Star trackers image targets with a large brightness range in a dark background. The premise of its work is to detect effective navigation stars and extract them from the background. The main difficulty and key point of star sensor imaging is to extract darker navigation stars, and the extraction of dark targets mainly depends on signal features and signal-to-noise ratio.
The common composition of star sensors includes two parts: the head of the star sensor and the electronic circuit processing box, with a single probe integrated structure.
The head of the star sensor is usually composed of a light shield, an optical system, and a detector. The main function of the light shield is to block external ambient light and stray light, preventing them from interfering with the normal operation of the star sensor after entering the field of view; The optical system is composed of optical lenses, mainly achieving the convergence of starlight energy; The detector mainly achieves the acquisition of star maps.
Electronic circuit processing boxes are usually composed of two parts: hardware circuits and software. The hardware circuit mainly includes detector driver circuit and signal processing circuit. The detector driving circuit mainly generates the driving control signal of the detector, completes initialization and monitoring with the detector; The signal processing circuit is the carrier for implementing star sensor algorithms, and can complete the implementation of all star sensor algorithms. The software mainly includes detector driver and star map acquisition, image preprocessing, star detection, star recognition, attitude calculation, and star tracking.
The CCD star sensor mainly consists of peripheral circuits, signal detection, analog signal processing, data acquisition and storage, data processing, and external interface.
Among them, the signal detection module includes several parts, including a light shield, optical system, CCD probe circuit, CCD photoelectric converter, and optical lens.
Working principle of star sensor:
The captured star is imaged through an optical lens, and then the CCD component converts the light energy of the star into an analog electrical signal. After processing this electrical signal, it is sent to the data acquisition and storage section for analog-to-digital conversion and data acquisition processing.
When the star map captured by the CCD camera is stored digitally in memory, the data processing module will perform star point extraction, star point coordinate calculation, and star map recognition on the digitized star map. The image points formed by the stars will be matched with the navigation star library. After analysis, the position coordinates of the stars corresponding to the image points in the celestial coordinate system can be obtained, and finally, the final determination of the carrier attitude can be completed from this.
1) Integrated star sensor configuration: mainly including a hood, lens, and electronic system.
2) Split type star sensor configuration: Optical Head (OH) and Electronic Unit (EU), which are connected through data transmission lines such as Low Voltage Differential Signal (LVDS).
Composition diagram: Taking the TYspace technology series products as an example.
Working principle of star sensor:
Star sensors are mainly divided into two parts in terms of working principle: imaging system and image processing system.
The star sensor first uses an optical lens and image sensor to image stars. After star point extraction and centroid positioning, the position and brightness information of the star points on the image sensor target surface are obtained. Then, through star map recognition, the corresponding stars of the star points in the star catalog are obtained. Finally, based on the recognition results, the three-axis attitude of the star sensor is obtained through attitude calculation, providing attitude data for the carrier control system to achieve carrier navigation.
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, and low power consumption.
CMOS image sensors based on APS technology have the following main advantages compared to CCD image sensors:
(1) Easy to integrate 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.
Given the outstanding advantages of APS detectors, APS image sensors have been widely used in star trackers.
The workflow of the star sensor mainly includes the six steps shown in the figure above. Below is a brief introduction to these 6 steps:
1. Detector Driver and Star Map Collection
After the detector is powered on, it is necessary to configure initialization registers and related parameters for the detector through driver code, and then enable the detector to work normally by providing clock drive and reset signals. During the process of outputting images from detectors, due to the diversity of image formats, it is often necessary to cache or align star maps in order to obtain a complete star map.
2. Star map preprocessing
After obtaining the star map input from the front-end detector, due to the noise of the detector itself or external environmental factors, there will be many other “interference factors” present on the star map besides the star point information. In this case, it is usually necessary to perform noise suppression through star map preprocessing operations to ensure that star points can be accurately extracted.
3. Star detection
Star point detection is mainly divided into two parts: coarse detection and centroid subdivision positioning. The coarse detection of star points is mainly to achieve preliminary positioning of the area where the star points are located; And centroid subdivision positioning is used to obtain accurate centroid coordinates of star points. The final output of star point detection is the precise sub pixel coordinate values of each star point in the star map.
4. Star point recognition
Star point recognition is sometimes referred to as star map matching. Star point recognition is to achieve feature matching between stars in the current star map and stars in the star library, and obtain effective information such as declination, declination, and star sign of stars in the inertial coordinate system in the current field of view through accurate matching relationships.
5. Attitude calculation
Based on the coordinate information of the star obtained from the observation coordinate system of the star sensor and the declination and declination values of the star obtained from the inertial coordinate system, the transformation matrix of the two coordinate systems can be obtained, thereby obtaining the attitude matrix or four element values of the attitude corresponding to the central axis of the current optical field of view in the inertial coordinate system.
6. Star tracking
After the star sensor can correctly recognize star points and calculate the correct attitude information, the star sensor enters the tracking and acquisition mode. In the tracking and capture mode, the star sensor continuously tracks the correctly identified stars and calculates the attitude of each frame in the tracking state.
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TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
