The embedded star simulator system introduced in this article works by using embedded chips to simulate the orbit and attitude of stars, search for stars in the field of view from the star catalog, and generate real-time star maps for star sensors to test. It has the characteristics of small size, convenient portability, and good real-time performance, and can be used for high and low temperature environmental experiments of star sensors, meeting the testing needs of star sensors in different environments.
Star sensors are highly accurate celestial sensors, which have greatly improved accuracy compared to other celestial sensors and play a crucial role in the development of China’s aerospace industry. Due to the limitations of various factors in the working environment of star sensors, developing equipment that can calibrate and test star sensors can solve the ground testing problem of star sensors. Looking at the development history of the entire star simulator, we can conclude that the problems that most star simulators have are: firstly, low accuracy and poor real-time performance; Secondly, most of them use optical devices to simulate stars at infinity, which are bulky and inconvenient to carry and test star sensors anytime and anywhere. The embedded star simulator system, similar to the traditional star simulator, solves the problem of limited testing environment for star sensors. Research is conducted to address several shortcomings of traditional star simulators, with the aim of improving accuracy, ensuring real-time performance, and miniaturization. The focus is not only on pure binary problems, but also on the analysis of non spherical gravitational perturbations of the Earth, atmospheric drag perturbations, direct solar radiation pressure perturbations, and daily and monthly perturbations. The design focuses on the research of high-precision simulation algorithms.
The static star simulator mainly consists of light sources, collimating optical system components, star point reticle components, electronic control components (including power supply and circuit box), vertical adjustment mechanisms, etc. The working principle and composition of the static star simulator are shown in Figure 1.
Figure 1 Working principle and physical photo of the static star simulator
The light emitted by the light source illuminates a star point reticle plate located on the focal plane of the collimating optical system, with several transparent micropores engraved, forming a simulated star point. The light generated by the simulated star point passes through the collimating optical system and is emitted in the form of parallel light, forming a complete star map at the entrance pupil of the star sensor, which realizes the simulation of the star map. Among them, in the fixed sky map, the single star angle and star diagonal distance are simulated using multiple small transparent micropores engraved on the star point dividing plate; The magnitude is simulated by illuminating the light source and adjusting the brightness of the light source; The simulation of starlight at infinity is achieved by a collimating optical system.
The infinite distance of stars is achieved through the design of optical systems. By using a collimating optical system, the star point reticle is placed at the focal plane position, and the star light vector is emitted in the form of parallel light, thus achieving infinite distance simulation of stars. In order to ensure clear and accurate imaging of all star points within the field of view of the simulated star map, the designed collimating optical system should have the characteristics of small distortion, small field curvature, and apochromatic aberration. In addition, to ensure that the energy of the star simulator can be accurately received by the star sensor when used in conjunction with the star simulator, parameter matching and energy connection between the two systems should also be considered. The optical system design parameters of the static star simulator are shown in Table 1.
Table 1 Main technical indicators of collimating optical system
The star points on the star point dividing board of the star simulator form a star light vector after passing through the optical system. Table 2 shows the constraint of the star position on the star board through the star light vector. Among them, azimuth and elevation angles refer to the positions of each star point relative to the spatial coordinate system, which can be obtained from the celestial catalog; The Cartesian coordinates of the star point theory are obtained based on the field of view and focal length of the optical system design, using the formula for calculating the position of the star point.
Due to the manufacturing and installation errors of the star point reticle, as well as the machining and assembly errors of the mechanical system, the machining and assembly errors of the optical system, and the influence of aberrations (including distortion, deviation between the energy center and the main beam position, spherical aberration, coma, and field curvature), the calculated theoretical Cartesian coordinate position of the star point has positional errors. In addition, when the star sensor is docked with the star simulator, there are also relative errors in the optical axis connection and adjustment errors in the fine-tuning device.
Due to the positional error of the star point dividing board, the accuracy deviation of the star simulator’s star map simulation cannot be avoided during the production of the star point dividing board. Therefore, a correction model can only be established to eliminate it during the testing phase of the star simulator through a large amount of test data. To test the accuracy of star map simulation for MN star sensitive static star simulator, a TM6100 theodolite was used to test the micro star sensitive static star simulator equipped with a new star point reticle. The difference between the theoretical interstellar angular distance value and the measured interstellar angular distance value is the simulation error of the star map. Based on the error distribution map, establish a correction model, use a theodolite to test the corrected star point reticle, and calculate the measured values of the inter star focal length through the formula to obtain the simulated error distribution map of the star map, as shown in Figure 6.
Figure 6 Star point position error diagram
From the figure, it can be seen that the inter star angular distance accuracy of all star points is better than 10 ″, meeting the technical requirements. This simulator can serve as a ground observation target to provide high-precision star simulation images for micro star sensors.
Starting from the requirements of ground calibration and testing equipment for star sensors, this article designs a static star simulator with star map simulation accuracy better than 10 ″. Firstly, the composition and working principle of the system were introduced; Then, the design method of high-precision collimation optical system was studied, and the optical system was designed using ZEMAX software. The star map simulation was achieved by using a star point reticle as the star map display device; Finally, a star position correction method was proposed to improve the accuracy of star map simulation. Thus, the development of a static star simulator with star map simulation accuracy better than 10 “has been completed, which can be used for ground testing and accuracy calibration of star sensors.
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TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
