Toshihiro Kubooka

Solar radiation pressure model for the relay satellite of SELENE

A new radiation pressure model of the relay satellite of SELENE has been developed. The shape of the satellite was assumed to be a combination of a regular octagonal pillar and a column. Radiation forces acting on each part of the spacecraft were calculated independently and summed vectorially to obtain the mean acceleration of the satellite center of mass. We incorporated this new radiation pressure model into the orbit analysis software GEODYN-II and simulated the tracking data reduction process of the relay satellite. We compared two models: one is the new radiation pressure model developed in this work and the other a so-called “cannonball model” where the shape of the satellite is assumed to be a sphere. By the analysis of simulated two-way Doppler tracking data, we found that the new radiation pressure model reduces the observation residuals compared to the cannonball model. Moreover, we can decrease errors in the estimated lunar gravity field coefficients significantly by use of the new radiation pressure model.

LEO-to-ground polarization measurements aiming for space QKD using Small Optical TrAnsponder (SOTA)

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.

A simulation study of effects of GRACE orbit decay on the gravity field recovery

The effects of satellite ground track changes of GRACE on monthly gravity field recoveries are investigated. In the case of a gravity field recovery using a relatively short period of a month or so, the variation of ground tracks affects the precision of the gravity field solutions. It is a serious problem when the solutions are employed for detecting temporal gravity changes which are almost at their detection limits. In this study, the recoveries of four-weekly gravity fields are simulated and the relation between the recovery precision and the ground track is investigated. The result shows that the GRACE ground track of the year 2003 was in good condition for four-week gravity field recovery, but it will sometimes appear as worse cases as the orbit altitude decays. In those cases, the global standard deviations of geoid height errors will be about one order worse than the best case. From our simulation, ground tracks of around altitudes of 473, 448, 399, 350 and 337 km give insufficient spatial resolutions, even for gravity field recovery up to degree 30.

Effective expansion of satellite laser ranging network to improve global geodetic parameters

The aim of this study is to find an effective way to expand the ground tracking network of satellite laser ranging on the assumption that a new station is added to the existing network. Realistic numbers of observations for a new station are numerically simulated, based on the actual data acquisition statistics of the existing stations. The estimated errors are compared between the cases with and without a new station after the covariance matrices are created from a simulation run that contains six-satellite-combined orbit determination. While a station placed in the southern hemisphere is found to be useful in general, it is revealed that the most effective place differs according to the geodetic parameter. The X and Y components of the geocenter and the sectoral terms of the Earth’s gravity field are largely improved by a station in the polar regions. A middle latitude station best contributes to the tesseral gravity terms, and, to a lesser extent, a low latitude station best performs for the Z component of the geocenter and the zonal gravity terms.

Introduction of a terrestrial free-space optical communications network facility: IN-orbit and Networked Optical ground stations experimental Verification Advanced testbed (INNOVA)

A terrestrial free-space optical communications network facility, named IN-orbit and Networked Optical ground stations experimental Verification Advanced testbed (INNOVA) is introduced. Many demonstrations have been conducted to verify the usability of sophisticated optical communications equipment in orbit. However, the influence of terrestrial weather conditions remains as an issue to be solved. One potential solution is site diversity, where several ground stations are used. In such systems, implementing direct high-speed optical communications links for transmission of data from satellites to terrestrial sites requires that links can be established even in the presence of clouds and rain. NICT is developing a terrestrial free-space optical communications network called INNOVA for future airborne and satellitebased optical communications projects. Several ground stations and environmental monitoring stations around Japan are being used to explore the site diversity concept. This paper describes the terrestrial free-space optical communications network facility, the monitoring stations around Japan for free-space laser communications, and potential research at NICT.

Current status of research and development on space laser communications technologies and future plans in NICT

The National Institute of Information and Communications Technology (NICT) has successfully conducted several laser communication experiments between geostationary earth orbit (GEO) and low earth orbit (LEO) satellites and optical ground stations. To date other organizations have also conducted many space laser communication demonstrations worldwide and the time has come when space laser communications can be used as operational systems. The NICT has recently carried out the first-ever successful data transmission from a 50-kg class micro-satellite via laser communication links. This paper presents recent activities on space laser communications in the NICT including the organizations future plans for next generation space laser communication research aiming to achieve 10 Gbps-class and 40 Gpbs-class laser communications at GEO and LEO distances.

SOFTSAT: RECONFIGURABLE COMMUNICATION SATELLITE SYSTEM

Satellite communication provides characteristics such as wide-area coverage and flexible network construction. Since enormous budgets and long lead times are required for satellite development, however, it is difficult to implement the latest techniques onboard. Even if state-of-the-art communications technology is implemented at the time of launch, paradigm shifts in terrestrial network technology can quickly make the onboard technology outdated. In this paper, we describe a reconfigurable communication satellite system based on the formation-flight technique and inter-satellite communication technology. This satellite system consists of a group of layered satellites that can operate together like a single satellite.

Satellite-to-ground optical communication system on Low Earth Orbit micro-satellite RISESAT

Within the scope of a Japanese FIRST (Funding Program for World-Leading Innovative R&D on Science and Technology) program led by Professor Nakasuka of University of Tokyo, Tohoku University is developing a 50kg-class international scientific microsatellite named RISESAT. In addition to various scientific instruments, RISESAT is also equipped with a laser communication terminal VSOTA, developed by Japanese National Institute of Information and Communications Technology (NICT). Tohoku University and NICT are now developing the engineering model of the satellite and undertaking its ground tests. VSOTA has two different wavelengths of laser outputs in 980 nm and 1540nm. The collimators for these are fixed with the satellite structure pointing toward the Earth direction. RISESAT aims to control the direction of the laser beams being precisely pointed toward the NICTs optical ground station with a pointing accuracy of better than 0.4 deg (3σ) during the fly-by. RISESAT can send actual scientific data obtained by payload instruments through this optical communication link. This will be the world first demonstration of microsatellite-to-ground optical downlink. This will bring innovation to misrosatellites system engineering, utilization, and communication network. This paper describes the detailed specification, system design strategy, and real-life implementation of laser communication system on the micro-satellite RISESAT.

Design status of the development for a GEO-to-ground optical feeder link, HICALI

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.

Development of a breadboard model of space laser communication terminal for optical feeder links from Geo

National Institute of Information and Communications Technology (NICT) has a long history of the R&D of space laser communications.