Star sensors generally consist of optical systems, imaging systems, data processing systems, and data exchange systems. The imaging system is an important component of the star sensor, and its performance determines the detection ability of the star sensor. The imaging system of star sensors mainly consists of charge coupled devices (CCD) or CMOS image sensors, which are used as cameras to capture images of the starry sky. Compared with CCD, CMOS image sensors have advantages such as high integration, low power consumption, comprehensive electrical functions, window selection and random readout, which are more in line with the requirements of miniaturization, lightweight, and low power consumption of space devices.
Currently, most star sensor products have adopted imaging systems based on CMOS image sensors.
In the 1970s, after the invention of the CCD image sensor by Willard S. Boyle and George E. Smith at Bell Laboratories in the United States, Goss first demonstrated the CCD star sensor at the Jet Propulsion Laboratory (JPL) in the United States and verified its advantages over the image tube star sensor. Afterwards, JPL successfully developed a STELLAR star sensor using CCD as the image sensor, with a field of view of 3 ° × 3 °, using Fairchild’s pixel array of 100 × The 100 CCD image sensor and Intel’s 8080 microprocessor can achieve a single star measurement accuracy of 10 ″. Due to the significant performance advantages of CCD, it gradually replaced the image tube as the main photoelectric conversion component of star sensors, and thus entered the first stage of star sensor development. After using CCD as an image sensor, the image resolution of the star sensor has been significantly improved, and a microprocessor has been embedded to enable the star sensor to perform autonomous operations. After the stars in the field of view are mapped onto the target surface of the image sensor, the microprocessor calculates the centroid coordinates of the star points in the star map and transmits the resulting data to the main control computer on the spacecraft, or saves it in memory for processing after returning to the ground. The field of view of the first generation star sensors is generally small, and due to the limitations of microprocessor computing power and memory storage capacity, they cannot perform star map recognition and attitude calculation without coarse attitude. Typical products in the first generation of star sensors include the ASTROS (Advanced Stellar and Target Reference Optical Sensor) star sensor developed by JPL, with a field of view of 5 ° × 5 °; MADAN (Multi Mission Attention Determination and Autonomous Navigation) star sensor developed by TRW in the United States, with a field of view of 7.4 ° × 7.4 °; HD1003 star sensor developed by HDOS in the United States, with a field of view of 8 ° × 8 °.
The research on CMOS (Complex Metal Oxide Semiconductor) star sensors predates CCD star sensors. However, due to the limitations of CMOS structure and semiconductor manufacturing technology at that time, the problem of low detection sensitivity of CMOS image sensors has not been solved. In the 1990s, with the invention of CMOS Active Pixel Sensor (APS) technology, the detection sensitivity of CMOS image sensors was significantly improved [16]. At the same time, miniaturization has become a trend in the development of spacecraft such as satellites. For such spacecraft, CCD star sensors have large power consumption, volume, and weight, making it difficult to meet the requirements of miniaturization. However, CMOS star sensors have gradually been widely used in spacecraft such as microsatellites due to their large field of view, low power consumption, high integration, and strong spatial radiation resistance, From then on, we entered the third stage of the development of star sensors [17]. Typical products in the third-generation star sensor include the AA-STR star sensor developed by Galileo Avionica, with a field of view of 20 ° × 20 °, data update rate of 10Hz, power consumption of 4-7W, attitude measurement accuracy of 12 ″, 12 ″, 100 ″ (2 σ）； The MAST star sensor developed by JPL has a field of view of 20 ° × 20 °, data update rate of 50Hz, power consumption of 69mW, attitude measurement accuracy of 7.5 ″ (1 σ）。
With the continuous maturity of star sensor technology, it has been widely used in space missions due to its high attitude measurement accuracy and no drift.
Star sensor is a high-precision attitude sensor widely used in attitude measurement of satellites, long-range ballistic missiles, and deep space exploration. The standard image sensor used in the previous generation of star sensors was CCD. With the advancement of CMOS active image sensor technology, replacing CCD with small size, low power consumption, and strong anti-interference ability in the image acquisition system of star sensors has become the goal of the development of the next generation of star sensors.
Disadvantages of CCD:
During the widespread application of CCD in star sensors, its shortcomings have gradually emerged. Firstly, the pixel units of CCD are composed of MOS capacitors, and the electronic effects excited by charges are easily affected by space radiation. Therefore, the ability of CCD to resist space radiation is relatively poor; Secondly, the manufacturing process of CCD is complex and cannot be compatible with the manufacturing process of general integrated circuits. Therefore, CCD requires support from more complex peripheral circuits; In addition, CCD requires a variety of power sources, and image charges need to be output in serial order to reach the output terminal. These shortcomings make it impossible for CCD based star sensor application systems to significantly reduce technical indicators such as volume, power consumption, and weight on the existing basis.
Advantages of CMOS:
The CMOS image sensor highly integrates the image sensing part and control circuit into the same chip. Its volume is significantly reduced, and its power consumption is also greatly reduced, which is only one-third to one-tenth of that of CCD. It has many advantages such as light weight, high reliability, high integration, low cost, wide dynamic range, radiation resistance, and no dragging, overcoming the inherent shortcomings of CCD, It fully meets the requirements of star sensors for image sensors.
Star sensors are mainly composed of optical systems, image sensor circuits, and control and data processing circuits. The image sensor part includes a CCD (or APS) image plane component, a driving circuit, a timing signal generator, and a video signal processor; The control and data processing circuit includes hardware and software such as digital signal processors (star image memory, star image address generator, program memory, star catalog memory, CPU) and interface circuits for connectivity analysis, subdivision algorithms, star map recognition, attitude angle calculation, and coordinate conversion. Hardware structure diagram
Star sensors, as an important component of satellite attitude and orbit control systems, play a crucial role in satellite attitude control during launch and in orbit flight The star sensor first takes a star map of the sky, performs star map preprocessing, star map recognition, and attitude calculation, and finally outputs attitude angle data for autonomous navigation of spacecraft such as satellites. The core component of the star sensor is the imaging system Early star sensors used charge coupled device image sensors as star imaging devices. With the increasing demand for low-power and miniaturization of star sensors and the advancement of complementary metal oxide semiconductor (CMOS) technology, star sensors based on CMOS image sensors have become the mainstream product in the current market.
The harsh spatial radiation environment can cause dark current and dark signal non-uniformity (DSNU) in complementary metal oxide semiconductor active pixel sensors (CMOS APS) The degradation of radiation sensitive parameters such as photo response non-uniformity (PRNU) and CMOS image sensor parameters leads to a significant increase in the background noise of the star map collected by the star sensor, which affects the performance of star point centroid positioning, star map recognition, attitude positioning accuracy, and other aspects of the star sensor
A study has found that the star maps collected by space working star sensors after irradiation exhibit performance degradation phenomena such as reduced signal-to-noise ratio and trailing star points, which affects the accuracy of star sensors in orbit attitude positioning At present, satellites are equipped with both star sensors and gyroscopes. When space radiation causes a decrease in the star map recognition ability of star sensors and abnormal attitude positioning, most engineering units adopt the solution of turning off star sensors and relying on gyroscopes for attitude positioning This solution can enable the satellite to continue operating, but there are certain risks because the attitude positioning accuracy of gyroscopes is lower than that of star sensors, and there is uncertainty in satellite attitude control In addition, image processing algorithms can also be used to remove the influence of radiation induced noise on star pattern recognition. However, the establishment of image processing algorithms is based on in-depth analysis of the mechanism of CMOS APS radiation induced noise and research on the mechanism of CMOS APS radiation effect on star pattern recognition of star sensors. Using only conventional noise reduction algorithms may also filter out star signal, or some unfiltered noise points may be mistakenly recognized as star points by star sensors Therefore, there is an urgent need to conduct research on the mechanism of CMOS APS radiation effect on star map recognition
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