Toshihiko Yamawaki

Long-term statistics of laser beam propagation in an optical ground-to-geostationary satellite communications link

A ground-to-space laser communications experiment was conducted to verify the optical interfaces between a laser communications terminal in an optical ground station and an optical payload onboard a geostationary satellite 38 000 km away. The end-to-end optical characteristics such as intensity, sensitivity, wavelength, polarization, and the modulation scheme of optical signals as well as acquisition sequences of the terminals were tested under fairly good atmospheric conditions. The downlinks bit error rate was on the order of 10/sup -10/ in spite of atmospheric turbulence. Atmospheric turbulence-induced signal fading increased the uplink bit error rate, the best value of which was 2.5 /spl times/10/sup -5/ because the turbulent layer near the earth surface affects the uplink signal more than it does the downlink one. The far-field optical antenna patterns were measured through the ground-to-satellite laser links. The long-term statistics of the optical signal data is in good agreement with the calculated joint probability density function due to atmospheric turbulence and pointing jitter error effects, which means the stationary stochastic process can be applied to not only the static link analysis but also the dynamic link performance of the optical communications link. The equivalent broadened optical beam pattern should be used for the fading analysis even though the atmospheric coherence length is larger than the antenna diameter or the optical beam diameter of the transmitter. From these results, a more accurate dynamic link design of the optical communications link can be performed that would be useful for system designers, especially for designers of commercial systems.

Ground-to-satellite optical link tests between Japanese laser communications terminal and European geostationary satellite ARTEMIS

Optical acquisition, tracking and communication tests were performed between a Japanese laser communications terminal placed within the ESAs optical ground station at Tenerife, Spain and a European optical payload onboard the ARTEMIS satellite in geostationary earth orbit at 21.5° East. The optical communications tests at Tenerife were to verify the end-to-end optical characteristics such as intensity, sensitivity, wavelength, and polarization, as well as the modulation scheme of optical signals and acquisition sequences of the terminals under fairly good atmospheric conditions. The downlinks bit error rate was on the order of 10-10 in spite of atmospheric turbulence. Atmospheric turbulence induced signal fading increased the uplink bit error rate, the best value of which was 2.5x10-5. The Japanese laser communication terminal itself autonomously established and maintained the ground-to-satellite optical link with the ESAs optical payload from the beginning to the end of a 20-minute session. The test results show that the laser communication terminal which is to be launched with the Japanese OICETS satellite is optically compatible with the optical communications payload onboard the European ARTEMIS satellite.

Conceptual design of a cryogenic system for the next-generation infrared space telescope SPICA

The conceptual design of the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) has been studied as a pre-project of the Japan Aerospace Exploration Agency (JAXA) in collaboration with ESA to be launched in 2018. The SPICA is transferred into a halo orbit around the second Lagrangian point in the Sun-Earth system, where radiant cooling is available effectively. The SPICA has a large IR telescope 3 m in diameter, which is cooled without cryogen to below 6 K by the radiant and mechanical cooling system. Therefore, the SPICA mission will cover mid- and far-IR astronomy with high sensitivity and spatial resolution during a long period of over 5 years for goal. Most heat radiation from the sun and spacecraft is blocked by the Sun Shield and thermal radiation shields covered with Multi-Layer Insulator (MLI) to limit heat radiation to the Scientific Instrument Assembly (SIA). The SIA, which is composed of the primary mirrors and optical benches equipped with Focal Plane Instruments (FPIs), is refrigerated to below 6 K by two sets of 4K-class Joule-Thomson (JT) cooler with a cooling power of 40 mW at 4.5 K. The Far-IR detector is refrigerated to 1.7 K by two sets of 1K-class JT coolers with a cooling power of 10 mW at 1.7 K. Improvements for the higher reliability and sufficient cooling performance are required in the development of SPICA mechanical cryocoolers. Thermal analysis indicates that the SPICA cryogenic system works effectively to limit the total heat load on the SIA to 41.2 mW. This paper describes the conceptual design of the SPICA cryogenic system, which was established with thermal feasibility for nominal operation mode.

Cooled Scientific Instrument Assembly onboard SPICA

The Space Infrared Telescope for Cosmology and Astrophysics (SPICA) is a 3.2m cooled (below 6K) telescope mission which covers mid- and far-IR waveband with unprecedented sensitivity. An overview of recent design updates of the Scientific Instrument Assembly (SIA), composed of the telescope assembly and the instrument optical bench equipped with Focal Plane Instruments (FPIs) are presented. The FPI international science and engineering review is on-going to determine the FPI suite onboard SPICA: at present the mandatory instruments and functions to perform the unique science objectives of the SPICA mission are now consolidated. The final decision on the composition of the FPI suite is expected in early 2013. Through the activities in the current pre-project phase, several key technical issues which impact directly on the instruments’ performances and the science requirements and the observing efficiency have been identified, and extensive works are underway both at instrument and spacecraft level to resolve these issues and to enable the confirmation of the SPICA FPI suite.

New cryogenic system of the next-generation infrared astronomy mission SPICA

We present the new design of the cryogenic system of the next-generation infrared astronomy mission SPICA under the new framework. The new design employs the V-groove design for radiators, making the best use of the Planck heritage. The new design is based on the ESA-JAXA CDF study (NG-CryoIRTel, CDF-152(A)) with a 2 m telescope, and we modified the CDF design to accommodate the 2.5 m telescope to meet the science requirements of SPICA. The basic design concept of the SPICA cryogenic system is to cool the Science Instrument Assembly (SIA, which is the combination of the telescope and focal-plane instruments) below 8K by the combination of the radiative cooling system and mechanical cryocoolers without any cryogen.

Preliminary structural design and key technology demonstration of cryogenic assembly in the next-generation infrared space telescope SPICA

Abstract. The infrared space telescope SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is a next-generation astronomical project of the Japan Aerospace Exploration Agency, which features a 3 m class and 6 K cryogenically cooled space telescope. This paper outlines the current status for the preliminary structural design of the SPICA payload module. Dedicated studies were conducted for key technologies to enhance the design accuracy of the SPICA cryogenic assembly and mitigate the development risk. One of the results is described for the concept of the on-orbit truss separation mechanisms, which aim to both reduce the heat load from the main truss assembly and isolate the microvibration by changing the natural frequency of the spacecraft.

Performance of lightweight large C/SiC mirror

Very lightweight mirror will be required in the near future for both astronomical and earth science/observation missions. Silicon carbide is becoming one of the major materials applied especially to large and/or light space-borne optics, such as Herschel, GAIA, and SPICA. On the other hand, the technology of highly accurate optical measurement of large telescopes, especially in visible wavelength or cryogenic circumstances is also indispensable to realize such space-borne telescopes and hence the successful missions. We have manufactured a very lightweight Φ=800mm mirror made of carbon reinforced silicon carbide composite that can be used to evaluate the homogeneity of the mirror substrate and to master and establish the ground testing method and techniques by assembling it as the primary mirror into an optical system. All other parts of the optics model are also made of the same material as the primary mirror. The composite material was assumed to be homogeneous from the mechanical tests of samples cut out from the various areas of the 800mm mirror green-body and the cryogenic optical measurement of the mirror surface deformation of a 160mm sample mirror that is also made from the same green-body as the 800mm mirror. The circumstance and condition of the optical testing facility has been confirmed to be capable for the highly precise optical measurements of large optical systems of horizontal light axis configuration. Stitching measurement method and the algorithm for analysis of the measurement is also under study.

Mechanical cooler system for the next-generation infrared space telescope SPICA

The Space Infrared Telescope for Cosmology and Astrophysics (SPICA) is a pre-project of JAXA in collaboration with ESA to be launched in the 2020s. The SPICA mission is to be launched into a halo orbit around the second Lagrangian point in the Sun-Earth system, which allows us to use effective radiant cooling in combination with a mechanical cooling system in order to cool a 2.5m-class large IR telescope below 8K. Recently, a new system design in particular thermal structure of the payload module has been studied by considering the technical feasibility of a cryogenic cooled telescope within current constraints of the mission in the CDF (Concurrent Design Facility) study of ESA/ESTEC. Then, the thermal design of the mechanical cooler system, for which the Japanese side is responsible, has been examined based on the CDF study and the feasible solution giving a proper margin has been obtained. As a baseline, 4K / 1K-class Joule-Thomson coolers are used to cool the telescope and thermal interface for Focal Plane Instruments (FPIs). Additionally, two sets of double stirling coolers (2STs) are used to cool the Telescope shield. In this design, nominal operation of FPIs can be kept when one mechanical cooler is in failure.

Thermal Study of Payload Module for the Next-Generation Infrared Space Telescope SPICA in Risk Mitigation Phase

The Space Infrared Telescope for Cosmology and Astrophysics (SPICA) is a pre-project of JAXA in collaboration with ESA to be launched around 2025. The SPICA mission is to be launched into a halo orbit around the second Lagrangian point in the Sun-Earth system, which allows us to use effective radiant cooling in combination with a mechanical cooling system in order to cool a 3m large IR telescope below 6K. The use of 4K / 1K-class Joule-Thomson coolers is proposed in order to cool the telescope and provide a 4K / 1K temperature region for Focal Plane Instruments (FPIs). This paper introduces details of the thermal design study for the SPICA payload module in the Risk-Mitigation-Phase (RMP), in which the activity is focused on mitigating the mission’s highest risks. As the result of the RMP activity, most of all the goals have been fully satisfied and the thermal design of the payload module has been dramatically improved.

Precision pointing control for SPICA: risk mitigation phase study

SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is an astronomical mission optimized for mid- and far-infrared astronomy with a 3-m class telescope which is cryogenically cooled to be less than 6 K. The SPICA mechanical cooling system is indispensable for the mission but, generates micro-vibrations which could affect to the pointing stability performances. Activities to be undertaken during a risk mitigation phase (RMP) include consolidation of micro-vibration control design for the satellite, as well as a number of breadboarding activities centered on technologies that are critical to the success of the mission. This paper presents the RMP activity results on the microvibration control design.