Star Tracker is a system which is made up of cameras and a complex electronics and software unit, which enable a satellite or space vehicle to know its orientation. Our star trackers are therefore the “guides” for satellites which operate in the Earth’s orbit or which explore our solar system.
The development and application of star sensors have gone through more than half a century, during which, with the continuous development of detection devices, star sensors have also been updated and replaced.
1) Star Sensor
In the 1950s and early 1960s, Photomultiplier tubes and image dissectors were an important part of early star tracker: Problems such as simulation stability, size, mass, magnetic effects, and high-voltage breakdown have limited their use and development.
2) Star Map Ritual Star Sensor
In the early 1970s, charge coupled devices (CCD) were introduced. Solid state image sensor technologies, such as CCD and charge injection devices (CID), have the characteristics of wide spectral response, high quantum efficiency, small size, low operating voltage, high sensitivity, low noise, high resolution, and good spatial stability.
3) Star Tracker
In the 1990s, the Jet Propulsion Laboratory (JPL) of the United States invented APS CMOS image sensors: small size, low power consumption, light weight, high integration, strong anti-interference ability, and flexible data readout methods.
CCD and APS CMOS image sensors have their own strengths and are both mainstream research directions. Different types of image sensors can be selected according to the needs of space missions.
Star sensors have been studied since the mid-19th century and have gone through four stages of development:
Tracing back to before 1970, due to the limitations of detectors such as photomultiplier tubes, image dissectors, or photoconductors, star sensors have many difficult problems to solve. At this stage, although the star sensor has a relatively simple structure, its performance and accuracy also have unstable defects. During this period, the weight and size of star sensors were relatively large, and the detectable field of view was relatively small. Reasonable calibration methods were needed to achieve an accuracy of 30 “. Star sensors based on image dissectors were widely used in space missions such as small astronomical satellites (SAS-C), International Ultraviolet Detectors (IUE), High Energy Astronomical Observatory (HEAO1,2,3), and MAGSAT.
Dating back to the 1970s, developed countries led by the United States implemented star sensors based on charge coupled devices (CCD) and applied them to actual models. In 1976, JPL developed the first star tracker STELLAR (Star Tracker for Economic Long Life Attention Reference) based on area array CCD image sensors, which used 100 × The 100 pixel CCD image sensor uses an 8080 microprocessor as the processor. It has the ability to track 10 stars simultaneously, And its accuracy has reached nearly 7 The optical field of view is 3 °. The main advantage of the first generation CCD star sensor is a small field of view, which means that the optical lens can have a larger focal length, making the star sensor have stronger weak star detection ability and higher star detection accuracy. However, due to the small field of view, more weak stars need to be detected, making the range of star magnitude detection by the star sensor larger and requiring a larger navigation catalog, resulting in star sensitivity The device consumes a lot of time in the recognition and attitude calculation processes, and cannot obtain real-time attitude calculation results, which is also the main problem faced by the first generation CCD star sensor. In addition, due to the small field of view, the number of stars in the general field of view is relatively small, so it is often necessary for other attitude sensors to provide initial coarse attitude information in order to enter the capture mode. Moreover, due to structural reasons, star map data is usually transmitted to the ground, and then attitude calculation and correction are carried out through ground equipment software. Therefore, star sensors in this mode lack the function of autonomous star point recognition and attitude calculation.
Time has come to the 1990s, during which, with the continuous progress of science and technology, the second generation CCD star sensor technology began to emerge. Compared to the first generation CCD star sensor technology, the second generation CCD star sensor has the ability to have a large field of view, a small navigation catalog, and can be independently recognized and calculated. Firstly, due to the advantage of a large field of view, there are more stars within the field of view, and only the brightest few stars need to be utilized to achieve star map recognition. In addition, combined with the application of large-area CCD devices and the implementation of a large number of high-performance microprocessors, real-time output of attitude angle calculation can be achieved while ensuring measurement accuracy. The volume, quality, and power consumption are also continuously reduced, and the overall performance is significantly improved. Of course, compared to the first generation CCD star sensor technology, the most significant change is the improvement of autonomous navigation capability.
The working performance of star sensors strongly depends on the photodetectors used in the system. Early star sensors used CCD detectors. Star sensors using CCD have been widely used due to their high sensitivity, large dynamic range, and low readout noise. However, the use of CCD detectors also has certain shortcomings: they cannot be compatible with the use of large-scale integration technology, as this part of the photosensitive pixel array can only be achieved on the CCD chip, and the timing and signal processing circuits on the periphery of the CCD cannot be integrated on the same chip, resulting in overly complex imaging systems and poor resistance to space radiation. Moreover, the CCD array requires multiple complex operating voltages for power supply, and special clock driven pulses are also required externally, and the image charge can only reach the output terminal through sequential readout. This makes it difficult to continue reducing the volume, weight, power consumption, and other aspects of its imaging system, making it unable to meet the future requirements for the miniaturization development trend of star sensors.
With the development of active pixel sensor APS technology, active pixel sensors, as an alternative to CCD technology, have been applied in the development of star sensors. The characteristic of active pixel sensors is that there is an amplifier inside each pixel, which can enhance the pixel signal. Compared with star sensors based on CCD detection, star sensors based on APS technology, also known as CMOS (Complex Metal Oxide Semiconductor) detectors, have advantages such as good radiation resistance, high integration [33-34], and low power consumption. In the 1990s, thanks to the active pixel sensor (APS) based star sensor experimental product Stracker developed by JPL laboratory, APS based star sensors gradually began to enter people’s vision. CMOS image sensors based on APS technology have the following main advantages compared to CCD image sensors: (1) easy integration and simple interface. Making it possible to integrate more functions within the chip; (2) Strong radiation resistance; (3) Single power supply with low power consumption can effectively improve the efficiency of power usage; (4) Flexible data reading, allowing for random reading of pixels within the area of interest, and enabling window opening of any size; (5) The frame rate is relatively higher.
According to the current development situation, the future development trend of star tracker is mainly reflected in the following directions: current star sensors have become a complete position and attitude measurement component, generally possessing the ability to solve the problem of “space lost” without prior information. It can complete star map recognition, star map matching, and attitude calculation, and directly output attitude angle data. In the future, it will be an inevitable trend for gyroscope free guidance systems to replace inertial guidance systems and satellite inertial integrated guidance systems. Highly integrated with low power consumption, small size, low cost, high precision, and high reliability. The current CMOS APS star sensors have reached a certain degree of miniaturization, with low power consumption and cost, but the accuracy needs to be improved. The design of split modular and redundant combination of multiple sensitive heads has also become a development direction. At present, star sensor information processing systems have inherent drawbacks such as long star map capture time and large internal catalog storage. Therefore, rapid capture, catalog compression, and algorithm improvement have become potential targets for star sensor information processing systems. Navigation multi-sensor information fusion. Developing a star sensor and its information processing system that can operate normally within a large dynamic range is an urgent need for the ballistic missile NS/CNS/GPS integrated navigation system.
The development trend of modern star tracker is constantly miniaturizing, lightweight, low-power, and high real-time, especially in reducing volume, weight, and power consumption, which has made a significant leap.
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TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
