) observatory in the implementation of a new kind of ground station. Among the main advantages that these telescopes provide are the much larger apertures needed to overcome the power limitation that ground-based gamma-ray astronomy and optical communication both have. Also, the large number of big telescopes that will be built for CTA will make it possible to reduce costs by economy-scale production, enabling optical communications in the large telescopes that will be needed for future deep-space links.