The working principle of star sensor based on the charged coupled device(CCD) technology was explained,as well as its important role playing in space positioning.The star sensor was needed to be calibrated by the star simulator using the ground calibration technique. According to the two basic parameters of the stellar light source,the simulation methods of the color temperature and the magnitude were proposed respectively.The realization of the star simulator was studied,and the electric light source,waveband light intensity controller,magnitude controller were designed.The entire system was assembled in laboratory, and the controlling and testing platform was set up.The experiments were carried out and the obtained results show that this system has high reliability.In the peak spectral range of 350nm~950nm,it can simulate the magnitude in 0~+6.5MI and the equivalent blackbody temperatures of the star:2 600K,3 600K,4 300K,5 000K,5 500K,6 000K,6800K,7 600K,9 800K,the relative intensity error is better than±0.10.
With the advancement of technology, China’s aerospace industry is also increasingly developing, and the requirements for aerospace equipment are also increasing. Among them, star sensors, which serve as the spatial position and attitude positioning of aircraft, have been widely used in the aerospace field, and the ground calibration device for star sensors, star simulators, has also emerged. Star sensors mainly use CCD technology to receive the light emitted by surrounding stars, process the data into observation star maps, and then compare the observation star maps with the navigation star maps stored in the database to obtain the position and attitude of space equipment in the starry sky. Star sensors have a wide range of applications, not only in aerospace technology, but also in precise guidance of military equipment such as missiles, and precise positioning of ships and submarines.
A crucial aspect of star sensor technology is the reception of light emitted by surrounding stars, making it particularly important to calibrate the receiving optical signal equipment. This calibration technique is called calibration. The calibration of star sensors can be divided into in orbit calibration and ground calibration. Directly calibrating in the starry sky is not only highly dangerous, but also extremely expensive. Therefore, the calibration of star sensors must be carried out on the ground. Therefore, it is necessary to simulate various stellar light sources in the ground laboratory, resulting in the emergence of star simulators.
The single star simulator consists of the following 6 parts:
1) Industrial computer. Connect to the control box through RS-485 serial bus to achieve unified control and management of the entire system. Inside the control box is the main control module, which communicates with the industrial control computer through RS-485 serial bus, receives instructions from the industrial control computer, sets various parameters in the current system, and uploads the current system status, such as the current opening and closing position of the variable light bar, star level control status, light source status, and light intensity of each band. By connecting with the light intensity controller, magnitude controller, and electric light source of each band through RS-485 serial bus, their control is achieved. At the same time, controlling human-machine interfaces such as keyboard display can achieve interactive dialogue with people.
2) Light source. Including 150W xenon lamp, power supply and controller, reflector, optical fiber bundle, etc.
3) Band light intensity controller. There are a total of 13 channels, each consisting of narrowband filters, beam splitters, photocells, variable light barriers, stepper motors, optical fiber bundles, and other components. Realize the control of the power of each color light, forming an equivalent color temperature.
4) Hexagonal prism integrating rod for mixing colored light.
5) Star magnitude controller. It is composed of a reflector group, a beam splitter, a photocell, a variable light bar, a stepper motor, a light transmission fiber bundle, and other components to achieve control of the total optical power and form an equivalent magnitude.
6) Collimating optical system and fiber optic holder. The overall system structure diagram is shown in Figure 1.
Firstly, the light source adopts a xenon lamp light source with LED supplementary lighting, so that the light source can cover the various wavelengths of light required for this design.
Secondly, the light source is connected to the band intensity controller by a one in multiple out optical fiber bundle, and the band intensity controller is connected to the hexagonal prism integrating rod by a multi in one out optical fiber bundle. The two fiber bundles used here belong to the same category. One in multiple out fiber bundles and multiple in one out fiber bundles can transmit light in reverse, but these two fiber bundles must be connected correspondingly, that is, the glass fiber bundle corresponds to the glass lens group, and the quartz fiber bundle corresponds to the quartz lens group.
Due to the large number of narrowband light sources that need to be controlled and the basic structure of each part is similar, a modular design is proposed, which involves designing 13 identical “band intensity controllers”. Each channel can achieve light filtering and intensity adjustment, simplifying the system structure, reducing fault rates, and facilitating testing and maintenance. All “band intensity controllers” are installed on the same platform as the light source, hexagonal prism integrating rod, and star level controller. They communicate with the main control module in the control box through serial communication. The main control module is responsible for centralized control of the light source, 13 band intensity controller, and star level controller. At the same time, the main control module is responsible for functions such as display, keyboard, environmental parameter detection, and communication with the upper computer.
The working process of a single star simulator is as follows: the light emitted by the electric light source is fed into a 13 channel band light intensity controller through a one in multiple out fiber bundle. After passing through the bandpass filter of each controller, a light with a certain wavelength as the center and a certain spectral band width is formed. The light bars of each channel light intensity controller are adjusted according to the given fitting coefficient to obtain multiple beams of different spectra Different intensities of light are then projected into and out of an additional fiber bundle into a hexagonal prism integrating rod for uniform mixing, thereby simulating the color temperature of stars. When the light with different color temperatures obtained through fitting enters the magnitude control system, the magnitude is controlled over a large range by a controllable light bar and a reflector. Using zero magnitude as the reference, three sets of reflectors divide the 0-6.5 stars into three segments, and then adjust the controllable light bar to attenuate the light intensity at the required rate. For every 2.512 times decrease in intensity, the magnitude increases by one, which can obtain the required brightness of the simulated star and achieve simulation of the brightness characteristics of the star.
(1) Selection of light sources
The system to be designed for this task requires a large wavelength range of light, ranging from 350nm to 950nm, spanning infrared, visible, and ultraviolet light. Therefore, xenon lamps are considered as the light source for this design. The energy reduction of xenon lamps mainly depends on the following three factors:
1) Evaporation of the internal metal coating of xenon lamps;
2) The metal coating of the xenon lamp filament reacts with the quartz sleeve (mainly hindering penetration);
3) Change in purity of filled xenon gas.
(2) Selection of light transmission system
The light transmission system mainly plays a connecting role, with the purpose of transmitting the light emitted from the previous mechanism through this unit without distortion to the latter unit. This part mainly includes the light transmission fiber bundle and the hexagonal prism integrating rod.
The wavelength span required for this design is 350nm~950nm, with each 50nm divided into a light control unit and a total of 13 branches. Therefore, 13 optical fibers are required for light splitting. Among them, 11 channels of light (450nm~950nm) are transmitted using multi-component glass fibers, while the 350nm and 400nm channels cannot achieve good transmittance due to the use of glass fibers and plastic fibers, so quartz fibers are used for transmission.
The basic principle of hexagonal prism optical integrating rod: When a light ray enters from one end face of the hexagonal prism integrating rod, it refracts into the internal wall of the integrating rod and forms a total reflection. Some light rays only need one total reflection to reach the other end face of the integrating rod, while others require two or more total reflections to reach the other end. Assuming that the incident light is a single light source, after multiple reflections, multiple beams of light will be formed. When the incident light is emitted, multiple light sources “mirror”, or sub light sources, will be formed. The sub light sources form a superposition at the exit to achieve the integration of light, which is equivalent to homogenizing a light source at the energy level.
Different equivalent blackbody temperatures are obtained by mixing light with different central wavelengths and a certain spectral band width. Find the best function match for the data by minimizing the sum of squared errors. The use of the least squares method can easily obtain unknown data and minimize the sum of squares of errors between these obtained data and actual data. In scientific experimental data processing, it is often necessary to find the functional relationship between the independent variable x and the dependent variable y based on a given set of experimental data. Due to the fact that there are always errors in observed data and the number of undetermined parameters is less than the number of given data points, it is different from interpolation problems.
We can obtain the fitting coefficients using the least squares method, and here we provide the fitting coefficients for some bands.
The structure of the star magnitude controller is generally the same as that of the band light intensity controller, with the main difference being the addition of a mirror combination controlled by another stepper motor to achieve adjustment of the large star magnitude range, as shown in labels 5 and 8 in Figure 3. By controlling the output light intensity to achieve different magnitudes, the adjustment range needs to be 0~+6.5MI, which is equivalent to a difference of 2.5126.5=398 times between the maximum and minimum light intensities. If a single control method is not easy to achieve, it is proposed to use a variable light barrier and a reflector group to achieve a fixed large proportion of attenuation; If the maximum opening diameter is selected Φ 23mm, with a minimum opening diameter of Φ A 1.5mm variable light bar can be achieved, and theoretically, the range of light intensity variation can be achieved as follows: (11.5 × 11.5)/(0.75 × 0.75)=235 times, the reflector group can be achieved with 1%, 10%, and 100% attenuation, which is generally divided into 3 segments and can be adjusted in magnitude with a variable light bar.
The laboratory has established a testing platform, with the main instrument being a spectrometer. The testing software used is OSM-Analyst. The testing platform is used to test the fitting curves of 3600K, 4300K, 5000K, 5500K, 6000 K, 6800K, 7600K, and 9800K. The following conclusion can be drawn from the test data: the fitting method used (i.e. the variable light bar and filter group used) can fit the corresponding curve, and the curve fitted by controlling the light intensity of 13 different center wavelengths and widths according to coefficients can roughly meet the standard color temperature curve, with an error basically meeting the requirements.
This article conducts simulation design from two aspects: color temperature and stellar magnitude, analyzes the simulation principle, and designs the optical mechanical system structure in detail. When simulating color temperature, the method of controlling different wavelengths of light separately and then synthesizing them is used. The system light source is filtered out once by a filter according to different wavelengths, and then these light branches are controlled by light bars for intensity control. Finally, the desired spectral composition can be successfully simulated by introducing a hexagonal prism integrating rod. This design aims to create a stellar light source in a laboratory environment that can simulate different magnitudes and spectral compositions for star sensor calibration.
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