Star Map Simulation of Star Sensors and Star Simulators

Star sensors refer to stars in the cosmic space light environment as reference targets, track, recognize, and extract different star maps, and then measure their own position and attitude. Through attitude calculation, the position of the spacecraft in the celestial coordinate system is determined. In the rendezvous and docking experiment, the Shen-11 spacecraft and the Tiangong-2 space laboratory approached and docked at a speed eight times faster than a bullet, relying on star sensors to achieve integration. Star sensors are the most accurate space optical attitude sensors in modern aerospace technology. Before operating in orbit, a complete performance test and accuracy calibration test must be conducted on the ground to ensure their high working accuracy.

From the research status and development trend of star sensors, it can be found that the star map recognition process of star sensors includes comparing the star positions and magnitude information when comparing the captured star points with the star points in the navigation star library. As the main component of ground performance testing and accuracy calibration equipment, the star simulator can achieve navigation star map simulation under laboratory conditions. However, due to factors such as the inability of existing light sources to achieve multi color temperature switching, the lack of basis for selecting incident light source spectra, the inability to apply spectral modulation and subdivision techniques in weak star spectral simulation, and the imperfect theory of radiation characteristics of the minimum fitting unit of stellar spectra, star simulators are unable to achieve wide spectrum, high-precision, multi color temperature, and multi star spectral simulation. The existing methods for simulating stellar spectra often adopt schemes such as multi-channel filter simulation, multi wavelength LED simulation, and spatial light modulator simulation with precise control of spectral dispersion. Due to the low accuracy of spectral simulation, the star sensor can only solve its accuracy calibration and performance verification problems through field observation experiments. However, the stargazing experiment has strict requirements for the experimental environment and consumes a lot of manpower, financial resources, and material resources, resulting in long calibration time and low efficiency.

This article focuses on the spectral simulation of star points in the all-sky spherical star map of the dynamic star simulator. Similar research has been conducted on the spectral simulation of star points in the all-sky spherical star map of the dynamic star simulator, namely the simulation of star light color in the star map. By deducing the relationship between the stellar color index and color temperature, combined with the blackbody radiation formula, the corresponding chromaticity coordinates of specific color temperature stars can be obtained to complete the simulation of starlight color in the star map. Although the principle of this method is simple, the error is large, and the star modeling data is too large. Moreover, according to astronomical knowledge, the inherent reason why stars exhibit different colors of starlight is caused by the different spectral energy emitted by the stars themselves. Therefore, spectral simulation of star points in the star map is more in line with the future development direction of star simulators.

1. Introduction to Star Sensors

Star sensors can be divided into two categories based on the application environment: missile borne star sensors and satellite borne star sensors. Missile borne star sensors are a type of star sensors that achieve precise guidance for aircraft and missiles by continuously tracking a fixed star in the field of view. Spaceborne star sensors are a type of star sensor that captures star maps in the field of view in space and identifies them, then calculates the specific positions of satellites and other spacecraft in the celestial coordinate system and controls their attitude.

The research on star sensors has gone through three stages. The first stage was the emergence of CCD star sensors after the successful development of CCD image sensors in the 1950s. The second stage was after the mid-1980s, when star sensors generally used mature large area array CCD as image sensors. The third stage was after the 1990s, when star sensors used CMOS APS image sensors with faster image acquisition speeds as imaging devices.

At present, the superior performance of foreign star sensors includes many advantages: high accuracy and attitude positioning accuracy better than 3 “; Large field of view and a working field of view generally exceeding 20 º; Small in size and gradually developing towards micro nano scale; High stability and strong anti-interference ability. Representative star sensors include the A-STR type star sensor developed by Galileo Avionica in Italy. ASTRO-APS and ASTRO-15 star sensors developed by Jena Optronik in Germany. The CT-601 star sensor developed by Ball Aerospace in the United States.

The research on star sensors in China started in the 1980s, and the Beijing Control Engineering Research Institute was the earliest unit to study star sensors in China, with a rich theoretical foundation and mature product series. Subsequently, research institutions such as Harbin Institute of Technology, Beijing University of Aeronautics and Astronautics, and the affiliated research institute of the Chinese Academy of Sciences conducted relevant research on the hardware and algorithms of star sensors. After nearly thirty years of effort, the performance of some star sensors developed in China has become outstanding, including research on optical systems and algorithms. However, there is still a certain gap in the overall performance compared to foreign countries, mainly due to the backward production level of image sensor components in China, resulting in a small working field of view and poor dynamic performance of the star sensor developed.

2. Introduction to Star Simulator

As the main equipment for ground performance testing and accuracy calibration of star sensors, star simulators can achieve navigation star map simulation under laboratory conditions. Star simulators can be divided into two categories based on their functions and purposes. One type is calibration type star simulators, also known as static star simulators, whose star maps are fixed and unchanging, usually only simulating a single star image or a limited number of representative star maps. The other type is a functional detection type star simulator, also known as a dynamic star simulator, whose star map can change in real time and can simulate the entire sky star map.

Due to the high cost and time-consuming process of space experiments, the calibration of star sensors is usually carried out on the ground. Two methods are usually used to simulate constant-star maps on the ground: the first is to build a physical model of the sky in a closed environment to simulate constant-star maps. By installing luminous devices on the dome of a closed environment to simulate stars of different magnitudes, multiple optical devices are used to simulate a star map. After placing the star sensor in the designated position, conduct on-site shooting, comparison, and recognition of the simulated star map. This method can simulate the size of star points sufficiently small through optical fibers, and can accurately simulate the absolute brightness of different star magnitudes, and even simply simulate the spectral information of stars. However, if you want to simulate a complex star map, there are a large number of luminescent devices required, and a large simulation environment needs to be built. At the same time, high-precision complex turntables need to be designed, which is difficult to achieve in actual star sensor calibration. The second method is currently widely used to simulate constant-star maps by displaying star maps through spatial light modulation devices and projecting parallel light through an optical system. The optical axis direction of the star sensor is randomly generated by the upper computer. In the all-sky spherical star map, based on the selected navigation star library, the star map is displayed through spatial light modulation devices and projected through the optical system. The star sensor recognizes the star map at the exit pupil of the optical system. This method has no limitations on the site and is low-cost and easy to implement. Therefore, currently, the calibration of star sensors usually uses the second method, which is usually achieved through star simulators.

Driven by the research on star sensors abroad, research on dynamic star simulators has also been carried out earlier, and the technology is currently relatively mature. The developed products have good performance matching with the detection indicators of star sensors, and gradually form a product sequence. Representative products include the OSI products of Jena optronik in Germany, the STOS products of Airbus DS, and the products of McDonnell Douglas Aerospace in the United States. The main representative products of foreign static star simulators are the multi tube star simulator developed by the original East German Zeiss, the static star simulator developed by Eastman Kodak in the United States, and a celestial field simulator developed by Hughes in the United States.

The research on dynamic star simulators started relatively late in China and was not conducted until the 1990s. Due to the small working field of high-precision star sensors independently developed in China, the requirements for the field of view of star simulators are relatively small. The representative research on static star simulators in China includes a set of integrated spherical light sources composed of brominated tungsten lamps and LEDs developed by Liu Hongxing and others from Changchun Institute of Optics and Mechanics in 2011. The brominated tungsten lamp is used as the base light source, and multiple LED spectra with different spectra are used for spectral compensation of the base light source. In 2015, Xi’an Institute of Optics and Precision developed a set of spectral adjustable star simulators, with a color temperature curve simulation range of 3900K~6500K, a simulation accuracy of no more than 15% between the simulated spectrum and the target spectrum in the continuous range of 3900K~6500K, a simulated magnitude of 0Mv~+6Mv, adjustable every 1Mv, and a magnitude brightness error of no more than ± 0.2Mv.