13-Po08 Collaboration of ranging and optical communication mission RISESAT

A 50kg-class micro satellite RISESAT has various missions for space research: a laser communication terminal called VSOTA, a 5m GSD multi-spectral high-resolution telescope called HPT, a corner cube retro reflector, and so on. In this paper, collaborate experiment plan of VSOTA, HPT, CCR and ground SLR stations for satellite attitude determination is introduced. Introduction Small satellites are expected to reduce the cost and period required for satellite development. In Japan, "Hodoyoshi" small satellite project has been proceeded to employ the advantage of small satellites efficiently. The word "Hodoyoshi" means "reasonable reliable" in Japanese. This concept tries to find out an appropriate design point which yields "high reliability per cost". Five small satellites have been developing in this project. The second satellite, RISESAT (Rapid International Scientific Experiment Satellite), has missions for space research proposed by several countries. Development of RISESAT is led by Tohoku University in collaboration with Hokkaido Univ., Kyoto Univ., and other organizations and companies. In Table 1, specifications of RISESAT are shown. Table 1 Specifications of RISESAT. One of the purposes of RISESAT is satellite-to-ground optical communication experiment. The advantages of optical communication systems compared to radio frequency communications are wider bandwidth, larger capacity, lower power consumption, more compact equipment, and protection against interference. Optical communication unit of RISESAT, Very Small Optical Transmitter for Component Validation (VSOTA), has been developed by National Institute of Information and Communications Technology (NICT) . Using VSOTA, RISESAT aims to demonstrate satellite-to-ground laser communication by means of accurate attitude control of the satellite body, i.e. the direction of the laser beams fixed to the satellite structure, with an attitude control accuracy of down to 0.04 deg (3σ). The desired maximum bitrate for this mission is 1 [Mbps]. This is the first step toward the establishment of future optical communication infrastructures. In addition to various scientific instruments, RISESAT carries a 5 [m] GSD multi-spectral high-precision Cassegrain telescope called HPT. It is planned that the HPT is utilized to determine the direction of the pilot signal sent from the ground station, which can be fed back to the attitude control system for achieving higher control accuracy. In this paper, concept of optical communication experiment is described. Moreover, collaborated experiment plan for evaluating attitude controll accuracy by use of the combination Laser transmitter on the ground and HPT is also shown. Optical Communication Experiment using VSOTA VSOTA is a dual-band (980 [nm] / 1550 [nm]) laser signal transmitter without gimbal mechanism or internal fine pointing mechanism. Therefore, in order to direct the laser beam to the desired direction, i.e. toward the ground station, it is necessary to change the attitude of RISESAT. This mode of attitude control is called as "Target Pointing Mode" shown in Fig. 1. Specification of VSOTA are summarized in Table 2. Fig. 2 illustrates mission instruments on RISESAT including VSOTA-COL, an optical collimator. VSOTA-E is the electric part which consists of the transmitter laser diode control unit. These two units are connected to each other via a pair of optical fibre. Four types of ground station will be used in this experiment: (1) 1.5 [m] telescope at Koganei, (2) 35 [cm] telescope at Kashima, (3) 1.0 [m] telescope at Koganei, Kashima, and Okinawa, and (4) portable telescope that aperture is about 20[cm] . Fig. 1 Target Pointing Mode of RISESAT. VSOTA is capable of transmit a NRZ waveform on/off keying signal with a maximum 1 [Mbps], however, due to the limitation in pointing direction accuracy, output power, and ground equipment, the maximum downlink rate of VSOTA is estimated to be up to a couple of 100 [kbps] or less. VSOTA can transmit PN pattern signal for BER evaluation, as well as the real scientific data obtained by scientific instruments. Dual-band (980 / 1550 [nm]) laser signal transmitter with a power 270 and 40 [mW] respectively at modulation allows us to monitor difference of atmospheric effect simultaneously. Table 2 Specifications of VSOTA. Fig. 2 Mechanical configuration of RISESAT. Table 3 Specifications of CCR onboard RISESAT. RISESAT also carries a Corner Cube Retro reflector (CCR) developed by NICT of which optical axis is aligned to the same as of VSOTA and of HPT. The specifications of CCR are also shown in Table 3. CCR can be used to detect the satellite when the attitude of RISESAT shifts from the nominal position by over several degrees. Moreover, from the high precision ranging data, orbit of RISESAT can be determined more precisely. The precise orbit leads to more accurate ephemeris which can be used for optical communications. Collaborated Experiment Plan using HPT and ground station HPT was designed as a science instrumentfor both Earth and astronomical observations. Diameter of main mirror is 100 [mm] and focal length is about 1000 [mm]. HPT is equipped with two Liquid Crystal Tunable Filters (LCTF) in continuous wavelengths of which step size is as small as 1 [nm]. Each LCTF is combined with a CCD matrix image sensor with pixel resolution of 5 [m] at an altitude of 700 [km]. In order to evaluate performance of body pointing, we have a plan to use the combination of HPT and the laser transmitter of NICT ground station. From the NICT's Koganei station, laser beam (532[nm]) for Satellite Laser Ranging (SLR) is discharged towards RISESAT. Using HPT, which is attached on the same plane as VSOTA-COL, the laser beam from Koganei can be detected. From the position of light spot in the HPT's field of view, we can estimate the attitude of RISESAT. In Fig. 3, the concept of this experiment is shown. Fig. 3 Configuration of assessment of body pointing accuracy using a pilot laser during experiment. In the Earth observation mode, RISESAT will conduct either nadir pointing or target pointing modes. In case of target pointing mode, HPT is able to take a selected spectral images of the target area FOV of 3.3 x 2.5 [km]. In this FOV, wavelength different from ones for the optical communication experiment, as a guide beam from optical ground station, optical image focal plane should be recorded above background noise moving around due to the attitude control variation in target pointing mode. Moreover, the position of pilot laser in FOV can be used by Attitude Control Unit (ACU) through recognizing brightest spot in CCD coordinates using FPGA logic and software