Star identification system, optical parameter determination method and program
Patent Information
- Application Number
- JP2022172223
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2022-10-27
- Publication Date
- 2025-08-28
AI Technical Summary
Existing star trackers face challenges in accurately determining optical parameters such as focal length due to thermal distortion, which affects attitude determination accuracy, especially in spacecraft where precise metal materials increase weight and cost, making them unsuitable for microsatellites.
A star tracker system that captures images of multiple stars, calculates their positions, and updates optical parameters using a parameter estimation method to minimize errors by selecting optimal parameters based on temperature data and star catalog comparisons.
Enables high-precision estimation of optical parameters, improving attitude determination accuracy and reducing the weight and cost of star trackers, suitable for various spacecraft types.
Smart Images
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Abstract
Description
[Technical field]
[0001] The present invention relates to a star tracker, an optical parameter estimation method, and a program. [Background technology]
[0002] Spacecraft such as artificial satellites and space probes are equipped with star trackers, which detect the positions of stars and determine the attitude of the spacecraft based on the detected positions of stars. The star tracker is one of the attitude sensors for spacecraft, and has an image sensor that captures star images, and detects star positions based on the star images captured by the image sensor. The detected star positions are then compared with star positions in a star catalog to identify the stars and determine the attitude of the spacecraft. The star tracker also detects the star direction based on the star positions. This detection requires optical parameter values such as the focal length and optical axis deviation of the optical system of the star tracker. Since it is difficult to obtain true values on the orbit of the spacecraft as optical parameter values, values obtained, for example, by experiments or simulations are often used as default values before the launch of the spacecraft.
[0003] Generally, star trackers are among the most accurate attitude sensors for spacecraft. Therefore, the accuracy required for star trackers is usually one digit arcsec. For example, if the actual focal length changes and there is a deviation of even 0.1 mm from the recognized focal length, the required accuracy will be exceeded and the attitude determination rate will drop significantly. In addition, in star trackers, the actual focal length and optical axis deviation are thought to change from moment to moment even during attitude determination. The main cause of this is thermal distortion of the optical system. For example, the temperature change during operation of a star tracker can be 80 degrees or more, and the focal length, etc. will change due to this temperature change.
[0004] There are star trackers that are designed to be robust using metallic materials, and those that use multiple metallic materials with different thermal expansion coefficients to suppress the effects of thermal strain (see, for example, Patent Document 1). This design to suppress thermal strain increases the weight of the device and increases production costs, and is therefore not suitable for star trackers mounted on ultra-small satellites. If there were a method to tolerate some thermal strain and estimate the resulting changes in focal length, etc., or to successively update the recognized value to a value that reduces the attitude error, it would be possible to realize a lightweight, inexpensive, and highly accurate star tracker. One such method is to measure or estimate the relationship between temperature and thermal strain, and calculate optimal parameters based on the relationship (see, for example, Patent Document 2). [Prior art documents] [Patent documents]
[0005] [Patent Document 1] Japanese Patent Application Publication No. 7-208979 [Patent Document 2] Japanese Patent Application Publication No. 7-270177 Summary of the Invention [Problem to be solved by the invention]
[0006] However, the method described in Patent Document 2 has a problem in that even if temperatures at multiple points are acquired, it is difficult to accurately correct thermal strain in the structure based on the temperature information alone.
[0007] The present invention has been made in view of the above problems, and has an object to provide a star tracker and an optical parameter estimation method that can estimate optical parameters such as focal length with high accuracy using a simple configuration. [Means for solving the problem]
[0008] In order to achieve the above-mentioned object, a star tracker of the present invention comprises an imaging means for capturing an image including at least two stars, a position detection means for detecting the position of each star in the image captured by the imaging means, a direction detection means for detecting the direction of the star in the image based on the detection result of the position detection means and a first optical parameter in capturing the image, an identification means for identifying a star in the image based on the detection result of the direction detection means, and a parameter updating means for updating the first optical parameter, wherein the parameter updating means comprises a selection means for selecting a plurality of second optical parameters as candidates for updating the first optical parameter, a calculation means for calculating the difference between a calculated value of the angle of separation between stars in the image obtained by a calculation based on the detection result of the position detection means and the second optical parameter and a theoretical value of the angle of separation between stars in the image obtained based on the identification result of the identification means, and a determination means for determining one optimal parameter from among the plurality of second optical parameters based on the difference calculated by the calculation means, and updates the first optical parameter to the optimal parameter. Effect of the Invention
[0009] According to the present invention, optical parameters such as focal length can be estimated with high accuracy using a simple configuration. [Brief description of the drawings]
[0010] [Figure 1] 11A and 11B are diagrams for explaining fluctuations in focal length due to thermal strain; [Diagram 2] 11 is a diagram showing the relationship between optical parameters during normal operation in a star tracker and optical parameters during parameter estimation operation. FIG. [Diagram 3] FIG. 2 is a functional block diagram of a star tracker. [Figure 4] 13 is a table for explaining the operation of a star separation error sum calculation unit. [Diagram 5] 4 is a table for explaining the operation of the optimal parameter determination software. [Figure 6]13 is a flowchart showing an optical parameter estimation process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0011] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. However, the configurations described in the following embodiments are merely examples, and the scope of the present invention is not limited to the configurations described in the embodiments. For example, each part or each means constituting the present invention can be replaced with any configuration that can exert the same function. In addition, any configuration may be added.
[0012] Hereinafter, an embodiment of the star tracker 100 will be described with reference to Figs. 1 to 6. The star tracker (STT) 100 is a device that can be mounted on a spacecraft flying in outer space, and captures images of stars in outer space to determine the attitude (orientation) of the spacecraft. The spacecraft is not particularly limited, and examples thereof include unmanned spacecraft such as artificial satellites and artificial planets, and manned spacecraft such as space probes, space planes, and space stations. The star tracker 100 is configured to be able to estimate optical parameters (optical system parameters) in capturing images of stars. In this embodiment, an example of the optical parameter is focal length, but is not limited thereto. In this embodiment, the optical parameters include a first optical parameter and a second optical parameter.
[0013] FIG. 1 is a diagram for explaining the variation of focal length due to thermal strain. FIG. 1(a) shows the state before thermal strain occurs. FIG. 1(b) shows the state after thermal strain occurs. As shown in FIG. 1(a) and FIG. 1(b), a lens 1, a cylindrical support part 3 supporting the lens 1, and an image sensor 2 housed in the support part 3 are arranged. The lens 1 focuses light from stars toward the image sensor 2. The image sensor 2 captures an in-image star pair 5 as an image including a star pair 4, which is two stars, and converts the light focused by the lens 1 into an electrical signal. Usually, the star tracker 100 estimates the direction and separation angle (angular distance) of the star pair 4 for the in-image star pair 5 based on the focal length from the principal point of the lens 1 to the focal point on the image sensor 2. Then, the estimated result is compared with a star catalog to identify the stars and determine the attitude of the spacecraft. However, it is difficult for the star tracker 100 to measure the true (actual) value of the focal length one by one. Therefore, attitude determination involves using an estimate of the focal length.
[0014] When the star tracker 100 becomes hot from the state shown in FIG. 1(a), thermal strain occurs. This results in the state shown in FIG. 1(b). In the state shown in FIG. 1(b), the support part 3 expands and the lens 1 moves to the upper side in the figure. This changes the focal length, and the position of the image star pair 5 on the image sensor 2 also changes compared to the position in the state shown in FIG. 1(a) (the image star pair 5 shown by the two-dot chain line in FIG. 1(b)). If the state shown in FIG. 1(a), i.e., the value before expansion, is used as the focal length estimate, there is a risk that the error in the direction and separation angle of the star pair 4 obtained based on the image star pair 5 will be large. Then, a problem occurs in that the attitude of the spacecraft will be shifted by the amount of the error. Conventional methods for solving this problem include, for example, a method of reducing thermal expansion by improving the structure of the star tracker, and a method of estimating the focal length based on information about the temperature of the star tracker. All of these methods are insufficient to realize a lightweight and highly accurate star tracker 100 as described above.
[0015] FIG. 2 is a diagram showing the relationship between the optical parameters during normal operation in the star tracker and the optical parameters during parameter estimation operation. FIG. 2(a) is a diagram showing a normal operation state. FIG. 2(b) is a diagram showing a parameter estimation operation state. FIG. 2(c) is a diagram showing the position of an in-image star pair on an imaging element. As shown in FIG. 2(a), a star pair 4-1, 4-2 is imaged as an in-image star pair 5-1, 5-2 on the imaging element 2. As shown in FIG. 2(c), on the imaging element 2, the in-image star pair 5-1 is located at 10 pixels (10PX) in the X-axis coordinate and 75 pixels (75PX) in the Y-axis coordinate, i.e., (X coordinate position, Y coordinate position) is (10, 75). The in-image star pair 5-2 is located at 50 pixels (50PX) in the X-axis coordinate and 25 pixels (25PX) in the Y-axis coordinate, i.e., (X coordinate position, Y coordinate position) is (50, 25). The estimated separation angle α' is an estimated value of the separation angle between the star pair 4-1 and 4-2 obtained by calculation based on the positions of the star pair 5-1 and 5-2 in the image. The known focal length value fr is the focal length, which is the distance between the lens 1 and the image sensor 2. The known focal length value fr is a known value recognized by the star tracker 100. For example, the known focal length value fr is 50 mm as an initial known value at room temperature. The star tracker 100 calculates the estimated separation angle α' between the star pair 4-1 and 4-2 based on the positions of the star pair 5-1 and 5-2 in the image and the known focal length value fr, and performs star identification by comparing it with a star catalog.
[0016] The star pair 4-3, 4-4 shown in Fig. 2(b) is identified by the above-mentioned process. The true separation angle α is the true value of the separation angle of the star pair 4-3, 4-4 calculated from the star catalog. The star separation angle calculated from the star catalog is generally very accurate, so in this embodiment, it is the true value. The focal length optimum value ft indicates the optimum value of the focal length calculated as a result of optimal parameter estimation by the parameter estimation unit 110 described below.
[0017] The star tracker 100 first calculates the estimated separation angle α' using the position information of the star pair 5 in the image and the focal length candidate value, and identifies the star. Next, the star ID of the identified star pair and the star catalog are referenced to calculate the true value of the star separation angle (true separation angle α). The estimated separation angle α' and the true separation angle α do not completely match due to focal length errors and star position calculation errors. However, depending on how the focal length candidate value is taken, there should be one that minimizes the difference Δα (=|α'-α|) between the estimated separation angle α' and the true separation angle α. The focal length candidate value that minimizes such difference Δα is set as the optimal focal length value ft. This optimal focal length value ft can be used as the recognized focal length value fr from now on. This allows the optimal focal length value ft, which is a focal length considered to be close to the true value, to accurately calculate the star direction, identify the star, and determine the attitude of the spacecraft, in other words, the accuracy of the star direction calculation, star identification, and attitude accuracy are improved. The star tracker 100 according to this embodiment is configured to execute optimal parameter estimation to calculate the optimal focal length value ft every time a star is identified. However, this is not limited to this, and it may be configured to obtain a focal length that is considered to be close to the true value when the change in temperature in a temperature acquisition unit such as a thermistor provided in the star tracker 100 exceeds a predetermined value, and update the first optical parameter to the second optical parameter as described later, that is, to update the focal length when it is estimated that the focal length has changed due to a temperature change.
[0018] Fig. 3 is a functional block diagram of the star tracker. As shown in Fig. 3, the star tracker 100 has an attitude determination operation unit 10, a parameter estimation unit 110, a star position / identification result storage unit (storage means) 200, and a temperature acquisition unit (temperature information acquisition means) 300. Although not shown, the star tracker 100 also has a CPU (main CPU) and a storage unit (storage means), and each unit in the above-mentioned functional blocks is realized by this CPU and storage unit. The storage unit has, for example, a ROM and a RAM, and stores various programs that are executed so that the CPU functions as each function. The storage unit also stores a star catalog (star catalog) in advance.
[0019] The attitude determination operation unit 10 has an imaging unit (imaging means) 20, a star position calculation unit (position detection means) 30, a star direction calculation unit (direction detection means) 40, a star identification unit (identification means) 50, and an attitude determination unit (attitude estimation means) 60. The imaging unit 20 has an imaging element 21 and a signal processing circuit 22. The imaging element 21 corresponds to the imaging element 2 in FIG. 2. The imaging unit 20 also has a lens 1 and a support part 3 that supports the lens 1 and has the imaging element 21 housed inside (see FIG. 2). The imaging element 21 captures a star image including at least two stars (star pairs) (imaging process). In this embodiment, two stars form a pair in a star image, and there are a plurality of pairs. The imaging element 21 is not particularly limited, and can be configured, for example, by a CMOS image sensor, a CCD image sensor, or the like. The imaging element 21 receives light incident through the lens 1 and outputs an analog signal according to the intensity of the light to the signal processing circuit 22. The signal processing circuit 22 converts the analog signal received from the imaging element 21 into a digital signal, and then generates image data. The signal processing circuit 22 outputs the image data to the star position calculation unit 30.
[0020] The star position calculation unit 30 detects the position of each star in the star image captured by the imaging unit 20 (position detection process). Specifically, the star position calculation unit 30 extracts star images in the star image captured by the imaging unit 20 and calculates the position of the star image. Information on the star position obtained by the star position calculation unit 30 is output to the star direction calculation unit 40 and the star position / identification result storage unit 200, and is used for star direction calculation and optical parameter estimation (optimum parameter estimation) for determining the attitude of the spacecraft. The star position calculation unit 30 has a brightness peak search circuit / software 31 and star position calculation software 32. The brightness peak search circuit / software 31 stores a pixel that has a brightness value higher than any of the adjacent pixels in the image data received from the signal processing circuit 22 as a peak pixel. The brightness peak search circuit / software 31 outputs information on the position of the peak pixel to the star position calculation software 32. The star position calculation software 32 calculates the star position by performing a brightness weighting calculation within a range centered on the position of the peak pixel stored in the brightness peak search circuit / software 31 among the image data output from the signal processing circuit 22. The calculation result is output to the star direction calculation unit 40 for calculating the attitude of the spacecraft, and is also output to the star position / identification result storage unit 200 for parameter estimation.
[0021] The star direction calculation unit 40 detects the direction of the star based on the detection result by the star position calculation unit 30 and the optical parameters (first optical parameters or second optical parameters) (direction detection process). The star direction calculation unit 40 has star direction calculation software 41 and an optical parameter storage unit (storage means) 42. The optical parameter storage unit 42 stores the optical parameters used for star direction calculation, i.e., the first optical parameters and the second optical parameters. The optical parameter storage unit 42 outputs the optical parameters to the star direction calculation software 41. Note that the optical parameter storage unit 42 stores the first optical parameters as initial values at the time of the initial attitude determination, but once the attitude is determined, the first optical parameters are updated by the operation of the parameter estimation unit 110 to values that reduce the attitude determination error, i.e., optimal parameters. The star direction calculation software 41 calculates the star direction in the star tracker 100 coordinate system based on the detection result by the star position calculation unit 30 and the first optical parameters. The calculation results are output to a star identification unit 50 and an attitude determination unit 60.
[0022] The star identification unit 50 has star identification software 51. The star identification software 51 identifies each star in the star image based on the detection results of the star direction calculation unit 40 and the star catalog (identification process). "Identification" refers to determining which star on the star image corresponds to an actual star in the universe. The identification results of the star identification software 51 are output to the attitude determination unit 60, and are also output to the star position and identification result storage unit 200 for optical parameter estimation.
[0023] The attitude determination unit 60 has attitude determination software 61. The attitude determination software 61 determines (estimates) the attitude of the spacecraft based on the detection results of the star direction calculation unit 40 and the identification results of the star identification unit 50. Specifically, the attitude determination software 61 compares the star direction in the inertial coordinate system with the star direction in the star tracker coordinate system, which is the output of the star direction calculation unit 40, and calculates a coordinate transformation matrix between the inertial coordinate system and the star tracker coordinate system. This calculation makes it possible to determine the attitude of the spacecraft. The calculation result is then output to the main computer of the spacecraft on which the star tracker 100 is mounted.
[0024] The star position / identification result storage unit 200 stores the detection results from the star position calculation unit 30 and the identification results (combinations of star IDs) from the star identification unit 50. This stored information is output to the parameter estimation unit 110.
[0025] The temperature acquisition unit 300 acquires temperature information on the temperatures of one or more of the components of the star tracker 100, and outputs the information to the parameter estimation unit 110. As the temperature information, it is preferable to use, for example, temperature information on the temperature of the support component 3 of the imaging unit 20, which has a large effect on the optical parameters, but this is not limiting. By using the temperature information on the temperature of the support component 3, the parameter estimation unit 110 can determine the extraction range of the second optical parameter according to the temperature, and can also extract a narrower range including the optimum value. The temperature acquisition unit 300 has a temperature sensor, for example, a thermistor.
[0026] The parameter estimation unit 110 is used when the first optical parameters need to be updated to optimal parameters. That is, it functions as a parameter update unit. The parameter estimation unit 110 has optical parameter candidate selection software (selection means) 120, star separation error calculation unit (computation means) 130, parameter / error sum calculation storage software (computation means) 140, and optimal parameter determination software (determination means) 150. After determining a plurality of second optical parameters as optimal parameter candidates, the parameter estimation unit 110 refers to data in the star position / identification result storage unit 200. This allows the star separation error sum to be calculated, and the second optical parameter that minimizes the error sum or has a value that satisfies a previously set condition to be extracted as the optimal parameter. The optimal parameter is output to the optical parameter storage unit 42 of the star direction calculation unit 40.
[0027] The optical parameter candidate selection software 120 selects values of a plurality of second optical parameters that can be optimal parameters, that is, selects a plurality of second optical parameters as candidates for updating the first optical parameters (selection step). The optical parameter candidate selection software 120 can select from a plurality of second optical parameters stored in the optical parameter storage unit 42. The optical parameter storage unit 42 stores in advance a plurality of second optical parameters set at a predetermined interval for the entire range that can be taken as the first optical parameter, or a plurality of second optical parameters having values close to the first optical parameters. This allows the optical parameter candidate selection software 120 to quickly select a plurality of second optical parameters suitable for updating. However, this is not limited to this, and the optical parameter candidate selection software 120 that receives the first optical parameters from the optical parameter storage unit 42 may generate the second optical parameters. In this case, the generated second optical parameters may be regarded as being selected as they are. In addition, temperature information acquired by the temperature acquisition unit 300 is input to the optical parameter candidate selection software 120. The optical parameter candidate selection software 120 can at least one of enlarging, reducing, and changing (offsetting) the selection range for selecting the second optical parameter based on the temperature information. For example, when the actual temperature acquired by the temperature acquisition unit 300 is close to the design reference value, the selection range can be narrowed. Conversely, when the actual temperature is far from the design reference value, the selection range can be widened. When the actual temperature is higher than the design reference value, the second optical parameter can be selected in a selection range offset in the direction in which the focal length becomes longer. Conversely, when the actual temperature is lower than the design reference value, the second optical parameter can be selected in a selection range offset in the direction in which the focal length becomes shorter. By such selection, the selection of multiple second optical parameters suitable for updating can be performed accurately and quickly. The selection results by the optical parameter candidate selection software 120, i.e., the multiple second optical parameters selected by the optical parameter candidate selection software 120, are output in order to the star separation error calculation unit 130.The selection result is also output to the parameter / error sum calculation and storage software 140 and stored therein.
[0028] The star separation angle error calculation unit 130 receives the detection result from the star position calculation unit 30 and the identification result from the star identification unit 50 from the star position and identification result storage unit 200. Based on these results, the star separation angle error calculation unit 130 can calculate the difference between the calculated value of the separation angle between two stars and the theoretical value of the separation angle between two stars (hereinafter referred to as the "separation angle error") (calculation step). The calculated value is a value obtained by calculation based on the detection result from the star position calculation unit 30 stored in the star position and identification result storage unit 200 and each second optical parameter selected by the optical parameter candidate selection software 120. The theoretical value is a value obtained based on the identification result from the star identification unit 50 stored in the star position and identification result storage unit 200. The star separation angle error calculation unit 130 has separation angle measurement value calculation software 131 that calculates the calculated value, separation angle theoretical value calculation software 132 that calculates the theoretical value, and separation angle error calculation software 133 that calculates the separation angle error. The calculation result by the angular separation error calculation software 133, that is, the angular separation error, is output to and stored in the parameter / error sum calculation and storage software 140.
[0029] FIG. 4 is a table for explaining the operation of the star separation error sum calculation unit. As described above, in this embodiment, a plurality of pairs of in-image star pairs 5-1 and 5-2 exist in the star image. As shown in FIG. 4, a serial number (No. 1, No. 2, No. 3, ...) is assigned to each pair. For example, in the case of the serial number No. 1, the star ID of the in-image star pair 5-1 is 12545, and the star ID of the in-image star pair 5-2 is 14578. The coordinates (X coordinate 134-1, Y coordinate 135-1) on the star image (see FIG. 2(c)) of the star ID 12545 are (1000, 1200). The coordinates (X coordinate 134-2, Y coordinate 135-2) on the star image of the star ID 14578 are (500, 826).
[0030] The separation angle measurement value calculation software 131 calculates a separation angle estimated value α' as a calculation value for each set with a serial number based on the information of (X coordinate 134-1, Y coordinate 135-1) and (X coordinate 134-2, Y coordinate 135-2) and the value of each second optical parameter from the optical parameter candidate selection software 120. This calculation result is linked to the star ID (or serial number) which is the star identification result, and is output to the separation angle error calculation software 133.
[0031] The theoretical separation calculation software 132 calculates the true separation α as a theoretical value for each pair of consecutively numbered stars based on the star ID and the star catalog. The calculation result is linked to the star ID and output to the separation error calculation software 133.
[0032] The separation error calculation software 133 calculates the separation error, which is the difference Δα (=|α'-α|) between the estimated separation value α' and the true separation value α, for each group with a serial number. The calculation result is linked to the star ID and the second optical parameter, and output to the parameter-error sum calculation and storage software 140.
[0033] The parameter / error sum calculation storage software 140 calculates (calculates) the sum of angular separation errors for each second optical parameter. After that, the parameter / error sum calculation storage software 140 associates each calculation result with each second optical parameter and stores them. For example, when there are three second optical parameters (second optical parameter A, second optical parameter B, and second optical parameter C) as the second optical parameters, each set, i.e., each set with serial numbers No. 1, No. 2, No. 3, . . ., includes an angular separation error A when the second optical parameter A is used, an angular separation error B when the second optical parameter B is used, and an angular separation error C when the second optical parameter C is used. Then, the parameter / error sum calculation storage software 140 calculates the sum of the angular separation errors A of the serial numbers No. 1, No. 2, No. 3, . . . Similarly, the parameter / error sum calculation storage software 140 calculates the sum of angular separation errors B whose serial numbers are No.1, No.2, No.3, . . . The parameter / error sum calculation storage software 140 also calculates the sum of angular separation errors C whose serial numbers are No.1, No.2, No.3, . . . The parameter / error sum calculation storage software 140 then links the second optical parameter A to the sum of angular separation errors A, links the second optical parameter B to the sum of angular separation errors B, and links the second optical parameter C to the sum of angular separation errors C, and stores them. When calculating the sum of errors, simple addition may be performed, or the sum of squares may be calculated. Although the term "error sum" is used, the method is not necessarily limited to a method of calculating a simple sum, and any other method that can be used as a calculation for treating the sum of errors may be similarly treated as a type of error sum.
[0034] FIG. 5 is a table for explaining the operation of the optimal parameter determination software. As shown in FIG. 5, the relationship between the second optical parameter and the sum of angular separation errors (total sum of angular separation errors) is shown for each serial number (No. 1, No. 2, No. 3, ...). For example, if the above example is applied, in pattern A, the second optical parameter is 50 mm, and the sum of angular separation errors is 2360 arcsec. In pattern B, the second optical parameter is 50.1 mm, and the sum of angular separation errors is 1463 arcsec. In pattern C, the second optical parameter is 49.9 mm, and the sum of angular separation errors is 3675 arcsec. Based on these sums of angular separation errors, the optimal parameter determination software 150 determines one second optical parameter (optimum parameter) that is the optimal value from among the multiple second optical parameters (determination step). Specifically, the optimal parameter determination software 150 refers to the data stored in the parameter / error sum calculation storage software 140, and determines the second optical parameters for which the angular separation error sum is the smallest or which satisfies a previously set condition as the optimal parameters. In the example shown in FIG. 5, No. 2, 1463 arcsec, for which the angular separation error sum is the smallest, is selected, and the second optical parameters of pattern B are determined (extracted) as the optimal parameters (=50.1 mm). This optimal parameter is written (reflected) in the optical parameter storage unit 42 of the star direction calculation unit 40. As a result, the first optical parameters are updated to the optimal parameters in the optical parameter storage unit 42. The first optical parameters updated to the optimal parameters are then used when performing star direction calculation in normal attitude determination operations.
[0035] 6 is a flowchart showing the optical parameter estimation process. As shown in FIG 6, in step S100, the image capturing unit 20 captures a star image.
[0036] In step S101, the attitude determination operation unit 10 judges whether or not the star position calculation in the star position calculation unit 30 and the star direction calculation in the star direction calculation unit 40 have been completed for all star candidates in the star image. If it is determined in step S101 that each calculation has been completed, the process proceeds to step S102. On the other hand, if it is determined in step S101 that each calculation has not been completed, the process proceeds to step S104 and step S105 in that order.
[0037] In step S104, the star position calculation unit 30 performs star position calculation for the uncalculated star candidates.
[0038] In step S105, the star direction calculation unit 40 performs star direction calculations on the uncalculated star candidates, and the process returns to step S101.
[0039] In step S102, the star identification unit 50 identifies stars in the star image captured in step S100 based on the star direction calculation result obtained in step S101. The star identification unit 50 associates the identified stars with star IDs in the star catalog.
[0040] In step S103, the attitude determining operation section 10 stores the identification result (star ID) in step S102 and the star position calculation result obtained in step S101 in the star position / identification result storage section 200, and the process proceeds to step S106.
[0041] In step S106, the attitude determination operation unit 10 judges whether the number of data items of star position and star identification results stored in the star position and identification result storage unit 200 is equal to or greater than the number (threshold) required for parameter estimation. This threshold is stored in advance in the star position and identification result storage unit 200, for example. If it is determined in step S106 that the number is equal to or greater than the threshold, the process proceeds to step S107. On the other hand, if it is determined in step S106 that the number is not equal to or greater than the threshold, the process returns to step S100, and the subsequent steps are executed in order.
[0042] In step S107, the optical parameter candidate selection software 120 selects a plurality of second optical parameters. Here, the simplest method is to select candidate values for each difference from the entire range of values that can be taken by the structure of the star tracker 100.
[0043] In step S108, the optical parameter candidate selection software 120 selects one second optical parameter to be used in the separation angle error sum calculation from the second parameter candidates selected in step S107. Then, the optical parameter candidate selection software 120 outputs the selected second optical parameter to the star separation angle error calculation unit 130 (separation angle measurement value calculation software 131) and the parameter / error sum calculation storage software 140.
[0044] In step S109, the star separation angle error calculation unit 130 calculates the separation error for all star pairs stored in the star position / identification result storage unit 200, and judges whether or not the separation angle error sum has been calculated in the parameter / error sum calculation storage software 140. If it is judged in step S109 that the calculation has been completed, the process proceeds to step S114. On the other hand, if it is judged in step S109 that the calculation has not been completed, the process proceeds to step S110.
[0045] In step S 110 , the star separation error calculation unit 130 extracts the position information and identification information of a predetermined star pair from the star position / identification result storage unit 200 .
[0046] In step S111, the separation angle measurement value calculation software 131 calculates the separation angle estimated value α′ based on the position information extracted in step S110 and the second optical parameter selected in step S107. The separation angle measurement value calculation software 131 outputs the separation angle estimated value α′ to the separation angle error calculation software 133.
[0047] In step S112, the theoretical separation calculation software 132 calculates a true separation α based on the identification information (star ID) extracted in step S110 and the star catalog. The theoretical separation calculation software 132 outputs the true separation α to the separation error calculation software 133.
[0048] In step S113, the separation angle error calculation software 133 calculates the difference Δα between the separation angle estimated value α' calculated in step S111 and the true separation angle α calculated in step S112. Then, the parameter / error sum calculation storage software 140 calculates the separation angle error sum corresponding to the second optical parameter used in step S111, and the process returns to step S109, and the subsequent steps are executed in order. In this case, the star separation angle error calculation unit 130 selects the remaining star pairs for which the separation error sum has not been calculated from among the star pairs stored in the star position / identification result storage unit 200, and executes the processes from step S110 onwards for the star pairs in order.
[0049] In step S114, the optimal parameter determination software 150 judges whether or not the optimal parameters can be determined based on the information obtained up to step S109. Specifically, the optimal parameter determination software 150 judges whether or not there is a second optical parameter whose angular separation error sum falls below a preset threshold value among the second optical parameters obtained up to step S109 (condition 1), or whether or not the angular separation error sum has been calculated for all second optical parameter candidate values (condition 2). If either of these two conditions is satisfied as a result of the judgment in step S114, the process proceeds to step S115. On the other hand, if neither of these two conditions is satisfied as a result of the judgment in step S114, the process returns to step S108, and the subsequent steps are executed in order.
[0050] In step S115, the optimum parameter determination software 150 extracts the second optical parameters that make the angular separation error sum equal to or minimize the threshold value, and sets the second optical parameters as the optimum parameters at that time point.
[0051] In step S116, the optimal parameter determination software 150 stores the optimal parameters determined in step S115 in the optical parameter storage unit 42 of the star direction calculation unit 40. This allows the optimal parameters to be used in processing in the attitude determination operation unit 10, thereby enabling the attitude of the spacecraft to be determined with high accuracy.
[0052] In this way, the star tracker 100 can have a simple configuration that is different from configurations that use, for example, metal materials that suppress the effects of thermal strain or that measure or estimate thermal strain, and can estimate optical parameters according to temperature changes in the imaging unit 20 with high accuracy. In addition, the true value of the star separation angle can be calculated with very high accuracy from data that has been obtained through detailed astronomical observations and is publicly available. Therefore, by using this data, it is possible to estimate optical parameters with higher accuracy than conventional techniques that use temperature information. This makes it possible to realize a star tracker 100 that is lightweight, low-cost, and highly accurate.
[0053] In the above, a case has been described in which the second parameter candidate is determined from the entire selection range of structurally possible values. In this case, it is considered possible to find a parameter that minimizes the error in a single parameter estimation operation. Also, the star tracker 100 can select parameter candidates by narrowing the selection value range to values close to the current estimated value. In this case, it may be difficult to find a parameter that minimizes the error sum in a single operation, but by repeating the imaging and parameter estimation processes multiple times, it is possible to ultimately estimate a parameter that minimizes the error sum.
[0054] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications and changes are possible within the scope of the gist of the present invention.
[0055] For example, in the above embodiment, the optimal parameters are estimated using the separation angle between the star pairs, but this is not limited to this, and includes calculating the error for each value obtained by changing the second optical parameter with respect to the true value calculated using a star catalog, and determining the optimal parameter from among the second optical parameters based on the error. [Explanation of symbols]
[0056] 20 Imaging unit (imaging means) 30 Star position calculation unit (position detection means) 40 Star direction calculation unit (direction detection means) 50 Star Identification Unit (Identification Means) 100 Star Tracker (STT) 120 Optical parameter candidate selection software (selection means) 130 Star elongation error calculation unit (calculation means) 150 Optimal parameter determination software (determination means)
Claims
1. A star identification system that detects the position of each star in an image containing at least two stars captured by an imaging means, and identifies the stars in the image based on the positions of the stars and optical parameters of the imaging means, A star identification system comprising a determining means for determining the optical parameters based on theoretical values of angular separation between identified stars.
2. The star identification system described in Claim 1, characterized in that the determination means determines the optical parameters based on the estimated angular separation between the identified stars calculated using candidate values for the optical parameters and the theoretical values.
3. A star identification system as described in Claim 2, characterized in that the determination means determines the optical parameters based on the difference between the estimated values and the theoretical values.
4. A selection means for selecting a plurality of the candidate values, 4. The star identification system according to claim 3, wherein the determining means determines the optical parameter based on the difference calculated for each of the plurality of candidate values selected by the selecting means.
5. the image includes a plurality of pairs of two stars in the image; 5. The star identification system according to claim 4, wherein the determining means determines the optical parameter based on the sum of the differences calculated for each of the plurality of candidate values for each of the sets.
6. a storage means for storing in advance the candidate values set at predetermined intervals over the entire range that can be taken by the optical parameter, or a plurality of candidate values close to the optical parameter; 5. The star identification system according to claim 4, wherein said selection means selects from a plurality of said candidate values stored in said storage means.
7. A storage means for storing the optical parameters determined by the determination means, 2. The star identification system according to claim 1, wherein the star is identified using the optical parameters stored by the storage means.
8. temperature information acquisition means for acquiring temperature information relating to the temperature of the imaging means; 3. The star identification system according to claim 2, wherein the determination of the optical parameter by the determination means is executed when the change in the temperature reaches or exceeds a predetermined value.
9. A selection means for selecting a plurality of the candidate values, The star identification system according to claim 8, wherein the selection means is capable of at least one of expanding, contracting, and changing the selection range for selecting the candidate value based on the temperature information acquired by the temperature information acquisition means.
10. the imaging means includes an imaging element that captures the image and a lens that condenses light toward the imaging element; 2. The star identification system according to claim 1, wherein the optical parameter is a focal length from a principal point of the lens to a focal point on the imaging element.
11. The star identification system described in Claim 1, characterized in that the star identification system is mounted on a star tracker.
12. the star tracker is mountable on a spacecraft; 12. The star identification system according to claim 11, further comprising an attitude estimation means for estimating the attitude of the spacecraft.
13. A star identification system that detects the position of each star in an image containing at least two stars captured by an imaging means, and identifies the stars in the image based on the positions of the stars and optical parameters of the imaging means, comprising: and a determination means for determining the optical parameters from a plurality of candidate values based on the error between the true value for the identified star calculated using a star catalog and the value calculated for each of the plurality of candidate values for the optical parameters.
14. An optical parameter estimation method for a star identification system, which detects the position of each star in an image including at least two stars captured by an imaging means, and identifies the stars in the image based on the positions of the stars and optical parameters of the imaging means, comprising: An optical parameter determination method comprising a determination step of determining the optical parameters based on theoretical values of the angular separation between identified stars.
15. A program for causing a computer to function as each means of the star identification system according to claim 1.