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The utilization of drones as flying base stations (BSs) in the sixth-generation (6G) cellular networks has attracted much attention recently. In this context, free-space optical (FSO) systems could be deployed to provide high-capacity fronthaul/backhaul links between drones and ground BSs. Particularly, a drone-to-ground FSO communication link can be established by equipping the drone with a modulating retro-reflector (MRR) to modulate the incoming optical beam and reflect the modulated beam back along the same path. This helps to alleviate stringent pointing requirements from the drone while satisfying the limited size, weight, and power (SWaP) consumption requirements. Nevertheless, the underlying physical channel effects of this aerial retro-reflected FSO system have never been experimentally explored in the literature. In this paper, we report, for the first time, the experimental channel statistics of a drone-to-ground retro-reflected FSO link, offering practical insights into the angle-of-arrival (AoA) fluctuations at the receiver, the channel coherence time, the probability of fade, the level crossing rate, the average fade duration, and the time-frequency channel characteristics. Our results are expected to serve as practical sources of reference for the theoretical performance analyses and engineering designs of drone-based retro-reflected FSO systems.

Free-space optical (FSO) systems are compulsory to realize high capacity and interference-free communication links from low-Earth orbit (LEO) satellite constellations as well as spacecraft and space stations to the Earth. To be integrated with high-capacity ground networks, the collected portion of the incident beam should be coupled into an optical fiber. To accurately evaluate the signal-to-noise ratio (SNR) and bit-error rate (BER) performance metrics, the probability density function (PDF) of fiber coupling efficiency (CE) must be determined. Previous studies have experimentally verified the CE PDF for a single-mode fiber, however, there is no such investigation for the CE PDF of a multi-mode fiber (MMF) in a LEO-to-ground FSO downlink. In this paper, for the first time, the CE PDF for a 200-μm MMF is experimentally investigated using data from an FSO downlink from the Small Optical Link for International Space Station (SOLISS) terminal to a 40-cm sub-aperture optical ground station (OGS) supported by a fine-tracking system. An average CE of 5.45 dB was also achieved given that the alignment between SOLISS and OGS was not optimal. In addition, using the angle-of-arrival (AoA) and received power data, the statistical characteristics such as channel coherence time, power spectral density, spectrogram, and PDFs of AoA, beam misalignments, and atmospheric turbulence-induced fluctuations are revealed and compared with the state-of-the-art theoretical background.

Free-space optical communication is becoming a mature technology that has been demonstrated in space a number of times in the last few years. The Japanese National Institute of Information and Communications Technology (NICT) has carried out some of the most-significant in-orbit demonstrations over the last three decades. However, this technology has not reached a wide commercial adoption yet. For this reason, NICT is currently working towards the development of a miniaturized laser-communication terminal that can be installed in very-small satellites, while also compatible with a variety of other different platforms, meeting a wide span of bandwidth requirements. The strategy adopted in this design has been to create a versatile lasercom terminal that can operate in multiple scenarios and platforms without the need of extensive customization. This manuscript describes the current efforts in NICT towards the development of this terminal, and it shows the prototype that has been already developed for the preliminary tests, which are described as well. These tests will include the performance verification using drones first with the goal of installing the prototype on High-Altitude Platform Systems (HAPS) to carry out communication links between HAPS and ground, and later with the Geostationary (GEO) orbit, covering this way a wide range of operating conditions. For these tests, in the former case the counter terminal is a simple transmitter in the case of the drone, and a transportable ground station in the case of the HAPS; and in the latter case the counter terminal is the GEO satellite ETS-IX, foreseen to be launched by NICT in 2023., 5 pages, 9 figures

The Japanese National Institute of Information and Communications Technology (NICT) is currently developing a high-performance laser-communication terminal for CubeSats aiming at providing a high-datarate communication solution for LEO satellites requiring to transmit large volumes of data from the orbit. A key part of the communication system is a high-power optical amplifier capable of providing enough gain to the transmitted signals to be able to close the link on its counterpart's receiver with the smallest impact in terms of energy and power on the CubeSat's platform. This manuscript describes the development of a miniaturized 2-W space-grade 2-stage Erbium-Doped Fiber Amplifier (EDFA) compatible with the CubeSat form factor, showing the best power-to-size ratio for a spaced-qualified EDFA to the best of the authors' knowledge. Performance results under realistic conditions as well as full space qualification and test are presented, proving that this module can support short-duration LEO-ground downlinks as well as long-duration inter-satellite links., 9 pages, 12 figures, 1 table

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.

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.

In National Institute of Information and Communications Technology (NICT) of JAPAN, an ultra high speed optical satellite communication equipment onboard the engineering test satellite IX has been developing. The satellite is planned to be launched to geosynchronous orbit in 2021. In this project, we are aiming for ultra high-speed data transmission at the world's highest level of 10 [Gbps] for both uplink and downlink between optical ground stations and geosynchronous satellite. This paper outlines the optical communication mission, the scheduled optical communication experiment, the examination of HICALI and the ground based system at the present time - the outline of the development situation is also explained.

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.

The Global Change Observation Mission (GCOM) is the next generation earth observation project of Japan Aerospace Exploration Agency (JAXA). GCOM concept will take over the Advanced Earth Observing Satellite-II (ADEOS-II) and develop into long-term monitoring of global climate change. The GCOM observing system consists of two series of medium size satellites: GCOM-W (Water) and GCOM-C (Climate). The Second-generation Global Imager (SGLI) on GCOM-C is a multi-band imaging radiometer with 19 spectral bands in the wavelength range of near-UV to thermal infrared. SGLI will provide high-accuracy measurements of Ocean, Atmosphere, Land and Cryosphere. These data will be utilized for studies to understand the global climate change, especially human activity influence on earth environments. SGLI is a suite of two radiometers called Visible and Near Infrared Radiometer (VNR) and Infrared Scanner (IRS). VNR is a pushbroom-type radiometer with 13 spectral bands in 380nm to 865nm range. While having quite wide swath (1150km), instantaneous field of view (IFOV) of most bands is set to 250m comparing to GLI’s 1km requirement. Unique observation function of the VNR is along-track ±45deg tilting and polarization observation for 670nm and 865nm bands mainly to improve aerosol retrieval accuracy. IRS is a wiskbroom-type infrared radiometer that has 6 bands in 1μm to 12μm range. Swath and IFOV are 1400km and 250m to 1km, respectively. This paper describes design and breadboarding activities of the SGLI instrument.

The Global Change Observation Mission (GCOM) aims to establish and demonstrate a global, long-term satelliteobserving system to measure essential geophysical parameters to facilitate understanding the global water circulation and climate change, and eventually contribute to improving future climate projection through a collaborative framework with climate model institutions. GCOM consists of two polar orbiting satellite observing systems, GCOM-W (Water) and GCOM-C (Climate). The first satellite, GCOM-W with Advance Microwave Radiometer -2 (AMSR-2), was already launched in 2012 and has been observing continuously. The follower satellite, GCOM-C with Second Generation Global Imager (SGLI), will be launched in Japanese fiscal year 2017. SGLI enables a new generation of operational moderate resolution-imaging capabilities following the legacy of the GLI on ADEOS-II (Advanced Earth Observing Satellite-II) satellite. The SGLI empowers surface and atmospheric measurements related to the carbon cycle and radiation budget, with two radiometers of Visible and Near Infrared Radiometer (VNR) and Infrared Scanning Radiometer (IRS) which perform a wide-band (380nm-12μm) optical observation not only with as wide as 1150-1400km FOV (field of view) but also with as high as 250-500m resolution. Also, polarization and along-track slant view observation are quite characteristic of SGLI, providing the sensor data records for more than 28 standard products and 23 research products including clouds, aerosols, ocean color, vegetation, snow and ice, and other applications. Sensor instrument proto-flight tests including optical characterization tests such as radiometric and geometric were completed, and satellite system proto-flight tests have finished including thermal vacuum, vibration and acoustic test. In this paper, the pre-launch phase instrument characterization of SGLI flight model and status of GCOM-C satellite system flight model along with the overview of them will be described. Especially we focus on the pre-launch geometric and radiometric performance test results, in-orbit calibration activities and methodologies: VNR's on-board calibrator, IRS's on-board calibrator and calibration maneuver, and in-orbit verification plan during a commissioning phase lasting approximately 3 months.

The Second-generation Global Imager (SGLI) is a mission instrument on the Global Change Observation Mission-Climate (GCOM-C) satellite. The SGLI includes two radiometers, the Visible and Near Infrared Radiometer (VNR) and Infrared Scanning Radiometer (IRS). The VNR is a multi-band optical imaging radiometer observing at wavelength range from 380 nm to 868.5 nm, including non-polarization and polarization observation. The test of the instruments consisting of VNR-SRU (Scanning Radiometer Unit) PFM (Proto-Flight model) has been finished, and the VNR-SRU PFM is under the phase of assembly and test. This paper describes the test result of the VNR-SRU PFM.

Global Change Observation Mission (GCOM) consists of two series of satellites, GCOM-W (Water) and GCOM-C (Climate) for long-term monitoring of earth environment. Second-generation Global Imager (SGLI), the onboard mission instrument of GCOM-C, is the wide FOV multi-spectral optical radiometer in the wavelength range of near-UV to thermal infrared. SGLI consists of two sensor units, Visible and Near Infrared Radiometer (SGLI-VNR) and Infrared Scanning Radiometer (SGLI-IRS). SGLI will provide high accuracy measurements of Ocean, Atmosphere, Land and Cryosphere. Manufacturing and sensor system integration of SGLI flight models were completed. The sensor system proto-flight tests (PFTs) are on-going including optical characterization tests such as radiometric tests using an integrating sphere and geometric and MTF tests using a collimator. This paper describes development and pre-launch test status of SGLI flight model. Especially we focus on the pre-launch radiometric performances such as the spectral response characteristics and signal to noise ratio.

A flight model of the free-space laser communications terminal onboard the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) was well developed by the National Space Development Agency of Japan, and the Proto-Flight Test was over in January 2001. At present, the flight model of the satellite is stored in the manufacturer and waiting for its launch. The system level test of the satellite is conducted every 6 months, and the movable devices are turned on every 3 months during this storage period. The optical pointing and tracking characteristics of a laser communications terminal called the Laser Utilizing Communications Equipment (LUCE) are tested again as the terminal level test in the thermal vacuum test. The pointing and tracking accuracy, the far-field pattern of the laser transmitter, the bit error rate, and so on are verified in this reconfirmation test. The acquisition time is also measured in the ambient and vacuum conditions. The LUCE terminal maintains the optical performances in order to establish the optical inter-satellite communication link between the OICETS and an European experimental data relay satellite.

Current development status of the small optical transponder (SOTA) to be installed into a small satellite is described, where the breadboard model, the engineering model and the protoflight model are respectively introduced. The tracking performance is estimated to show that the angular error is low enough in comparison to the divergence angle of the transmitted beam.

This paper explains some results of research and development of high speed optical communication technology for observed data transmission from satellites/airplanes. The amounts of data acquired on observation satellites or airplanes have become larger with the rapid improvements of resolutions of sensors. In order to transmit the vast amounts of data, a communication system that combines free space optical communication system and terrestrial optical networks is proposed. A specifically designed communication protocol is developed to realize efficient and stable link quality under the special characteristics of free space optical link such as interruption by rain or cloud. Also digital coherent detection scheme is proposed to be implemented to the system to overcome the degradation by atmospheric scintillation. Based on the system design, a mobile onboard laser communication terminal is developed and some experimental results of the system are presented.

NICT initiated R & D activities of Small Optical Transponder (SOTA) for micro-satellites to demonstrate attractive features of optical technology. Development of the SOTA started based on the bread board model (BBM). Design review of BBM revealed the necessity of engineering model (EM) for critical technologies of 2-axes gimbal and receiving optics. Evaluation of the EM carried out and final design of the SOTA was fixed. This paper describes design progress of the SOTA.

This paper explains a plan of research and development of high speed optical communication technology for observed data transmission from satellites/airplanes. The amounts of data acquired on observation satellites or airplanes have become larger with the rapid improvements of resolutions of sensors. In order to transmit the vast amounts of data, a communication system that combines free space optical communication system and terrestrial optical networks is proposed. Particular communication protocol is planned to be developed to realize efficient and stable link quality under the special characteristics of free space optical link such as interruption by rain or cloud. Also digital coherent detection scheme is proposed to be implemented to the system to overcome the degradation by atmospheric scintillation. The plan includes the development of mobile onboard laser communication terminal and the demonstration experiments of the total system.

The new tracking control system has been developed for Free-Space Optical Communications. It consists of the agile two-axis gimbals, the high performance fine-pointing-mechanism, the optical coarse acquisition sensor, and the optical fine tracking sensor. In the conventional tracking control systems, the two-axis gimbals is combined with the narrow range actuator for the fine pointing control, and it have the role of the coarse pointing control based on the GPS position and the body attitude. The each motion of the tracking mechanisms, therefore, is mutually disturbed by the individual control algorithms, and the obtained tracking accuracy is restricted by each motion. To improve the control performance of this two-stage type, the synchronized tracking-control system is proposed. The features of the system are the cooperative/predictive control using the each control/detective signals, and the improvement of the performance for disturbances, with the hollow-structured and rapid two-axis mirror gimbals. This control scheme is applied for the developed tracking control system, and makes it possible to achieve Free-Space Optical Communications. The paper shows the system configuration, the control algorithms and strategy for Free-Space Optical Communications, and the effect of the proposed control system by the several test results.1

The Second-generation Global Imager (SGLI) on the Global Change Observation Mission (GCOM) is a multi-band optical imaging radiometer in the wavelength range from near-UV to thermal infrared. SGLI will provide high accuracy measurements of Ocean, Atmosphere, Land and Cryosphere. SGLI will provide the global scale multi spectral data with about 2 days frequency. The observation data over the land area is 250m resolution with more than 1000km swath, and the ocean area in 1km resolution. SGLI also has a unique tilting data to realize the directional polarized observation with red and near infrared wavelength. This paper describes the operation concept and current status of the SGLI instrument development.

The Second-generation Global Imager (SGLI) on the Global Change Observation Mission (GCOM) is a multi-band imaging radiometer in the wavelength range of near-UV to thermal infrared. SGLI will provide high accuracy measurements of Ocean, Atmosphere, Land and Cryosphere. SGLI project successfully completed its Bread Board Model (BBM) evaluation last year and currently under Engineering Model (EM) development phase. This paper describes current development status of the SGLI instrument.

The Global Change Observation Mission (GCOM) is the next generation earth observation project of Japan Aerospace Exploration Agency (JAXA). GCOM concept will take over the Advanced Earth Observing Satellite-II (ADEOS-II) and develop into long-term monitoring of global climate change. The observing system consists of two series of medium size satellites: GCOM-W (Water) and GCOM-C (Climate). The Second-generation Global Imager (SGLI) on GCOM-C is a multi-band imaging radiometer in the wavelength range of near-UV to thermal infrared. SGLI will provide high accuracy measurements of Land, Ocean, Atmosphere, and Cryosphere. This paper describes design and breadboarding activities of the SGLI instrument.

The Japan Aerospace Exploration Agency (JAXA) has the plan of the Global Change Observation Mission (GCOM) for monitoring global environmental change. Second generation Global Imager (SGLI) is a mission instrument to be installed on the satellite of GCOM Mission Climate (GCOM-C) satellite. SGLI is the optical radiometer observed to the frequent Global, Ocean, Land, Cloud and Ice sphere to help determine the Earth's climate change. SGLI is a suite of two radiometers called VNR and IRS. The VNR is employing a wide swath (1150km) push-bloom scan with line CCD detector. IRS is employing a conventional cross-track mirror scan system (1400km swath) with cooled infrared detector. We report the SGLI preliminary design and special feature. The current SGLI is BBM development phase which is underway to confirm the feasibility of the design.

The experimental results of an inter-orbit laser communication performed under an atmospheric influence is presented. The demonstration was planned so that the optical link was supposed to graze the earth's rim because of the satellite revolution around the earth. The trial was successfully carried out on 5th April, 2006. The measured experimental data are introduced to show the temporal behavior of the OICETS's optical terminal. The atmospheric influence on the optical link is calculated with a theoretical model to obtain a probability density of normalized intensity as a predictive value. The probability density is also estimated from the experimentally measured data. The comparison shows that the theoretical prediction well describes the experimental results.

The experiment results on the inter-orbit laser communications between OICETS and a geostationary satellite and the results of two kinds of orbit-to-ground laser communications between OICETS and ground stations are summarized. The geostationary satellite for the inter-orbit demonstrations is the European Space Agency's geostationary satellite, ARTEMIS, and the ground stations for the orbit-to-ground demonstrations are of the National Institute of Information, and Communications Technology (NICT) in Japan and the German Aerospace Center (DLR), respectively. The descriptions of those experiments contain some statistically analyzed results as well as data samples measured during the demonstrations. The authors present the overview of these demonstration progresses and discuss on the results.

The tracking and pointing performance of the Laser Utilizing Communications Equipment (LUCE) equipped on the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) is presented. The operation characteristics of LUCE observed in the ground-satellite communication demonstrations are focused on. Since the angular movement of LUCE's optical antenna required in the ground-satellite trials exceeds the specification demanded in the inter-satellite communications, the marginal performance for proper tracking and pointing can be observed. During the ground-satellite communication trials, the ground stations found periodical discontinuances in the optical link. By looking at the LUCE's telemetries, the cause of the repeated breaks is revealed.

Ground-to-satellite laser communication experiments between the optical ground station located in Koganei of downtown Tokyo and a low earth orbit (LEO) satellite, the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) called "Kirari", were successfully performed in March and May, 2006. The optical communication demonstration experiment at the optical ground station was conducted in cooperation between the Japan Aerospace Exploration Agency (JAXA) and the National Institute of Information and Communications Technology (NICT). Ten trials over the course of two months were conducted during the test campaign. Acquisition and tracking of the satellite were successful on seven days out of the assigned test days. The tests were unsuccessful during three out of the four days when it was cloudy or rainy, but they were successful on all six of the days when partly clear skies were predominant. For the uplink, the fluctuation of the received signal power was well minimized by using multi-beam laser transmissions. The bit error ratio (BER) on the downlink was measured to be as low as 10-5. The applicability of the onboard optical terminal was demonstrated, aiming not only for geostationary earth orbit (GEO)-LEO links but also for ground-to-LEO optical links.

The initial verification results of the in-orbit free-space laser communication functions equipped on the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) are presented. The operation system for OICETS is introduced with some characteristics on the radio-frequency communication links which connects OICETS to the ground facilities and satellites. The verification procedures of optical functions are carried out with a scenario that begins with measurements of optical devices' responses and moves to confirmations of automatic sequences in acquisition and tracking stars. Finally, all the OICETS functions required for the in-orbit laser communications with ARTEMIS are found to be in good conditions. We describe here the story of the initial verifications with typical examples measured by these tests.

We present the results of the on-orbit free-space laser communications between the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) and the Advanced Relay and Technology Mission (ARTEMIS) geostationary satellite. We first introduce the history of developing OICETS and add some descriptions on characteristics of OICETS, where the performance of the optical communication terminal named the Laser Utilizing Communications Equipment (LUCE) is also stated. We summarize the on-orbit investigation results of LUCE's function and finally make a report on the inter-orbit laser communication experiment carried out by ARTEMIS and OICETS.

Next-Generation LEO System (NeLS) Research Center is now conducting continuous effort to demonstrate feasibility of key technologies for optical inter-satellite links in space. Evaluation of critical components for the NeLS optical terminal, such as Wide-range FPM, RX-collimator combined with a fine tracking sensor and devices for optical receiver, were carried out using trial models. In this paper, performance evaluation results are presented including mechanical environmental test and radiation test.

This paper presents outline of the optical terminal for Next-generation LEO System (NeLS) in-orbit demonstration, which will be conducted as part of Phase 2 of NeLS project. Two small satellites are assumed to launch into GTO orbit changing distance between them from 500km to 3000km. Acquisition and tracking experiments with a star or planet and 2.4Gbps data transmission between two SmartSat is also planned. The design of optical terminal is briefly presented.

Optical Inter-orbit Communications Engineering Test Satellite (OICETS) is under development by NASDA to verify the laser communications technology in space. The in-orbit experiment will be done by establishing inter-orbit communication between the OICETS and European geostationary satellite ARTEMIS in cooperation with ESA. These satellites will be launched in to the orbit in 2000. Laser communications system in space is a promising technology for future space activities, but it has many research matters. Acquisition tracking and pointing system of a laser terminal performs a prominent role to keep the laser communication. This paper describes the ATP system strategy of the laser terminal.

An offset feed-back type optical beam point-ahead mechanism (PAM)was studied for the application for the laser communicationequipment (LCE) of the Engineering Test Satellite VI (ETS-VI).For the calibration of the point-ahead angle, a small fraction ofthe transmitting beam is split back using a corner cube reflector, andis fed into the tracking sensor of an Si four quadrant detector (4QD) to make a precise alignment with the in"coming receiving optical signal. In this system a tone frequency is superimposed on each of the transmitting and receiving main communication optical signal. Both of the tone frequencies are detected in the tracking receiver simultaneously and the receiving beam is adjusted to point the centerof the 4QD sensor using a fine—pointing mechanism. The transmittingbeam is controlled to point a direction with an offset equivalent to the point-ahead angle determined by almanac data, against the receiving beam using a closed feed—back technique. A small lever-type mirror deflectingmechanism with a maximumdeflection angle of 3.5 mrad and a resolution less than 3 ,urad wasdeveloped for the PAM using a multilayer piezo-electric actuator.

A new type of the fine pointing mechanism (FPM) was examined as part of the optical ISL studies conducted by NASDA. Compact piezo-electric mirror deflecting mechanism (MDM) , having a very high resolution (less than 3 irad) was fabricated, and the performance of the MDM was evaluated aiming for application in the ISL optical FPM. The fundamental performance of the fine pointing assembly (FPA) in optical ISL with the multi-layered piezo—electric MDM was confirmed sufficient for optical ISL having pointing error by disturbance (± 0.75 irad) and pointing angle (± 720 Mrad). It was confirmed that the examined FPM is suitable for the fine pointing of optical ISL, by combining appropriately designed optics and coarse pointing assembly (CPA) , through a preliminary system design.