The star simulator is used to provide a point light source at an infinite distance from the measured object as the simulated star point, which can simulate the size, star equivalence and spectral characteristics of the target star. The research of conventional star simulator mainly focuses on the research of its simulation accuracy. In recent years, with the development of star sensor technology, star module technology has gradually transferred to the simulation of the real radiation characteristics of the target star, namely the stellar spectrum and the real magnitude. The precision of star simulator is more and more demanding, so it is of great significance to develop a high precision multi-star simulator with large field of view.
In recent years, with the development of space technology, satellites, manned spacecraft and other spacecraft need to continuously improve the accuracy and reliability of in orbit attitude control, thus promoting the emergence of a new type of space optical attitude sensor, namely star sensor. Star sensors play a significant role in space attitude correction for reconnaissance satellites, civilian communication satellites, space shuttles, space stations, and the Harley Telescope. The attitude correction of these instruments in outer space requires high accuracy, so we need to use high-precision control instruments to control their spatial position. In order to avoid the impact of environmental changes and atmospheric airflow, a device that can simulate the true state of the starry sky is usually made, simulating its magnitude value, star point size, color temperature, etc. This is known as a star simulator, also known as a star simulator. Therefore, conducting research on star simulators plays an important role in the development of star sensors. The research on star simulators is not yet well-developed in China. The existing star simulators in China have a small field of view and low accuracy. The new star simulators require a large field of view and high accuracy in simulating star magnitude. The development of high-precision star simulators has certain theoretical and practical value.
As a product of star sensors, the development of star simulators mainly depends on the development needs of star sensors. The earliest successful development of star trackers, also known as early star sensors, was in the late 1940s to early 1950s, mainly used for guidance of aircraft and missiles. In the mid-1960s, star sensors were mainly used in satellites and other space vehicles. In the early 1970s, the emergence of charge coupled devices (CCD) led to significant development of CCD star sensors. By the mid-1990s, it had developed into the second generation CCD star sensor. In recent years, with the successful development of active pixel image sensor star sensors, star sensors have rapidly developed. The development of star simulators has gradually evolved with the development of star sensors, from single star simulators initially used for calibrating star trackers to various types of dynamic and static star simulators with multiple stars, large field of view, wide spectrum, and multiple magnitudes. The following will introduce and analyze the current research status of satellite simulators both domestically and abroad.
(1) Research progress abroad
The research on satellite simulator technology has matured in the field of aerospace abroad. The star simulator developed by Hughes Company in the United States based on liquid crystal light valve technology transfers real-time simulated star maps from computers to high-resolution liquid crystal displays, achieving the goal of detecting aircraft and monitoring their attitude, and timely transmitting them through light tubes to star sensors for star map position verification.
The CT631 star simulator produced by Ball Company in the United States has a field of view of 20 ° * 20 °, a magnitude detection accuracy of 0.25, and an angle measurement accuracy of 20 ″. The star simulator developed by Eastman Kodak in the United States simulates three star point images, guiding light with different color temperatures to the focal plane of the collimating tube, and adding different sets of filters on the optical path to simulate -2~8 magnitude. McDonnell Douglas Aerospace has developed a star simulator for ground testing on a star tracker, which has three unrelated simulated star fields. Field of view size 25 ° × 25 °, using 4096 × 4096 high-resolution display with a refresh rate of 1000Hz, and each display can provide 50 stars at a time, simulating 2-8 stars, and simulating a single star angle of 100 ″.
The dynamic star simulator product developed by Jena optronik in Germany is called Optical Sky Simulator (OSI). The refresh frequency of this star simulator is greater than 60Hz, the dynamic rate of the star map exceeds 3 °/s, the full field of view is 20 °, and the simulated magnitude range is 2-6.5. It can be used in conjunction with star sensors for testing in a thermal vacuum environment. There are two existing models of OSI for practical detection, the first with a resolution of 800 × 600, single star position accuracy better than 11.88 ″; The second resolution is 1280 × 1024, the positional error of the star point is better than 27 ″. OSI can handle complex testing environments and is one of the internationally leading aerospace testing equipment. The physical photos are shown in Figure 1.3, and Figure 1.1 (a) and (b) respectively show two different resolutions of OSI.
The Star Tracker Optical Simulator (STOS) produced by Airbus DS (Airbus defense and space) is a product with high accuracy and good dynamic performance among the large field of view dynamic star simulators. The physical image of STOS is shown in Figure 1.2. Its field of view is 25 º, and the inter star angular distance error is better than 18 ″. It simulates 1-6 stars, and the image resolution is 1280 × 1024, the dynamic performance of star map simulation can reach 18 º/s.
(2) Domestic research progress
The technology of star simulators started relatively late in China, and the development of star simulators began in the late 1970s. After the 1990s, with the development of star sensor research in China, the research on star simulators has also become increasingly perfect. The research on star simulator systems in China mainly includes Changchun Institute of Optics, Mechanics and Physics, Chinese Academy of Sciences, China Aerospace Science and Technology Group Research Institute, Beijing Astronomical Observatory, Harbin Institute of Technology, Changchun University of Science and Technology, and other units.
In 1988, Harbin Institute of Technology developed a multi star simulator resembling a laptop computer. Size 193mm (6.74 °) × 145mm (5.06 °), with a long focal length of 1638.77mm and a relative aperture of 1:32.77 for optical projection. Figure 1.3 shows the structural diagram of a multi star simulator.
In 1996, the Beijing 502 Institute and the Institute of Optoelectronics of the Chinese Academy of Sciences jointly developed a small dynamic star simulator. Using liquid crystal light valve technology, a star simulator with a size of 26.88mm * 20.16mm and pixels of 640 * 480 was produced. Two liquid crystal light valves were used to splice the display screen, resulting in a total pixel size of 960 * 640 and a star map field of view of 6.5 ° * 6 °.
In 2003, a small star simulator was developed by the University of Electronic Science and Technology and the Institute of Optoelectronics Technology of the Chinese Academy of Sciences, as shown in Figure 1.4. It uses a TFT-LCD display device as the core component, and the field of view of the star map is 8 ° × 6 °, single star angle
In 2004, Beijing University of Aeronautics and Astronautics collaborated with Aerospace Times Electronics to design a semi physical simulation system for astronomical navigation, and conducted static and dynamic experiments on star map recognition algorithms and star sensor performance. The star simulator in this simulation system can simulate 2-7 magnitude stars with a magnitude accuracy of ± 0.5 magnitude, an inter star angular distance accuracy of no more than 20 ″, and a field of view angle of 10 ° × 8 °, the parallelism of the emitted light is less than 15 ″.
In 2012, Changchun University of Science and Technology and Changchun Institute of Optics and Mechanics jointly developed a dynamic star simulator. The star map display device uses two pieces of LCOS for optical splicing, achieving high-resolution star map simulation with a field of view size of 10.2 ° × 10.2 °, with an inter satellite angular distance error of no more than 22 ″, and a simulated magnitude range of -1-6Mv, as shown in Figure 1.5. In 2015, the school developed a dynamic star simulator with a simulated field of view of 20.2 ° × 20.2 °, magnitude simulation range is 2-8Mv.
In 2015, the TFT-LCD star simulator developed by Tsinghua University achieved high-precision star position simulation with a field of view of 8.8 º × 8.1 º, with an accuracy of better than 10.45 ″ for inter satellite angular distance. However, due to the limitations of fixed LCD parameters, it is difficult for the entire machine to achieve miniaturization and high dynamic requirements.
Through comparative analysis, it can be found that the accuracy of domestic extraterrestrial simulators in simulating magnitude is relatively low. With the development of space attitude sensors, there is an urgent need for star simulators that can achieve high-precision simulation of magnitude.
With the application of high-precision optical sensors in the aerospace industry, equipment related to ground performance testing and calibration has also rapidly developed. With the development of high-precision navigation technology applications such as deep space exploration projects, high-precision aircraft attitude adjustment and control have basically adopted star sensors as the core equipment. Therefore, star simulators play a very important role in ground calibration and detection of star sensors.
Star simulators are generally used to provide infinitely distant light sources as simulated star points to simulate the size, magnitude, spectral characteristics, etc. of target stars. Star simulators are divided into standardized and functional detection types according to their functions. A calibrated star simulator, also known as a static star simulator, is composed of a point light source and a set of filters. It focuses on simulation accuracy and can strictly simulate the size, magnitude, and spectrum of star points. It is commonly used for ground calibration of the detection ability, spatial resolution, and other aspects of star sensors, and does not require high real-time performance; The functional detection type star simulator, also known as the dynamic star simulator, requires a large number of star points and various levels of star brightness (i.e. magnitude), so the development accuracy of the star simulator is relatively high. The following aspects need to be considered when developing a star simulator: the distance from the star to the Earth is relatively long, and its relative radiation angle to the Earth is extremely small; There are many types of radiation spectra of stars, and upon reaching the ground, the energy received is very weak; High precision star map simulation must consider the contrast between the luminous stars in the star map and the sky, and try to eliminate the interference of radiation generated by other celestial bodies on the simulated star map background; In order for the designed star simulator optical system to have higher performance requirements, a collimating optical system that simulates infinity must have a relatively large field of view and aberration correction ability.
The Large Field of View Multi Star Simulator belongs to the static star simulator used to calibrate star sensors. It uses 65 infinitely distant targets to simulate stars in the sky, establishes a celestial coordinate system, converts star coordinates in the J2000.0 celestial coordinate system into star simulator coordinates, calculates the orientation of the parallel light tube’s optical axis, designs the light tube installation base, and places all parallel light tubes on the light tube installation base according to the calculation results, as shown in Figure 2.1, Each collimator target source simulates the corresponding star position. Star magnitude adjustment is achieved through the use of adjustable constant current driving technology with duty cycle adjustment. A fixed magnification attenuation plate is used to match the simulated star magnitude with a white LED, and the driving current is adjusted by adjusting the duty cycle of the LED constant current control circuit to adjust the LED illumination output, achieving automatic star magnitude adjustment function and simulating different constant stars.
The large field of view multi star simulator mainly consists of a collimator component, a light tube installation base, an adjustment base component, and an electronic control system. The collimator component simulates an infinitely distant target, i.e. a star, by installing a star hole at the focal plane; The installation base of the light tube is equipped with mechanical installation interfaces for different angles of the light tube, and the direction of this interface is the direction represented by the stellar coordinates in the simulator coordinate system. The entire installation is connected to the adjustment base through bolts, and the adjustment base has adjustment position and leveling functions, used to match different star sensors; The precision position adjustment auxiliary fixture is used to adjust the movement of the sky zone simulator within ± 70mm in the horizontal and vertical axis directions. The parts are connected by bolts for easy disassembly. After the auxiliary sky zone simulator is adjusted to the precise position, disassembly processing is carried out. The control system can control the switch of the star simulator and adjust the magnitude within a certain range. The composition diagram of the Tianqu Star Simulator is shown in Figure 3.1.
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