A method for evaluating the performance of a guided camera for asteroid defense

Abstract

This invention discloses a method for evaluating the performance of a guidance camera used in asteroid defense, belonging to the field of deep space exploration technology. The method includes calculating the apparent magnitude, and based on the apparent magnitude and Posen's formula, calculating the irradiance of the target asteroid; calculating the upper limit of the integration time for point target detection and recognition by the guidance camera based on satellite platform stability, guidance camera focal length, detector pixel size, and point target detection spot size; calculating the effective number of electrons within the pixel array at a specified integration time, using the point target detection pixel array as the calculation unit; calculating the noise equivalent number of electrons, using the point target detection pixel array as the calculation unit; and calculating the detection limit apparent magnitude and the detectable distance of the target asteroid based on the target detection signal-to-noise ratio threshold, or calculating the surface image integration time based on the surface image signal-to-noise ratio threshold. This invention can provide certain reference for the selection of target asteroids and the demonstration of overall guidance camera performance in asteroid defense missions.

Description

Technical Field

[0001] This invention belongs to the field of deep space exploration technology, specifically relating to a method for evaluating the performance of a guidance camera used for asteroid defense. Background Technology

[0002] Near-Earth asteroids are those whose perihelion distance is less than 1.3 AU. Among them, near-Earth asteroids with a closest approach to Earth of less than 0.05 AU and a diameter greater than 140 meters are called potentially hazardous asteroids (PHAs). Near-Earth asteroids are numerous, but due to their small size and faint appearance, they are difficult to observe. Although approximately 30,000 near-Earth asteroids have been cataloged globally, it is estimated that this represents only 1% of the true number. Near-Earth asteroid impacts are extremely destructive. The scientific community generally believes that a near-Earth asteroid impact 65 million years ago caused the extinction of 75% of all species worldwide, including the dinosaurs. Since records began, numerous regional asteroid impact events have caused significant harm to human life.

[0003] In asteroid defense missions, due to the limited accuracy of ground-based telescopes in tracking target asteroids, impactors need to possess autonomous target detection and identification capabilities. During the long-range asteroid detection phase, after the impactor detects and identifies the target asteroid using its onboard guidance camera, it performs multiple maneuvers to ensure impact effectiveness. To guarantee the impactor's maneuvering time margin, the guidance camera is required to have an excellent signal-to-noise ratio, improve the target detection limiting magnitude, and arrive at the target's detectable time and distance as early as possible. During the asteroid's flyby phase, the target observation produces surface images, which are used by the impactor to select the optimal impact point. This requires the guidance camera to have appropriate exposure times and a large dynamic range to preserve target image details.

[0004] Compared to star sensors used in traditional spacecraft for observing stellar targets, the design and performance evaluation of guidance cameras for asteroid defense missions face the following challenges: First, the target asteroids in asteroid defense missions are on the order of tens of meters in diameter and are extremely faint. The attitude stability of the satellite platform limits the integration time, so the guidance camera design focuses more on detecting limiting stars and assessing the target's detectable time and distance. Second, in asteroid defense missions, the target's apparent magnitude changes with the distance between the impactor and the target asteroid, requiring the guidance camera to evaluate the dynamic target observation performance. Finally, when the impactor approaches the target, the integration time at a specified signal-to-noise ratio needs to be evaluated. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for evaluating the performance of a guiding camera used in asteroid defense. When the impactor is far from the asteroid, the guiding camera operates in point target detection and identification mode. Based on existing general satellite camera signal-to-noise ratio evaluation methods, physical quantities such as asteroid absolute magnitude, asteroid distance, satellite platform stability, and detector spot size are introduced to evaluate the guiding camera's ability to detect and identify distant asteroid targets. When the impactor is close to the asteroid, the guiding camera's performance in observing close-range surface targets is evaluated through formula transformation.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for evaluating the performance of a guidance camera used in asteroid defense, the method comprising:

[0008] Step 1: Calculate the apparent magnitude at the impactor's location based on the target asteroid's absolute magnitude, albedo, solar phase angle, distance between the impactor and the target asteroid, and distance between the sun and the target asteroid. Calculate the target asteroid's irradiance based on the apparent magnitude and the Posen formula.

[0009] Step 2: When detecting and identifying distant point targets, the guiding camera uses n n detector pixel arrays are used for point target detection, n≥2; when observing near-range surface targets, n=1. Based on the satellite platform stability, the guide camera focal length, detector size and point target spot size, calculate the upper limit of the integration time for point target detection and recognition by the guide camera.

[0010] Step 3: Using the point target detection pixel array as the calculation unit, introduce the target irradiance, guiding camera aperture, system transmittance, and detector quantum efficiency to calculate the effective number of electrons in the array under the specified integration time upper limit.

[0011] Step 4: Using the point target detection pixel array as the calculation unit, calculate the noise equivalent electron number under the combined effects of quantization noise, readout noise, dark noise, and shot noise.

[0012] Step 5: Calculate the limit magnitude of the guiding camera and the detectable distance of the target asteroid based on the target detection signal-to-noise ratio threshold; or calculate the integral time of the surface image based on the surface image signal-to-noise ratio threshold.

[0013] The beneficial effects of this invention are as follows:

[0014] This invention establishes a comprehensive target observation performance evaluation method encompassing numerous characteristics, including the target, impactor platform, and guidance camera payload. The method covers mission scenarios ranging from long-range point target detection and identification to close-range surface target observation. Its excellent versatility ensures applicability for asteroid defense missions where the characteristics of impact targets are uncertain. This invention can also provide valuable reference for asteroid selection, satellite overall design, and guidance camera overall performance evaluation in asteroid defense missions. Attached Figure Description

[0015] Figure 1 This is a flowchart of a method for evaluating the performance of a guidance camera used in asteroid defense according to the present invention.

[0016] Figure 2 A schematic diagram showing the relative positions of the asteroid, the sun, and the impactor;

[0017] Figure 3 This is a schematic diagram showing the relative positions of the point target detection array and the diffused light spot. Detailed Implementation

[0018] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0019] like Figure 1 The diagram shows a flowchart of a method for evaluating the performance of a guidance camera used in asteroid defense, which includes the following steps:

[0020] Step 1: Calculate the characteristics of the asteroid. First, calculate the apparent magnitude based on the target asteroid's absolute magnitude, albedo, solar phase angle, distance between the impactor and the target asteroid, and distance between the sun and the target asteroid. Then, calculate the irradiance of the target asteroid based on the apparent magnitude and the Posen formula.

[0021] Step 2: Calculate the integration time of the guiding camera. When detecting and identifying distant point targets, the guiding camera uses n... n detector pixel arrays are used for point target detection to increase detection energy acquisition, thereby improving the signal-to-noise ratio of target detection and recognition. n is the number of rows and columns of the pixel array used for detection and recognition; n≥2 for long-range point target observation and recognition, and n=1 for short-range surface target observation. The upper limit of the integration time for point target detection and recognition by the guiding camera is calculated by introducing satellite platform stability, guiding camera focal length, detector size, and point target spot size.

[0022] Step 3: Calculate the effective signal of the guiding camera. Using the point target detection pixel array as the calculation unit, calculate the number of effective electrons in the array under the specified integration time upper limit by introducing physical parameters such as the guiding camera aperture, system transmittance, and detector quantum efficiency.

[0023] Step 4: Calculate the noise of the guiding camera. Using the pixel array of point target detection as the calculation unit, calculate the noise equivalent number of electrons under the combined effect of noise terms such as shot noise, readout noise, dark noise, and quantization noise.

[0024] Step 5: Evaluate the performance of the guidance camera. Based on the target detection signal-to-noise ratio threshold, important information such as the limit magnitude that the guidance camera can detect and the detectable distance of the target asteroid can be calculated; or the surface image integration time can be calculated based on the surface image signal-to-noise ratio threshold.

[0025] Specifically, in step 1, the relative positions of the asteroid, the sun, and the impactor are as follows: Figure 2 As shown.

[0026] Apparent magnitude of an asteroid The calculations are about the absolute magnitude of asteroids. Sun-star distance r, planet-star distance l, and solar phase angle The function. This method uses the widely adopted two-parameter function. The magnitude system serves as a magnitude model for asteroids.

[0027] (1)

[0028] in, To describe the slope parameter of an asteroid's surface material in terms of its ability to scatter sunlight, it is usually assumed that... , and It is a phase function;

[0029] (2)

[0030] Where A1=3.332, A2=1.862, B1=0.631, B2=1.218.

[0031] In the absence of asteroid absolute magnitude When the asteroid's diameter and albedo are used, the apparent magnitude of the asteroid can be obtained by calculating it using equation (1).

[0032] (3)

[0033] Where D is the diameter of the asteroid; P is the surface albedo of the asteroid, which is generally taken as 0.14.

[0034] Asteroid corresponding spectral irradiance Based on the apparent magnitude of asteroids Solar apparent stars, etc. Corresponding spectral irradiance of the sun The result is obtained using the Posen formula:

[0035] (4)

[0036] In step 2, the detector pixel identification array and the relative positional relationship of the point target observation diffuse spots are as follows: Figure 3 As shown.

[0037] In the long-range point target detection mode, calculate the upper limit of the integration time of the navigation sensor. At this time, the detector of the guidance camera is at n... In the n-pixel array recognition mode, due to the instability of the satellite platform's attitude, to ensure point target detection and recognition, the displacement range of the imaging spot is controlled within n during the integral time. Within an n-pixel array.

[0038] At this moment, the impactor platform's attitude change angle Angle of field of view corresponding to the diffuse spots observed by the asteroid point target camera Angularity of the field of view corresponding to the target recognition array The following relationship should be satisfied:

[0039] (5)

[0040] The field of view subtended by the diffuse spot observed by the asteroid point target camera. The angular resolution of the guidance camera is a result of the camera's optical design and can be determined by measuring the diameter of the blur spot. and camera focal length The calculation yielded:

[0041] (6)

[0042] Target recognition array corresponding field of view angle It can be achieved through single pixel size and camera focal length The calculation yielded:

[0043] (7)

[0044] Points Time Limit Angle of change in the attitude of the impactor platform and platform stability The calculation yielded:

[0045] (8)

[0046] In step 3, the spectral radiant flux received by the guiding camera detector under a certain aperture is first calculated based on the illuminance corresponding to the apparent star magnitude, thereby calculating the equivalent number of electrons of the effective signal received within the upper limit of the integration time.

[0047] The spectral flux reaching the guiding camera detector is calculated as follows:

[0048] (9)

[0049] in, To guide the system transmittance of the camera, To guide the occlusion rate of the camera system, The effective aperture for guiding the camera.

[0050] The number of electrons generated by the radiation from the target asteroid received by the guiding camera within the upper limit of the integration time is:

[0051] (10)

[0052] in, The number of electrons generated by the radiation from the target asteroid received by the guiding camera is equivalent to the number of electrons in the effective signal. For single photon energy, For the quantum efficiency of the detector For wavelength, Let be Planck's constant. It is the speed of light.

[0053] In step 4, during the process of quantizing the analog electronic signal generated by the observed asteroid into a digital signal by the guiding camera, the noise components mainly include shot noise, readout noise, dark current noise, and quantization noise. Shot noise is related to the Poisson distribution of photons, and its magnitude is equal to the standard deviation of the number of signal electrons. Dark noise is generated by the thermal motion of charge carriers in the detector, also exhibiting a Poisson distribution and positively correlated with the integration time. Readout noise is noise related to the detector design and is superimposed on the effective signal. Quantization noise is the noise factor caused by photon quantization error during analog-to-digital conversion, and is related to the electronic gain and the number of quantization bits. n is used for point target identification. The number of noise electrons in an n-pixel array is:

[0054] (11)

[0055] (12)

[0056] in, The noise equivalent number of electrons; As for single-pixel dark noise, the total dark noise increases accordingly due to the use of area array recognition mode; Noise is read out for a single pixel, regardless of the operating mode; To quantize noise; The number of saturated electrons; For electronic gain; Quantize the detector to a specific number of bits.

[0057] In step 5, based on the target detection signal-to-noise ratio evaluation calculation of the guidance camera, a guidance camera performance evaluation method that is related to the entire chain of factors such as the magnitude of the target asteroid, the distance between the impactor and the asteroid, the stability of the satellite platform, the aperture of the guidance camera, the focal length of the guidance camera, the quantization efficiency of the detector, and the pixel size of the detector is established and can be used for asteroid defense.

[0058] When detecting and identifying asteroid targets at long distances, the detector image signal-to-noise ratio (SNR) is used as input, with the target identification image SNR threshold as input (typically a threshold of 3:1 for ground processing of the original image and 5:1 for on-board autonomous identification). This allows for the calculation of the detectable and identifiable distance for a specific asteroid. When observing asteroid targets at close range, the image SNR threshold (which can be 100:1) is used as input to calculate the adjustment of the integration time as the target approaches.

[0059] (13)

[0060] This invention can be used to evaluate the performance of guided cameras for asteroid defense. Based on the background of kinetic impacts in asteroid defense missions, it establishes a comprehensive target observation performance evaluation method encompassing numerous characteristics of the target asteroid, the impactor platform, and the guided camera payload. Specifically, in implementing this performance evaluation method, given parameters such as the scattering characteristics of the target asteroid, solar phase angle, satellite platform attitude stability, guided camera system transmittance, focal length, detector pixel size, and spot size, the signal-to-noise ratio of the guided camera's observation of the target asteroid at a certain distance is calculated, serving as the criterion for guiding camera performance evaluation. The observable distance of the target directly determines the impactor's maneuver time margin and maneuver scheme formulation in asteroid defense missions, playing a decisive role in mission success or failure. Furthermore, this method, through formula evolution, can be further used to calculate other key parameters in asteroid defense mission implementation plans, such as the detectable limiting magnitude, the detectable distance of the target asteroid, and the surface image integration time. For example, for the assessment of the detectable limiting magnitude, the observable distance of the guiding camera to the target asteroid can be calculated based on parameters such as the scattering characteristics of the target asteroid, the solar phase angle, the attitude stability of the satellite platform, the transmittance of the guiding camera system, the focal length, the detector pixel size, the spot size, and the target observation signal-to-noise ratio threshold, and then the target detection limiting magnitude can be calculated.

[0061] This invention establishes a comprehensive target observation performance evaluation method encompassing numerous characteristics of the target, platform, and guidance camera payload. Its excellent versatility ensures applicability to asteroid defense missions where the characteristics of impact targets are uncertain. This invention can also provide valuable reference for asteroid selection, satellite overall design, and guidance camera overall performance evaluation in asteroid defense missions.

[0062] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for evaluating the performance of a guiding camera used in asteroid defense, characterized in that, The method includes: Step 1: Calculate the apparent magnitude at the impactor's location based on the target asteroid's absolute magnitude, albedo, solar phase angle, distance between the impactor and the target asteroid, and distance between the sun and the target asteroid. Calculate the target asteroid's irradiance based on the apparent magnitude and the Posen formula. Step 2: When detecting and identifying distant point targets, the guiding camera uses n n detector pixel arrays are used for point target detection, n≥2; when observing near-range surface targets, n=1. Based on the satellite platform stability, the guide camera focal length, detector size and point target spot size, calculate the upper limit of the integration time for point target detection and recognition by the guide camera. Step 3: Using the point target detection pixel array as the calculation unit, introduce the target irradiance, guiding camera aperture, system transmittance, and detector quantum efficiency to calculate the effective number of electrons in the array under the specified integration time upper limit. Step 4: Using the point target detection pixel array as the calculation unit, calculate the noise equivalent electron number under the combined effects of quantization noise, readout noise, dark noise, and shot noise. Step 5: Calculate the limit magnitude of the guiding camera and the detectable distance of the target asteroid based on the target detection signal-to-noise ratio threshold; or calculate the integral time of the surface image based on the surface image signal-to-noise ratio threshold.

2. The method for evaluating the performance of a guidance camera for asteroid defense according to claim 1, characterized in that, Step 1 includes calculating the apparent magnitude of the asteroid using the following formula (1). : （1） in, Indicates the absolute magnitude of an asteroid. Indicates the distance between the sun and stars. Indicates the distance to the satellite. Indicates the solar phase angle, The slope parameter is used to describe the scattering characteristics of sunlight on the surface material of an asteroid. and It is a phase function; （2） in, =3.332, =1.862, =0.631, =1.218; asteroid absolute magnitude Calculate using the following formula: （3） In the formula, D is the diameter of the asteroid; P is the albedo of the asteroid surface; Calculate the spectral irradiance of the asteroid : （4） In the formula, Indicates the apparent magnitude of the sun. This indicates the corresponding spectral irradiance of the sun.

3. The method for evaluating the performance of a guidance camera for asteroid defense according to claim 1, characterized in that, In step 2, to ensure the detection and identification of long-distance point targets, under the preset attitude stability condition of the satellite platform, the displacement range of the imaging spot is controlled within n within the integral time. n-pixel array, impactor platform attitude change angle The field of view angle corresponding to the imaging spot of the asteroid point target Angularity of the field of view corresponding to the target recognition array The following relationship must be satisfied: （5） Among them, the field of view angle corresponding to the imaging spot of the asteroid point target is This is the result of the optical design of the guiding camera, determined by actual measurement of the spot diameter. and camera focal length The calculation yielded: （6） Target recognition array corresponding field of view angle By single pixel size The angular resolution of the guiding camera is calculated from the camera focal length f, where n=1. （7） Upper limit of integration time for point target detection and recognition Angle of change in the attitude of the impactor platform and platform stability The calculation yielded: （8）。 4. The method for evaluating the performance of a guidance camera for asteroid defense according to claim 3, characterized in that, Step 3 includes: Spectral flux reaching the target detection pixel array of the guided camera detector The calculation is as follows: （9） in, To guide the system transmittance of the camera, To guide the occlusion rate of the camera system, To guide the effective aperture of the camera; The number of electrons generated by the radiation from the target asteroid received by the guiding camera within the upper limit of the integration time is: （10） in, To guide the camera to receive the effective number of target asteroid photons, For single photon energy, For the quantum efficiency of the detector For wavelength, Let be Planck's constant. It is the speed of light.

5. The method for evaluating the performance of a guidance camera for asteroid defense according to claim 4, characterized in that, In step 4, n is used for point target detection. The noise equivalent number of electrons in an n-pixel array is: （11） （12） in, The noise equivalent number of electrons; This represents single-pixel dark noise. Noise is read out for a single pixel; To quantize noise; The number of saturated electrons; For electronic gain; Quantize the detector to a specific number of bits.

6. The method for evaluating the performance of a guidance camera for asteroid defense according to claim 1, characterized in that, In step 5, the target detection signal-to-noise ratio is calculated using the following formula: （13）。

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