This article proposes a small dynamic star simulator scheme with a liquid crystal light valve as the core device. The optical splicing method is used to overcome the difficulties of its small and dynamic size, and it is verified through principle experiments. Finally, its performance is analyzed.
The GNC system of the aircraft usesstar sensors to improve the accuracy of its in orbit attitude determination and provide the initial attitude reference for the return of the strapdown inertial navigation system. In GNC system testing and aircraft detection, a star simulator is required to provide the input signals required by the star sensor, and to provide simulated star maps within the field of view of the star sensor at any time, under any aircraft attitude and orbital position conditions, Provide precise star positions and angular distances between stars.
Due to the fact that the starlight simulator needs to simultaneously consider two different test conditions: system testing in the laboratory and testing in the final assembly plant and base plant, the equipment must be small in size, light in weight, and simple in structure. From this, it can be seen that a small-scale, dynamic, and precision required star simulator is needed, that is, a small dynamic star simulator. At present, the existing star light simulators both domestically and internationally are large static calibration equipment, and there have been no reports on small dynamic star simulators.
The small dynamic star simulator has the following significant characteristics: ① high accuracy, ② good dynamic performance, and ③ miniaturization. To meet the above requirements, the star simulator needs to overcome the following difficulties:
High precision and miniaturization require the use of small, high dot matrix, and high contrast display devices.
The real-time performance requires the star simulator to have a smaller star map sampling period.
Therefore, developing or seeking display devices that meet the above requirements and designing fast dynamic star map generation software has become a key issue. According to the performance of the star sensor and the requirements of ground testing, the star simulator must meet the following technical indicators:
Star map field of view: 6.5 ° X 5 °
Single star angle: ＜ 30 arcseconds
Minimum inter star angular distance: 1 minute
Accuracy requirement: 30 arcseconds (3 σ）
Simulated magnitude: 2 class stars to 6.5 class stars, magnitude accuracy ± 0.3 class
Spectral range: light source spectrum 0.4 μ M~l μ M
The spectral design range of the optical system matches the star sensor: (0.46~0.7l μ M)
Star map sampling period: ＜ 20ms
Probe: maximum length
Weighing up to 5kg
Working voltage
(1) Working principle:
The working principle is shown in Figure 1. The main computer of the experimental system calculates the declination and declination of the star sensor optical axis and the field of view direction of the star sensor based on the real-time orbit parameters, attitude parameters, and installation orientation of the star sensor. The navigation star within the field of view of the star sensor at that time is extracted, and the liquid crystal light valve is driven by the interface and driving circuit to display the star map. After imaging by the optical system, parallel dynamic multi star light is generated.
(2) Composition of Star Simulator
The star simulator can be functionally divided into three parts: a star map generator, a star map display, and an optical system
1) Star map generator
Implemented by controlling the computer host and star map generation software.
2) Star map display
Its hardware is distributed in the control computer and probe, and also includes necessary display software
3) Optical system
As shown in Figure 2, the optical system consists of two parts: a liquid crystal light valve splicing optical system and a collimator. The splicing section will be explained in detail in the next section. The design and processing quality of the collimator will affect the final starlight quality. The determination of the diameter of the collimator emitted from the collimator should be considered in conjunction with the optical system parameters of the sensor. The exit pupil of the collimator should coincide with the entrance pupil of the sensor to ensure that the light emitted from various star points in the full field of view is the same as the total light flux at the entrance pupil of the sensor.
——The exit pupil of the light tube should coincide with the entrance pupil of the sensor to ensure that the light emitted by star points in various parts of the full field of view coincides with the entrance pupil of the sensor
——The size of the exit pupil of the light tube is the same as the size of the entrance pupil of the sensor. If the exit pupil of the light tube is too large, it becomes impossible to increase the total light flux due to the limitation of the sensor’s pupil, and causes an increase in the structure and weight of the light tube. If the exit pupil is too small, it will cause energy waste of luminous stars on the LCD.
The distance between the exit pupil and the pupil of the light tube is determined by referring to the position of the sensor’s entrance pupil and the degree to which the simulator can approach the sensor.
(1) Dynamic star map display
1) Liquid product light valve and driver
The starlight simulator requires miniaturization and dynamic display of star maps, which requires high requirements for star map display devices. Therefore, it has been decided to use the world’s advanced liquid crystal light valve.
The main parameters are:
Screen size: 26.88mm x 20.16mm, pixel count: 640x 480, pixel size: 42 μ Mx 42 μ m. Overall dimensions: 38mm x 42mm x 4.3mm, color: black and white, contrast ratio: 200: 1, gray scale: 16 levels, light source: internal fluorescent light source, back lighting.
The following is an analysis of the display performance that can be achieved using this device:
A displays the field of view and resolution of the star map: 6.5 ° X 5 °, with a single star angle of 30 arcseconds, and an inter star angular distance of 1 arcminute. The minimum number of display pixels in the 6.5 ° direction is (6.5 ° X 3600 ′)+30 ′=780, and the minimum number of display pixels in the 5 ° direction is (5 ° X 3600 ′)+30 ′=600. The currently selected LCD light valve has a pixel count of 640x 480, but still cannot meet the minimum pixel count requirements mentioned above. Therefore, it is decided to use a two piece splicing method, which can reach (960 X 640) and meet the requirements.
Requirements for simulated magnitude: 2 to 6.5 magnitude stars, with an accuracy of 0.3 magnitude stars.
At present, the contrast of the LCD light valve can reach 200:1, and after testing, the absolute brightness can reach 2 stars.
Simulate 2 to 6.5 stars, with an accuracy of 0.3 stars, and a contrast ratio of only 2.5145:1=63:1. It can be seen that the selected device meets the requirements.
C Star map display time: Star map sampling period of 20m; The requirement is 320m relative to the star sensor; The integration time is proposed. Due to the fact that the star map generator takes a considerable amount of time to complete a series of calculations during a sampling cycle, it is required to minimize the display time.
The experimental results show that the refresh time of star data in the display memory of the two probes in the star simulator’s field of view is 3.08 In’i, which meets the requirement of a star map sampling period of 20ms.
2) Optical splicing
Due to the inability of a single liquid crystal light valve to meet the requirements of field of view and resolution, at least two pieces need to be spliced, so it was decided to use optical splicing method. Fix the LCD screen LCDl and LCD2 symmetrically on its two sides using a splitter prism. The splicing principle is shown in Figure 3.
(2) Dynamic star map generation software
1) Correction and partitioning of star catalogs
Correction of a catalog
The SAO catalog provided by the observatory is not the one we need at the desired time. We need to make corrections to the catalog, taking into account the effects of precession and nutation. The precession and nutation data can be calculated using theoretical formulas and corrected for the catalog.
Partition of B starlight
Based on the known position of the optical axis of the star sensor and the direction of the field of view, in order to quickly identify the navigation star within the field of view, it is necessary to partition the navigation star light to ensure that the star map sampling period is less than 20ms. Given that the expanded field of view is 6.5 ° X 5 ° and the diagonal is 8.5 °, including the attitude error offset of 0.1 °, we can consider the range of 9 ° X 9 °. After analysis, it is appropriate to divide the star table into a sub region of 4.5 ° X 4.5 °. Within each sub area, there are generally around 2 navigation stars with a rating of 6.5 or higher. Compare the declination and declination of all navigation stars in these 9 sub regions with the optical axis declination and declination 8, and exclude stars outside the field of view. After removal, the navigation stars in the field of view and the declination and declination of each star are obtained.
2) Establishment of field of view star map
The simulated star position in the field of view of the star sensor is solved by calculating the position of the navigation star corrected for precession and nutation in the celestial inertial coordinate system, as well as the declination and declination of the optical axis of the star sensor and the X-axis of the CCD plane in the inertial coordinate system provided by the main computer of the system experiment.
The purpose of the principle experiment is to verify the feasibility of the liquid crystal light valve and its splicing scheme, mainly involving the following issues:
Performance of a liquid crystal light valve and its driving circuit
Can the liquid crystal light valve be effectively received by a star sensor (simulated using a CCD camera) after passing through optical devices and simulate the magnitude of the stars.
C LCD light valve optical splicing performance.
The experimental plan and equipment are shown in Figure 6.
——Image quality: The displayed image is polarized light;
Clear imaging quality; Imaging is a mirror image that needs to be inverted into a positive image through computer and software or optical system hardware.
——Brightness and contrast: Design software to display the darkest and brightest star points, and use the BM-2 color brightness meter to measure. The measurement results are as follows: the brightest star is 43nit, the darkest star is 0.2nit, and the contrast is 215: 1. After calculation, the brightness of the brightest star is equivalent to that of a second class star, and it is lower than the brightness of a sixth class star when it is darkest.
——Dynamic response time
Drive the LCD light valve using a 486 microcomputer (main frequency 66M) and an 8900VGA card. The time to display a star point is 0.22ms, and according to analysis, the sampling time meets the requirements.
2) Receiving performance using CCD camera
The simulated starlight emitted by the liquid crystal light valve, after passing through the optical system, is simulated using a CCD camera as a star sensor. The received image is displayed on a monitor, which can clearly display the star points.
3) Optical splicing performance of liquid crystal light valve
According to the optical splicing method of the liquid crystal light valve mentioned earlier, two optical images can be clearly spliced together, and can be clearly displayed through a CCD camera and monitor.
The accuracy of a series of main technical indicators of the star simulator is related to the accuracy of the simulated star spatial angular position. Using existing splicing schemes, this accuracy indicator is related to factors such as the unit angular resolution of the LCD in the system, optical system correction error, splicing error, correction error, star table calculation error, and the accuracy of docking with sensors during use.
(1) Resolution error:
Given that the angular distance of each unit is 28.13 ‘勹 and the angular distance between two adjacent units is also 28.13’/, if the spatial resolution is defined by the unit (i.e., a single unit simulates star points), then in mathematical sense, the continuous star point spatial position is replaced by a discrete system with an angular distance of a, and the resulting position error is called resolution error.
(2) Optical system distortion correction residual and splicing error correction residual:
Due to the errors that occur throughout the entire process of optical system design, processing, and alignment, this error becomes a system error related to the field of view position after the alignment is completed. Splicing error is also a systematic error. Therefore, it can be corrected. After correction, the error is 3.6 ”.
(3) Catalog calculation error: 2 ”.
(4) Docking and adjusting errors with star sensors
The adjustment error mainly includes the relative error with the optical axis of the star sensor and the adjustment error of the fine-tuning device. In summary, the angular position accuracy of the star simulator and the inter star angular distance accuracy within the field of view are 29.4 ”, which meets the technical specifications of 30 “.
This article has determined a small dynamic star simulator scheme with a splicing liquid product light valve as the core component. Using imported devices, optical splicing and star map display principle experiments have been conducted. The experimental results show that the star simulator can meet the requirements of GNC system testing and aircraft ground detection, and has a simple structure and excellent performance.
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