57 JSTS Vol. 28, No. 1 [10] Narasaki, K., Tsunematsu, S., Ootsuka, K., Kanao, K., Okabayashi, A., Mitsuda, K., Murakami, TECHNICAL CHALLENGES AND SOLUTIONS TO H., Nakagawa, T., Kikuchi, K., Sato, R., Sugita, H., Sato, Y., Murakami, M., Kobayashi, M., PRECISION AUTONOMOUS STAR TRACKER FOR AGILE SPACECRAFT “Lifetime test and heritage on orbit of coolers for space use.” Cryogenics 2012;52:188–95. [11] Sato, Y., Shinozaki, K., Sugita, H., Mitsuda, K., Yamasaki, N., Takei, Y., Nakagawa, T., Shuichi MATSUMOTO1), Takanori IWATA1), Hiroshi KAWAI1), Takeshi SEKIGUCHI1) Fujimoto, R., Murakami, M., Tsunematsu, S., Otsuka, K., Yoshida, S., Kanao, K., Narasaki, K., Isamu HIGASHINO2), Kazuhide NOGUCHI2), Koshi SATO2) and Yasunobu TORIKAI3) ASTRO-H SXS team, “Development of mechanical cryocoolers for the cooling system of the Soft X- ray Spectrometer onboard Astro-H.” Cryogenics 2012 52 (2012) 158-164 1) Japan Aerospace Exploration Agency, Tsukuba, Ibaraki, 305-8505, Japan, +81-50-3362-7281 [12] Sato, Y., Sugita, H., Mitsuda, K., Nakagawa, T., Fujimoto, R., Murakami, M., Otsuka, K., 2) NEC TOSHIBA Space Systems, Ltd., Fuchu, Tokyo, 183-8551, Japan, +81-42-354-4768 Tsunematsu, S., Kanao, K., Narasaki, K., “Development of mechanical cryocoolers for Astro-H/SXS.” 3) NEC Corporation, Fuchu, Tokyo, 183-8501, Japan, +81-42-333-3991 Cryogenics 2010;50:500–6. ABSTRACT Recently, scientific satellites and earth observation satellites have required more accurate pointing and higher angular rate maneuver capabilities. Autonomous attitude determination without a priori information has also became important, reflecting the need to simplify satellite systems and operations. To realize these requirements, we have been developing a precision autonomous star tracker for agile spacecraft, which is named the Next-Generation Star Tracker (NSTT). NSTT provides high-accuracy attitude determination results: random error is less than 4 arc seconds (3σ), while bias error is respectively less than 6 and 4 arc seconds (3σ) for wide and narrow temperature range. NSTT is able to track and acquire stars under high attitude-rate of 2 deg/s with 99.9% probability. A qualification model of NSTT has been manufactured and its functions and performances have been evaluated by qualification tests. NSTT is planned to be installed on the next X-ray observation satellite, ASTRO-H. This paper describes the technical challenges of NSTT and our solutions for them. This paper also presents the system design, manufacturing results and some test results of NSTT. 1. INTRODUCTION In Japan, the first generation star trackers which had the function of imaging stars were developed and used for Japanese spacecraft, such as ASTRO-C (GINGA), MUSES-B (HARUKA), MUSES-C (HAYABUSA), ASTRO-EII (SUZAKU), ALOS (DAICH) and ASTRO-F(AKARI). These star trackers made a substantial contribution to Japanese space activities to date (Ref 1, Ref. 2). Recently however, scientific satellites and earth observation satellites require accurate pointing capability to fulfill their advanced missions, for which accurate attitude determination by the star tracker is essential. High resolution earth observation satellites have higher angular rate attitude maneuver capability to perform off-nadir observations, for which the availability of star trackers at a high angular rate is key technology. In addition, since the satellites change their attitude widely at the attitude maneuvers, bright objects such as the sun, earth and moon tend to come close to the field of view (FOV) of star trackers making it even more important to ensure a small stray light avoidance angle for such bright objects. Similarly, it is also important for star trackers to keep their performance at wide temperature range because the sun’s rays come from various directions during attitude maneuvers and star tracker temperature may vary significantly. At the same time, for recent satellites, efforts have been made to reduce ground operation load and improve their onboard automatic operation capability. Corresponding to this trend, autonomous attitude determination without a priori information is required for star trackers (Ref 3, Ref 4). Furthermore, because star trackers in low earth orbit observe many false stars above the south Atlantic anomaly (SAA) region, it is necessary for star trackers to have robust star acquisition and tracking capability above SAA region. Taking account of these requirements, we extracted the following technical challenges to be solved for the second generation Japanese star tracker: 1) Accurate attitude determination 2) Availability at high angular rate 3) A small stray light avoidance angle for bright objects 4) Robust star acquisition and tracking capability above the SAA region 5) Autonomous attitude determination ⓒ Japanese Rocket Society 58 We had solved the technical challenges. And combining the solutions and the technology of the first generation Japanese star trackers, we have been developing a precision autonomous star tracker for agile spacecraft, which is named the Next-Generation Star Tracker (NSTT). 2. TECHNICAL CHALLENGES AND SOLUTIONS (1) ACCURATE ATTITUDE DETERMINATION In order to achieve accurate attitude determination for wide temperature range, -25 ~ 55 ºC, we carefully analyzed thermal distortion on the optical head and the thermo-mechanical interaction between the optical head, electric circuit, hood and satellite structure. We chose separate configurations of the optical module (STO), electrical circuit module (STE) and hood (STH) as shown in Fig. 1 to avoid STO’s thermal distortion caused by heat from the electrical circuit module and hood. The optical module is typically attached to an STT bracket, to which the hood is also attached through a conductive heat insulator as shown in Fig. 1. The optical head and CCD driver circuit on STO are also isolated mechanically and thermally, as shown in Fig. 2. The optical head is mounted on kinematic mounts to minimize the thermo-mechanical interaction between the optical head and the satellite structure. We chose Titanium for the material of the optical head structure to minimize the thermal distortion for the lens because the coefficient of linear expansion of Titanium is similar to that of the material of the lens. The optics of NSTT is specially designed for high performance, with low optical distortion and low thermal distortion to improve bias accuracy. NSTT uses a low noise back-illuminated CCD to increase the number of visible stars and improve random accuracy. The bias Error analysis results of attitude determination for NSTT is shown in Table 1. Thermal distortion includes focal length variation by temperature and optical axis distortions by thermal potential, thermal distribution and structural deformation. The linear thermal distortion which is proportion to temperature can be compensated by the dedicated correction algorithm for thermal distortion. The errors caused by optical distortion and chromatics aberration also can be corrected by correction algorithms using optical characteristic data taken by optical test. Error casused by stray light and centroid shift error at sampling are estimated less than 1 arc seconds. And residual of aberration of light correction and error caused by relative time error are estimated less than 0.01 arc seconds. Hood STT Bracket Optical Head CCD Driver Circuit Hood Fig. 1 Configuration of NSTT Fig. 2 Conceptual diagram of STO Table 1 Bias error analysis results of attitude determination Wide temperature range Narrow temperature range Error source (-25 ~ 55 ºC) (reference ± 5 ºC) Residual of thermal distortion correction 2.6 arc seconds 1.0 arc seconds Residual of optical distortion correction 3.0 arc seconds 3.0 arc seconds Residual of chromatic aberration 1.5 arc seconds 1.5 arc seconds Error caused by stray light 1.0 arc seconds 1.0 arc seconds Residual of aberration of light correction 0.01 arc seconds 0.01 arc seconds Error caused by relative time error 0.01 arc seconds 0.01 arc seconds Centroid shift error at sampling 1.0 arc seconds 1.0 arc seconds Total error 4.5 arc seconds 3.7 arc seconds 59 JSTS Vol. 28, No. 1 (2) AVAILABILITY AT HIGH ANGULAR RATE As an angular rate becomes larger, amount of light for each pixel becomes weaker. Since a shape of star is elongated at high angular rate, exposure time may limit for star identification. Thus the key for the availability at high angular rate is how to gather amount of light for star identification. To meet star acquisition and tracking requirements at high angular rate for the whole celestial sphere, NSTT uses a high performance large aperture lens with bore diameter of 45mm and F number of 1.05, low noise back-illuminated CCD and a special star acquisition and tracking algorithm. NSTT changes its exposure time according to angular rate and uses the CCD in binning mode to increase the amount of light for a pixel. Figure 3 shows the percentage area with fewer than four visible stars, which means star acquisition fails, for each threshold apparent magnitude of the star tracker with FOV of 16 × 16 degrees. The threshold apparent magnitude of NSTT at 2 deg/s is 5.7 [magnitude] at the end of life (EOL), while that at 3 deg/s is 5.2 [magnitude] at EOL. Figure 3 shows the NSTT potential for star acquisition at 2 and 3 deg/s, namely 100% and 98.7% respectively for the whole celestial sphere. A similar analysis for star tracking, which requires more than two visible stars, shows that the NSTT star tracking potential at 3 deg/s is 99.9% for the whole celestial sphere. NSTT is at 3 deg/s NSTT is at 2 deg/s Fig. 3 Probability of star acquisition failure for the threshold apparent magnitude of a star tracker (3) SMALL STRAY LIGHT AVOIDANCE ANGLE FOR BRIGHT OBJECTS Since agile spacecraft change their attitude widely at the attitude maneuvers, bright objects such as the sun, earth and moon tend to come close to the field of view (FOV) of star trackers making it even more important to ensure a small stray light avoidance angle for such bright objects.