Star sensors are one of the most commonly used attitude determination instruments.The research on star sensor technology began in the 1950s, and up to now, many different types of star sensor products have been developed and successfully applied.
The indicator parameters related to star sensors include field of view, focal length, ultimate magnitude, and three-axis attitude measurement accuracy. Among the above performance parameters, when the hardware platform parameters related to the star sensor are determined, the main performance parameters such as attitude measurement accuracy are closely related to the algorithm for processing data. The following is a performance parameter analysis:
The size of the image sensor and the focal length of the optical system determine the field of view of the star sensor. In addition, the focal length of the optical system also determines the pixel angular resolution of the star sensor. As the focal length increases, the angular resolution will be improved.
The maximum value of the spatial range of star sensor imaging is the field of view. Typically, the optical system of a star sensor has a circular field of view ranging from a few degrees to thirty degrees, and the edge length A of the image sensor determines the field of view angle ω 。 Reducing the field of view angle can help improve the accuracy of measuring single stars. However, for small field of view star sensors, it is required to ensure that the star sensors are in normal working conditions and meet the requirements for the number of star points. Therefore, the optical system aperture needs to be further expanded, and the exposure time can also be increased. By taking the above measures, the total number of dark stars detected by the system can be increased. However, large aperture systems can cause an increase in the mass and volume of optical systems, leading to a larger overall scale and unfavorable design; Increasing the exposure time will result in a decrease in the frequency of updating attitude data, and there will also be tailing issues with star targets, which can interfere with the quality of the star map. In addition, the increase in the total number of detected dark stars will lead to a rapid increase in the number of star points in the navigation star library. The star map recognition process requires more complex calculations, and the recognition difficulty will also greatly increase.
The ultimate magnitude is mainly a measure of the ability of star sensors to detect stars with the lowest brightness, and also a representation of star detection sensitivity. It is a very important parameter indicator in measuring the performance of star sensors. The limit magnitude depends on parameters such as exposure time and the effective value of the lens aperture. And as the limit magnitude increases, it indicates that the star sensor has stronger detection ability. The maximum magnitude can be estimated based on the target signal-to-noise ratio.
For star sensors, a very important parameter is the accuracy of three-axis attitude angle measurement. As the accuracy of positioning star points improves, the number of star points required in the attitude calculation process will also increase, and the final solution result will be more accurate.
There is a difference between the continuous output of attitude information from inertial navigation components, mainly due to two factors that determine the frequency of star sensor update data, namely the speed of processing star map data and exposure time. As the exposure time is extended, the corresponding signal-to-noise ratio of the output star map will be increased, which can improve the accuracy of determining the position of low and medium detection star points. However, it is also possible to increase positioning errors due to excessive saturation of brightness. In addition, as the exposure time increases, there may be star trailing issues during the shooting process, which can interfere with the quality of the star map and also cause a decrease in the frequency of updating attitude data. If the exposure time is shortened, it can further increase the update frequency, but it will also reduce the accuracy of detecting by target and determining the position of star points, thereby reducing the total number of available star points in attitude settlement and star map recognition. Therefore, in practice, when determining the exposure time, it is necessary to take into account the task’s requirements for measurement accuracy and update frequency. The current attitude measurement and control system requires the time required for a single measurement data. Therefore, based on the current level of photoelectronic technology, it is required to minimize the time required to calculate star map data, in order to ensure the frequency of updating attitude data.
By understanding the technical parameters of star sensors, we can better understand the main working mechanisms of star tracker.
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TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
