The star sensor is mainly used to provide the attitude control system (ADCS) with the current attitude information of the satellite, which is equivalent to the satellite’s eyes and plays a role in navigation and controlling the satellite’s attitude. By solving the quaternion output from the star sensor, it can accurately know the position of the satellite and its attitude in space. Its main functions include the following parts:
After the system is powered on, the CMOS image sensor will receive a reset signal from the DSP and perform a reset operation on it. After the reset is completed, the DSP configures the registers of the CMOS image sensor to make it in the expected working state. After the register configuration is completed, the CMOS image sensor begins to collect the star map and stores the collected star map data in SRAM through CPLD.
Image processing is the core function of star sensors, and all star map processing and recognition algorithms are completed here. The DSP communicates with CPLD through the EMIFA interface to read out the star map temporarily stored in SRAM. The image processing algorithm first filters and preprocesses the original star map to remove the noise mixed in the original star map. Segmentation of the filtered star map involves separating the foreground stars from the background sky. Then, high-precision centroid subdivision positioning is performed on the separated star points to obtain the sub pixel level centroid coordinates of the star points, which are the precise coordinate positions of the star points. Finally, the positioned star points are identified to obtain specific star information for each navigation star in the field of view. The recognition method depends on the specific algorithm, and generally, the recognition strategy based on angular distance is the most commonly used recognition algorithm.
Due to the need to transmit the actual star map image, it is necessary to temporarily store the real-time collected star map data for transmission. When processing star maps, due to the large amount of data in the map, image data cannot be processed in one round, and intermediate data needs to be cached in memory. Before the satellite is launched into the sky, it is necessary to pre store the star catalog data in the star sensor, and the star catalog is designed based on specific star map recognition algorithms. Therefore, sometimes the data volume is large and external storage devices are needed for pre storage.
After completing star map recognition, the vector information of the navigation star in the inertial coordinate system and the ontology coordinate system in the field of view is obtained. As the satellite’s attitude information is included in these two vector information, the satellite’s attitude information can be output through calculation.
The attitude system of a space vehicle refers to the parameters of the vehicle’s rotational motion around the center of mass, which are commonly described by the orientation or direction of a fixed body coordinate system relative to a reference coordinate system. The attitude of a satellite refers to its pointing angle and angular velocity in space relative to a certain coordinate system. Satellite application tasks usually require satellites to maintain high pointing accuracy in space, which first requires describing the satellite’s attitude. Attitude description is the establishment of satellite attitude kinematics and dynamics equations for satellite motion. The variation characteristics of attitude parameters are given by attitude kinematics. The satellite attitude control system consists of the satellite body, controller, actuator, and attitude sensor, all of which form a closed-loop loop.
A rigid body moving in space has six degrees of freedom, of which three are the centroid motion described by positional degrees of freedom, namely orbital motion; The degrees of freedom of the other three satellite bodies rotating around the center of mass describe the attitude motion of the rigid body, so satellite control can be divided into orbit control and attitude control. For most satellites, orbit control and attitude control can be considered separately. The attitude control system studies the attitude motion of satellites rotating around the center of mass, by applying torque to the satellite to rotate around the center of mass, maintaining or changing the satellite’s orientation in space as needed. The task of attitude control system includes two aspects: attitude determination and attitude control. The principle structure is shown in Figure 1-1.
Figure 1-1 Principle of attribute control
Satellites operating in orbit undertake certain tasks of exploration, development, and utilization of space. In order to complete these tasks, various requirements are put forward for the satellite’s attitude, which can be summarized as attitude control. The attitude determination system is an important component of the attitude control system, and its accuracy is a decisive factor affecting the accuracy level of the attitude control system. The main task of attitude determination is to accurately estimate the three-axis attitude information of the satellite through the measurement information of the attitude sensor. On the one hand, it provides information feedback for the attitude control system to better control the satellite’s attitude; On the other hand, it is provided for the use of payloads. The attitude determination system mainly consists of attitude sensors and corresponding attitude information processing algorithms. The accuracy of attitude determination not only depends on the hardware performance and accuracy of the attitude measurement system, but also on the attitude estimation algorithm. In traditional satellite attitude determination systems, gyroscopes are generally configured to provide angular velocity information. Due to the drift error of gyroscopes, general satellite attitude control systems use gyroscopes as the reference and use other attitude sensors to correct the gyro drift.
The star sensor is currently the most advanced equipment among the satellite sensors in use, which can achieve very high attitude measurement accuracy, reaching an accuracy of one thousandth of one degree, thus achieving a needle tip to wheat awn in space. However, its use is also limited. On the one hand, it is too sensitive and afraid of the sun, so it is necessary to actively avoid the sun. On the other hand, due to the high requirements for measurement and the need for calculations, satellites move too quickly and cannot be used.
With the development trend of high precision, long lifespan, and high reliability for satellites, the requirements for satellite attitude determination and control systems are also increasing. Therefore, high-precision attitude determination, high-precision star sensor and control systems have naturally become a hot research topic for scholars.
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TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
