Technical Challenges and Solutions to Precision

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[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

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

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(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. The requirements of the angle for NSTT are shown in Table 2. The stray light analysis had shown that the existing black coating technology for space, Ravi Black, whose reflection coefficient is 3.46%, cannot meet the required attenuation for stray light without stray light reduction algorithm as shown in Fig. 4. Thus NSTT uses the world's lowest reflectivity black coating technology, Ultra Black, for its hood. The reflection coefficient of Ultra Black is less than 1%. Micro photography of Ultra Black is shown in Fig. 5. Although Ultra Black is originally developed for small flat objects used in ground environment, we applied this black coating technology for large curved objects using in space environment such as a star tracker hood and evaluated its reflection coefficient during space environment tests. The use of Ultra Black for the NSTT hood, which is small, single-stage hood and 40cm long, means NSTT can achieve the required small stray light avoidance angles for bright objects shown in Table 2.

Table 2 Requirements of stray light avoidance angles for bright objects Avoidance angle for Sun 25 degrees Avoidance angle for Earth 20 degrees Avoidance angle for Moon 15 degrees

0 10 Lens only

-2 Lens + Ravi Black 10 Lens + Ultra Black

10-4

10-6 Attenuation 10-8 (b)

10-10 (a)

-12 10 Fig. 5 Micro photography of Ultra Black 0 10 20 30 40 50 [Multiplying power: 500] Avoidance Angle (deg) (a) Required attenuation for sun light without stray light reduction algorithm (b) Required attenuation for sun light with stray light reduction algorithm Fig. 4 Stray Light Analysis

(4) ROBUST STAR ACQUISITION AND TRACKING ABOVE THE SAA REGION When a satellite on low earth orbit passes through above the SAA region, a star tracker observes many false stars above the region. Figure 6 shows the full-pixel images taken by the star tracker of the Japanese earth observation satellite, “DAICHI”, when it passed above the SAA region (Ref. 5). Although the interval between two images is only 1 second, the shapes and positions of the emerging star-like objects are dissimilar, because of the numerous false stars which appear and disappear in a very short time. As a means of countering the false stars above the SAA region, we developed a robust star acquisition and tracking algorithm using frame correlation between consecutive observed images. The algorithm eliminates anomalous objects on CCD which do not resemble stars, extracts star-like images, and calculates centroids of the star-like images. Then the algorithm matches the centroids from three consecutive image frames and finds real stars which exist on all three image frames as shown in Fig. 7. Figure 8 shows all the observed centroids overlapped by three consecutive image frames. In the Fig. 8, there are true stars in almost the same position in three consecutive image frames, as well as false stars which emerge in one or two image frames. Figure 9 shows extracted true stars using the frame correlation technique.

Fig. 7 True star images and false star images of 3 consecutive flames

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[Image: STT3_0501] [Image: STT3_0502] Fig. 6 Full-pixel images taken by the star tracker of “DAICHI” at 1 second intervals above the SAA region

Fig. 8 Centroids overlapped by three Fig. 9 Extracted stars using the consecutive image frame frame correlation method

(5) AUTONOMOUS ATTITUDE DETERMINATION To realize autonomous attitude determination without a priori attitude information, NSTT uses a dedicated image processing circuit for star image extraction, a high performance space-qualified 64bit RISC MPU, HR5000, and onboard software for star centroid processing, star identification and attitude determination. NSTT changes its operation mode autonomously in accordance with the observed star number, interference with bright objects, and its error status. Table 3 shows the operation mode of NSTT and Fig. 10 shows the state transition diagram of NSTT’s operation mode. The technical challenges of the autonomous attitude determination are accurate star centroid processing, robust and rapid star identification without a priori attitude information, and continuous accurate attitude determination at a high attitude rate.

(a)Accurate star centroid processing After taking a star image, NSTT conducts labeling and grouping of observed pixels whose signal level are larger than threshold signal level, checks the labeled groups by size, shape and total signal of the group, and extracts of star candidates from the labeled groups. Processing stray light reduction algorithm, NSTT calculates star centroids for the star candidates. To improve an accuracy of the centroid processing, NSTT chooses two stage star centroid method.

(b) Robust and rapid star identification without a priori attitude information After the star centroid processing, NSTT determines that each star candidate is a real star or not by the frame correlation method described in the previous section. NSTT chooses maximum 10 bright observed

stars for star identification, compares the elongations calculated from the observed stars with those from the stars on star catalog which are visible at an assumed FOV of NSTT. And NSTT repeats this star identification process changing the assumed FOV until NSTT finds matched sets of the elongations, which is the goal of the star identification.

(c) Continuous accurate attitude determination at a high attitude rate After the star identification, NSTT calculates the aberration of light correction, optical distortion correction, chromatic distortion correction and thermal distortion correction, corrects star centroids using them, and makes accurate star direction vectors for attitude determination. Using the star direction vectors, NSTT estimates its attitude using QUEST method (Ref. 6) and outputs an attitude quarternion for the J2000.0 inertial coordinates.

Table 3 NSTT operation mode Mode Symbol Actions Off Mode OFF NSTT is in power off. Initial Mode IPL NSTT initializes itself in this mode. Standby Mode STBY This mode is standby state for transition to other modes. NSTT captures image, calculates the centroid of stars, and identifies stars by Star Acquisition Mode ACQ pattern matching with a star catalog. NSTT captures the image, calculates the centroid of stars, and estimates attitude Star Tracking Mode TRK by direct matching with a star catalog. If NSTT detects some anomaly, NSTT shifts to this mode and retains error information inside NSTT. Users can choose the next action in emergency mode, Emergency Mode EMM whether NSTT should await ground operations or revert to the initial mode and attempt to recover. Area Data Output ADO NSTT outputs CCD area data by a command in this mode. Mode

Fig. 10 State transition diagram of the NSTT’s operation mode

Fig. 11 System block diagram of NSTT

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3. SYSTEM DESIGN OF NSTT Using the technical solutions described in section 2 and the technologies developed for the first generation Japanese star trackers, we performed the system design and analysis of NSTT, the main functions and performance specification of which are shown in Tables 4 and 5 respectively. The NSTT system block diagram is shown in Fig. 11.

Table 4 NSTT main functions Function Overview 1 Imaging of stars NSTT takes star images in FOV in accordance with the imaging timing signal. Detection of star position NSTT extracts stars and calculates the centroid in terms of CCD coordinates and 2 and brightness the stars’ brightness. NSTT compares the detected star information with the onboard star catalog to find 3 Star Identification matched sets of elongation between stars and identifies detected star direction in the inertial coordinate using the onboard star catalog. After processing star position corrections such as those for optical distortion and 4 Attitude determination light aberration, NSTT determines the precise attitude using the QUEST method and output quaternion in the J2000.0 inertial coordinate. 5 Angular rate estimation NSTT estimates the angular rate using movements of the star positions. Using the estimated attitude and angular rate, NSTT calculates tracking windows 6 Tracking of stars for next star imaging. NSTT continuously estimates attitude and angular rate at 4Hz. In case the star tracking is interrupted, NSTT keeps propagation of the tracking 7 Re-acquisition window using the last attitude and angular rate for swift re-acquisition for a certain duration. Autonomous mode NSTT switches its operation mode autonomously in accordance with the observed 8 transition star number, interference with bright objects and its error status. NSTT eliminates false stars in imaging above the SAA region using the frame 9 False star rejection correlation technique. Memory error detection NSTT detects and corrects memory errors by using the memory patrol and error 10 and correction correcting code.

Table 5 NSTT performance specification Item Performance specification Attitude Accuracy Cross bore-sight axis Less than 4 arc seconds (3σ, *1,*2) (Random) Bore-sight axis Less than 40 arc seconds (3σ, *1,*2) Less than 6 arc seconds (3σ, *2) Attitude Accuracy Cross bore-sight axis Less than 4 arc seconds (3σ, *3) (Bias*4) Bore-sight axis Less than 20 arc seconds (3σ, *2) Maximum acquisition rate More than 2 deg/s (99.9% of the whole celestial sphere) Maximum tracking rate More than 3 deg/s (99.9% of the whole celestial sphere) FOV 16 × 16 degrees Output rate 4 Hz Sun avoidance angle 25 degrees Maximum power consumption Less than 20 W MASS (STO, STE and STH） Less than 6.2 kg Vibration tolerance More than 20 Grms Shock tolerance More than 1000 Gsrs Operation Temperature -25 ~ 55 ºC (*1)Angular rate is less than 0.3 deg/s. (*2)Accuracy is defined within operation temperature. (*3)Accuracy is defined within the range of a reference temperature ± 5 ºC . (*4)Bias error sources includes optical distortion, thermal distortion, chromatic aberration and stray light.

4. NSTT QUALIFICATION MODEL To verify the NSTT design, we manufactured an NSTT qualification model (QM). NSTT QM was made by the same design and manufacture process of NSTT flight model. The only difference between the QM and the flight model is the quality assurance level of parts. NSTT QM consists of an optical module (STO), electric circuit module (STE), hood (STH) and onboard software. Figures 12, 13 and 14 show the picture of the STO, STE and STH of the NSTT QM. The static optical stimulator for NSTT, which is used for functional verification test of NSTT after installation on satellites, is also shown in Fig. 14. We also developed the electrical star simulator for NSTT and the dynamic optical star simulator which can project star images on FOV of a star tracker in accordance with satellite motion and can be used for NSTT and other star trackers. Figures 15 and 16 show the system diagram and picture of the dynamic optical star simulator.

Fig. 12 STO of NSTT QM Fig. 13 STE of NSTTQM Fig. 14 STH of NSTT QM

Fig. 15 System diagram of dynamic optical star simulator Fig. 16 Dynamic optical star simulator

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4. NSTT QUALIFICATION MODEL 5. NSTT TEST RESULTS To verify the NSTT design, we manufactured an NSTT qualification model (QM). NSTT QM was made by Currently, the functions and performances of the NSTT QM have been being evaluated by qualification tests. the same design and manufacture process of NSTT flight model. The only difference between the QM and the A formal NSTT qualification tests will be completed by March 2014. In this section, we present some results of flight model is the quality assurance level of parts. NSTT QM consists of an optical module (STO), electric the qualification tests. circuit module (STE), hood (STH) and onboard software. Figures 12, 13 and 14 show the picture of the STO, STE and STH of the NSTT QM. The static optical stimulator for NSTT, which is used for functional (1) ATTITUDE DETERMINATION TEST verification test of NSTT after installation on satellites, is also shown in Fig. 14. We also developed the We evaluated attitude determination accuracy of NSTT using the electrical star simulator. Figure 17 and electrical star simulator for NSTT and the dynamic optical star simulator which can project star images on FOV Table 6 show the attitude determination result at an inertial fixed attitude. NSTT can satisfy the attitude of a star tracker in accordance with satellite motion and can be used for NSTT and other star trackers. Figures accuracy requirements. Figure 18 and Table 7 show the attitude determination result at an angular rate of 3.0 15 and 16 show the system diagram and picture of the dynamic optical star simulator. deg/s. NSTT can track stars at the maximum angular rate requirement and satisfy the attitude accuracy requirement at 3.0 deg/s.

Attitude Error (Angular rate = 0.0 deg/s) 4 2 0 -2 -4 X-axis Error(arcsec) 0 50 100 150 200 250 300

4 Table 6 Random error at inertial fixed attitude 2 Random Error 0 Cross Bore-sight X 1.55 arc seconds (3σ) -2 Axis Y 1.51 arc seconds (3σ) -4 Fig. 12 STO of NSTT QM Fig. 13 STE of NSTTQM Fig. 14 STH of NSTT QM Y-axis Error(arcsec) 0 50 100 150 200 250 300 Bore-sight Axis Z 14.69 arc seconds (3σ)

20 10 0 -10 -20

X-axis Error(arcsec) 0 50 100 150 200 250 300 Time (sec)

Fig. 17 Attitude determination result at inertial fixed attitude.

Attitude Error (Angular rate = 3.0 deg/s) 20 10 0 -10 -20 X-axis Error(arcsec) 0 50 100 150 200 250 300

20 Table 7 Random error at 3.0 deg/s 10 Random Error 0 Cross Bore-sight X 7.38 arc seconds (3σ) -10 Axis Y 7.76 arc seconds (3σ) -20 Fig. 15 System diagram of dynamic optical star simulator Fig. 16 Dynamic optical star simulator Y-axis Error(arcsec) 0 50 100 150 200 250 300 Bore-sight Axis Z 77.02 arc seconds (3σ)

100 50 0 -50 -100

X-axis Error(arcsec) 0 50 100 150 200 250 300 Time (sec)

Fig. 18 Attitude determination results at an angular rate of 3.0 deg/s.

(2)AVOIDANCE ANGLE TEST Using NSTT hood and the illuminant which simulates sunlight, we evaluated the NSTT sun avoidance angle requirement. During the test we changed the angle between the illuminant and optical axis, and measured the light intensity to calculate attenuation ratio. If the attenuation ratio is smaller than 1.52 × 10-8, NSTT can satisfy a sun avoidance angle of 25 degrees. Figure 19 shows the result of the avoidance angle test. Although the attenuation ratio calculated by the measurement results is not as good as that of the analysis, NSTT meets the required sun avoidance angle. The difference between the measurement and analysis results is due to the small amount of dust in the test clean room. We will evaluates the attenuation ratio in orbit.

Fig. 19 Avoidance angle test results

6. CONCLUSION We have been developing a precision autonomous star tracker, NSTT, for advanced scientific satellites and earth observation satellites which require higher accurate pointing capability, higher angular rate maneuver capability, and autonomous attitude determination capability without a priori information. We had designed NSTT to solve the technical challenges discussed in this paper. NSTT had been manufactured and its functions and performances have been being evaluated by qualification tests. A formal NSTT qualification tests will be completed by 2014. NSTT will be installed on the next X-ray observation satellites, ASTRO-H, which will be launched in 2015.

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