The main technical indicators of star sensor product performance include field of view, maximum detection magnitude, angular resolution, single star measurement accuracy, three-axis attitude measurement accuracy, all sky coverage, data update frequency, volume, quality, power consumption, etc. Among the above performance parameters, volume, power consumption, and quality mainly depend on electronic and optical systems. When the hardware platform parameters related to star sensors (field of view, maximum detection magnitude, angular resolution) are determined, key performance parameters such as attitude measurement accuracy, all sky coverage, and update frequency are closely related to data processing algorithms.
The field of view refers to the maximum spatial range that can be imaged by a star sensor, which determines the size of the captured star map. The optical system of star sensors generally has a circular field of view, with design values ranging from a few degrees to thirty degrees, and a field of view angle θ Determined by the edge length L of the image sensor (formula):
The reduction of field of view angle helps to improve the accuracy of single satellite measurement. However, for small field of view star sensors, it is necessary to ensure the required number of star points during normal operation. Therefore, it is necessary to increase the aperture of the optical system or increase the exposure time to enable more dark stars to be detected by the system. However, the large aperture system leads to an increase in the volume and mass of the optical system, which is not conducive to the overall design of miniaturization; The extension of exposure time reduces the frequency of updating attitude data, and the star target appears to be trailing, which affects the quality of the star map. In addition, the increase in the number of dark stars detected has led to a rapid increase in the number of star points contained in the navigation star library, resulting in a sharp increase in the computational complexity and difficulty of star map recognition. Therefore, in the design of miniaturized star sensors, medium and large field of view lenses should be selected.
The ultimate detection magnitude is used to measure the ability of a star sensor to detect the darkest stars within its field of view. It is the most critical indicator parameter that characterizes the detection ability, mainly determined by the effective aperture of the lens, the exposure time of the captured star map, and the quantum efficiency of the image sensor. The higher the limit magnitude, the stronger the star sensor’s ability to detect stars, and its value can be estimated based on the signal-to-noise ratio of the target star point on the detector plane.
The accuracy of single star measurement is the foundation of the attitude measurement accuracy of star sensors, and its numerical value directly affects the accuracy level of the final output attitude data. The accuracy of single star measurement is determined by three factors: field of view size, image sensor resolution, and the accuracy of star sub pixel positioning algorithm. After the field of view size of the star sensor is determined with the resolution of the image sensor, the angular resolution corresponding to a single pixel is determined, and combined with the accuracy of the star sub pixel positioning algorithm, the single star measurement accuracy can be obtained.
The measurement accuracy of the three-axis attitude angle output by the star sensor is one of the most important technical parameters of the star sensor, which is the yaw angle, pitch angle, and rolling angle error of the star sensor. The higher the accuracy of star point positioning, the more stars participate in attitude calculation (i.e. the number of recognized stars output by the star map recognition algorithm), and the higher the attitude calculation accuracy of the star sensor. Therefore, from an algorithmic perspective only, the implementation of high-precision measurement of star sensors needs to be approached from two aspects: improving the accuracy of star sub pixel positioning algorithms and increasing the number of star recognition algorithms in star map recognition.
The coverage rate of the entire celestial sphere refers to the percentage of the sky area that successfully recognizes the captured star map by the star sensor and calculates the correct attitude data for the entire celestial sphere. To meet the requirements of autonomous navigation and reliability, it is generally required that the star sensor can operate within the entire celestial sphere, with a coverage rate of 100%, and at least three observation stars in the captured star map are correctly identified. To meet the above requirements, the star sensor field of view needs to have sufficient star points to meet the basic requirements of star map recognition algorithms. In addition, in practical applications, star map recognition algorithms should have a sufficient success rate in star map recognition throughout the entire celestial sphere, and have a certain degree of robustness against various interference factors to ensure the successful completion of star map recognition tasks in complex space environments.
Unlike inertial navigation components that continuously output attitude information, the data update frequency of star sensors is mainly determined by two factors: exposure time and star map data processing speed. As the exposure time increases, the signal-to-noise ratio of the output star map increases, which helps to improve the positioning accuracy of dark stars and medium brightness detection stars. However, bright stars may have larger positioning errors due to oversaturation. In addition, if the exposure time is too long, the star points may appear trailing during the shooting of the star map, which affects the quality of the star map and leads to a decrease in the frequency of attitude data updates. The reduction in exposure time helps to improve the update frequency, but the accuracy of star point positioning and the sensitivity of dark target detection decrease, and the number of available star points for star map recognition and attitude calculation decreases. Therefore, in practical applications, the determination of exposure time needs to fully consider the task’s requirements for update rate and measurement accuracy. The current attitude measurement and control system has certain requirements for single data measurement time. Therefore, under the current level of optoelectronic technology, it is necessary to shorten the star map data calculation time as much as possible to ensure the update frequency of attitude data.
With the rapid development of deep space exploration and small satellite technology, stricter requirements have been put forward for the volume, quality, and power consumption of star sensors. Among them, the volume and mass parameters are mainly determined by the optical system and electronic system of the star sensor, while the power consumption is mainly determined by the electronic system. The selection of optical systems is determined based on actual indicator requirements, which has a significant impact on the overall volume and mass reduction of star sensors. The electronics system consists of two parts: image sensors and signal processing circuits. APS CMOS sensors have high integration, single power supply, simple peripheral circuits, and power consumption only 1/100~1/10 of CCD, making them the first choice for current miniaturized star sensor design due to their significant advantages in reducing volume, quality, and power consumption; At present, there are two main architectures for signal processing circuits: FPGA+DSP and ARM. The former has been widely used in engineering practice, but compared to the ARM architecture, its overall structure is more complex, and its volume, weight, power consumption, and cost are relatively high. Therefore, the selection of key components in electronic systems has a significant impact on the volume, quality, power consumption, and other performance of star sensors.
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TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
