An on-orbit absolute radiometric calibration method for full field of view and full dynamic range of optical satellites

Abstract

A method for on-orbit absolute radiometric calibration of an optical satellite with full field of view and full dynamic range. The method belongs to the field of aerospace-to-earth optical remote sensing technology, and specifically relates to the field of on-orbit absolute radiometric calibration of satellites. The method comprises the following steps: 1. The satellite maneuvers to radiometric calibration posture 1, sets the exposure time and rotation angle of the spectral radiometer in this posture, and continuously collects the spectral radiance of direct sunlight; 2. Expands the diffuse reflection plate to angle #imgabs0#, sets the exposure time and rotation angle of the spectral radiometer in this posture, and continuously collects the spectral radiance of diffusely reflected sunlight; 3. The satellite maneuvers to radiometric calibration posture 2, expands the diffuse reflection plate to angle #imgabs1#, sets the exposure time and rotation angle of the space camera in this posture, and continuously collects the spectral response value of the space camera; 4. Based on the data collected in steps 1‑3, calculates the absolute calibration coefficients of all spectral segments of the space camera, and completes the on-orbit absolute radiometric calibration of the optical satellite.

Description

Technical Field

[0001] The present invention belongs to the technical field of aerospace-to-earth optical remote sensing, and in particular to the technical field of satellite on-orbit absolute radiation calibration. Background Art

[0002] In-orbit calibration of optical satellites generally uses a diffuse reflector to reflect sunlight as a uniform and stable light source to achieve full-aperture calibration of the camera. In recent years, the following public literature has introduced methods for in-orbit radiation calibration of satellites:

[0003] China's invention patent application "A device for on-orbit calibration of a satellite-borne atmospheric major greenhouse gas detector" (publication number: CN117848977A) uses the earth's shielding effect to achieve signal acquisition at different brightness levels. It can only be used for the calibration of atmospheric measurement payloads, but is not suitable for on-orbit calibration of multi-spectral and hyperspectral payloads. The introduction of additional atmospheric absorption errors will lead to reduced calibration accuracy.

[0004] China's invention patent "A satellite-borne imaging spectrometer on-board calibration system" (authorization announcement number: CN106352979B) uses three diffuse reflection plates at angles to each other to achieve backup and precision calibration. However, it is not comparable to timing measurement equipment such as radiometers, and cannot obtain accurate attenuation coefficients or achieve full dynamic calibration.

[0005] China's invention patent "A Calibration Optical System Based on Geosynchronous Orbit Differential Absorption Spectrometer" (Authorization Announcement No.: CN109374547B) uses the opening and closing of the sunlight blocking door assembly to switch between imaging and calibration conditions, avoiding direct exposure of the diffuse reflection plate to space and effectively reducing attenuation. However, it can only provide single-point calibration of a specific radiance and cannot achieve full dynamic radiation calibration.

[0006] The Chinese invention patent, "On-orbit Full Dynamic Range Radiometric Calibration Method for Reflection Band Space Optical Remote Sensing Instruments" (Authorization Announcement No. CN113945278B), uses integration time to adjust the camera's response DN value, while maintaining constant illumination intensity. While this achieves full dynamic range calibration, its calibration results are highly dependent on the image sensor's variable integration time calibration equation. This approach fails to eliminate on-orbit image sensor characteristic degradation caused by space radiation exposure, particularly the effects of changes in dark current characteristics. Furthermore, this calibration process offsets the exposure time, a key imaging parameter, significantly from the nominal exposure time used for ground-based push scanning, hindering radiometric calibration accuracy. This approach is inconsistent with the ground-based method of achieving full dynamic range calibration by adjusting the brightness of an integrating sphere, making it difficult to compare and calibrate the calibration results.

[0007] In summary, current on-orbit radiation calibration technologies are unable to provide a uniform light source with different radiances to achieve full dynamic range calibration. Instead, they need to rely on prior information such as the radiation response linearity and exposure time linearity measured during the ground-based radiation calibration process to trace the radiation characteristics. As the satellite operates in orbit, these characteristics may change due to space irradiation and other characteristics, affecting the accuracy of radiation calibration. Summary of the Invention

[0008] In order to solve the technical problems that the current on-orbit absolute radiometric calibration method for optical satellites cannot achieve full dynamic range radiometric calibration and relies on the ground radiometric calibration process, resulting in low calibration accuracy, the present invention provides an on-orbit absolute radiometric calibration method for optical satellites with full field of view and full dynamic range, the method comprising the following steps:

[0009] S1: The satellite maneuvers to radiation calibration attitude 1, sets the exposure time and rotation angle of the spectroradiometer in this attitude, and continuously collects the spectral radiance of direct sunlight;

[0010] S2, expand the diffuse reflection plate to the angle , in this posture, the exposure time of the spectroradiometer and the rotation angle of the diffuse reflection plate are set to continuously collect the spectral radiance of the solar diffuse reflection light and the angle data of the diffuse reflection plate;

[0011] S3, the satellite maneuvers to the radiation calibration attitude 2, and unfolds the diffuse reflector to the angle , set the space camera exposure time and diffuse reflection plate rotation angle in this posture, and continuously collect space camera image data and diffuse reflection plate angle data;

[0012] S4. Based on the data collected in steps S1-S3, the absolute calibration coefficients of the full field of view and full dynamic range of the space camera are calculated to complete the absolute radiation calibration of the optical satellite on orbit.

[0013] Furthermore, the radiation calibration posture 1 is specifically as follows: the entire satellite posture maneuvers so that the sun vector is located in the XOZ plane of the space camera and forms an angle of 10° with the +X axis.

[0014] Furthermore, in the radiation calibration posture 1, the exposure time of the spectroradiometer is 0.1ms, and the rotation angle of the spectroradiometer is a uniform rotation from 90° to 5° between the optical axis of the spectroradiometer and the +X axis of the XOZ plane of the space camera.

[0015] Further, expand the diffuse reflector to an angle When the exposure time of the spectroradiometer is 10ms, the diffuse reflection plate rotates at a constant speed of 1° / s to -90° with the +X axis. Greater than .

[0016] Furthermore, the radiation calibration posture 2 is specifically as follows: the entire satellite posture maneuvers so that the sun vector is located in the XOZ plane of the camera and forms an angle of 0° with the +X axis.

[0017] Furthermore, in the radiation calibration posture 2, the exposure time of the space camera is the nominal line hour, and the rotation angle of the diffuse reflection plate is that the diffuse reflection plate is retracted at a uniform speed of 1° / s to -90° with the +X axis of the XOZ plane of the space camera.

[0018] Furthermore, the spectral radiance of direct sunlight is calculated by Obtain, among which, Indicates the spectroradiometer The spectrum is under direct sunlight The response value at the moment, and Spectroradiometer The start and end wavelengths of the spectrum band, represents the solar radiance, Indicates the exposure time of the spectroradiometer. represents the spectral response function of the spectroradiometer, Indicates the wavelength of light.

[0019] Furthermore, the spectral radiance of the sun's diffuse reflected light is calculated by Obtain, among which, Indicates the spectroradiometer The spectrum is in the diffuse sunlight The response value at the moment, Indicates the exposure time of the spectroradiometer. represents the spectral reflectance function of the diffuse reflector, Represents the angle between the sun vector and the diffuse reflector.

[0020] Furthermore, the spectral response value of the space camera is obtained by Obtain, among which, Indicates the space camera The spectrum is The response value at the moment, and Represents the space camera The start and end wavelengths of the spectrum band, Represents the spectral response characteristic function of the space camera.

[0021] Furthermore, the absolute radiation calibration of the optical satellite on-orbit is achieved by Conduct, among which Indicates the The solar radiance of the spectral channel is a known quantity, represents the spectral resolution of the spectroradiometer, which is a known quantity. Indicates the diffuse reflector The reflectance of the spectral channel is a measured quantity, Represents the absolute calibration coefficient of the mth spectral segment of the space camera, which is the quantity to be solved. is a known quantity, so the space camera can be The absolute calibration coefficient of the spectrum segment is solved and the transformation is performed as needed. The absolute calibration coefficients of all spectral bands of the space camera can be calculated from the numerical value, that is, the absolute calibration coefficients of the full field of view and full dynamic range of the space camera.

[0022] The method described in the present invention has the following beneficial effects: It proposes a full-dynamic-range on-orbit radiometric calibration method. By utilizing the changes in solar incidence angle caused by the retraction of the diffuse reflector, a full range of radiance light fields, from full brightness to full darkness, is generated. A miniature spectroradiometer is then used to fully record the solar spectral radiance and the spectral radiance of the diffuse reflector throughout the entire retraction process, which is then used to establish an absolute radiometric benchmark. This method achieves full-dynamic-range radiometric calibration, improving relative radiometric accuracy while also possessing high-precision traceability for absolute radiometric calibration. It also monitors the reflectivity attenuation characteristics of the diffuse reflector in various spectral segments, enabling precise calibration of the channel attenuation characteristics of high-spectral payloads. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a flow chart of an on-orbit absolute radiometric calibration method for an optical satellite with full field of view and full dynamic range according to an embodiment of the present invention;

[0024] Figure 2 spectral radiance curves of the diffuse reflector at different solar altitude angles collected in an embodiment of the present invention;

[0025] Figure 3 Schematic diagram of the radiation calibration process in an embodiment of the present invention;

[0026] Figure 4 This is a comparison chart of the on-orbit radiation calibration and diffuse reflection panel radiance monitoring working conditions in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0028] Example 1

[0029] like Figure 1 As shown, the present invention provides an on-orbit absolute radiometric calibration method for an optical satellite with full field of view and full dynamic range, the method comprising the following steps:

[0030] S1: The satellite maneuvers to radiation calibration attitude 1, sets the exposure time and rotation angle of the spectroradiometer in this attitude, and continuously collects the spectral radiance of direct sunlight;

[0031] S2, expand the diffuse reflection plate to the angle , in this posture, set the exposure time of the spectroradiometer and the rotation angle of the diffuse reflection plate, the spectral radiance of the solar diffuse reflection light and the angle data of the diffuse reflection plate;

[0032] S3, the satellite maneuvers to the radiation calibration attitude 2, and unfolds the diffuse reflector to the angle , set the space camera exposure time and diffuse reflection plate rotation angle in this posture, and continuously collect space camera image data and diffuse reflection plate angle data;

[0033] S4. Based on the data collected in steps S1-S3, the full-field-of-view and full-dynamic-range absolute calibration coefficients of all spectral bands of the space camera are calculated to complete the on-orbit absolute radiation calibration of the optical satellite.

[0034] The radiation calibration posture 1 is specifically: the satellite is maneuvered so that the sun vector is located in the XOZ plane of the space camera and forms an angle of 10° with the +X axis.

[0035] When the satellite maneuvers to radiometric calibration attitude 1, the optical axis of the space camera must be pointed toward the darkness of deep space, and the sun vector must remain stable. Because the relationship between the sun vector and the camera only defines one axis, and the Earth must be kept out of the sphere of influence, the camera's optical axis must be pointed toward deep space to avoid the influence of reflected light from the Earth.

[0036] In the radiation calibration posture 1, the exposure time of the spectroradiometer is 0.1ms, and the rotation angle of the spectroradiometer is the angle between the optical axis of the spectroradiometer and the +X axis of the XOZ plane of the space camera, which rotates uniformly from 90° to 5°.

[0037] Expand the diffuse reflector to the angle When the exposure time of the spectroradiometer is 10ms, the diffuse reflection plate rotates at a constant speed of 1° / s to -90° with the +X axis. Greater than , in this embodiment, Take 45°, Take 55°.

[0038] The radiometric calibration posture 2 is specifically as follows: the entire satellite is maneuvered so that the sun vector is located in the XOZ plane of the camera and forms an angle of 0° with the +X axis.

[0039] When the satellite maneuvers to radiometric calibration attitude 2, it is necessary to ensure that the optical axis of the space camera points toward the darkness of deep space and that the sun vector remains stable. Because the relationship between the sun vector and the camera only defines one axis, and the Earth must be kept out of the sphere of influence, the camera's optical axis must be facing deep space to avoid the influence of reflected light from the Earth.

[0040] In radiometric calibration attitude 2, the exposure time of the space camera is the nominal flight time, and the diffuse reflection plate rotates at a uniform speed of 1° / s until it is -90° to the +X axis of the XOZ plane of the space camera. For example, the nominal flight time of the Jilin-1 spectral satellite is 700us, so the corresponding space camera exposure time is 700us.

[0041] The spectral radiance of direct sunlight is Obtain, among which, Indicates the spectroradiometer The spectrum is under direct sunlight The response value at the moment, and Spectroradiometer The start and end wavelengths of the spectrum band, represents the solar radiance, Indicates the exposure time of the spectroradiometer. represents the spectral response function of the spectroradiometer, Represents the wavelength of light waves, which will continue to attenuate with factors such as space radiation. The imaging time and wavelength are correlated, but in the same on-orbit radiation calibration process, the time is close, and it can be assumed that their response functions are consistent; since the radiance value after direct sunlight is much larger than that after diffuse reflection, in order to take into account the measurement accuracy of the two working conditions, the exposure time of the spectral radiometer is set to and , ensure that the spectroradiometer works at the optimal working point.

[0042] The spectral radiance of the sun's diffuse reflected light is Obtain, among which, Indicates the spectroradiometer The spectrum is in the diffuse sunlight The response value at the moment, Indicates the exposure time of the spectroradiometer. Represents the spectral reflectance function of the diffuse reflector, the spectral reflectance function of the diffuse reflector It will continue to decay due to factors such as space radiation, and requires on-orbit monitoring. Represents the angle between the sun vector and the diffuse reflection plate, such as Figure 2 Shown are the spectral radiance curves of the diffuse reflector at different solar altitude angles.

[0043] Will and Perform calculations to obtain the attenuation function of the diffuse reflector:

[0044] ;

[0045] Since the spectral resolution of the spectroradiometer is very high and the bandwidth of each spectrum is very narrow, the Spectral center wavelength The relationship between the attenuation function at and the test data can be approximately expressed in discrete form as:

[0046] ,in, represents the spectral resolution of the spectroradiometer, Indicates the diffuse reflector Reflectance of the spectral channel.

[0047] The spectral response value of the space camera is obtained by Obtain, among which, Indicates the space camera The spectrum is The response value at the moment, and Represents the space camera The start and end wavelengths of the spectrum band, The spectral response characteristic function of the space camera will continue to decay with factors such as space radiation, showing time and wavelength correlation.

[0048] Optical satellite on-orbit absolute radiometric calibration passed Conduct, among which Indicates the The solar radiance of the spectral channel is a known quantity, represents the spectral resolution of the spectroradiometer, which is a known quantity. Indicates the diffuse reflector The reflectance of the spectral channel is a measured quantity, Represents the absolute calibration coefficient of the mth spectral segment of the space camera, which is the quantity to be solved. is a known quantity, so the space camera can be The absolute calibration coefficient of the spectrum segment is solved and the transformation is performed as needed. The absolute calibration coefficients of all spectral bands of the space camera can be calculated from the numerical value, that is, the absolute calibration coefficients of the full field of view and full dynamic range of the space camera.

[0049] A single calibration process generates dynamic image samples across all spectral bands, from bright to dark. The absolute calibration coefficients for each pixel column and m spectral bands are then calculated using the optical satellite on-orbit absolute radiometric calibration formula. This formula can be applied to any pixel column or to the average absolute calibration coefficient for all columns.

[0050] Example 2

[0051] This embodiment further limits the embodiment 1. The hardware preparation operation during the on-orbit absolute radiation calibration process is described to facilitate more accurate completion of the steps in the embodiment 1.

[0052] In the process of radiation calibration, the hardware equipment required includes three parts: space camera, diffuse reflection plate, and spectral radiometer. The space camera is generally a multispectral imager or hyperspectral camera, which is a space optical camera with high requirements for quantitative radiation remote sensing. The diffuse reflection plate is generally composed of a polytetrafluoroethylene diffuse reflection surface, a structural frame and a rotating shaft mechanism, which converts sunlight into diffuse reflection light with good Lambertian properties, which is used for full-aperture radiation calibration of the space optical camera. The spectral radiometer collects direct sunlight and diffuse reflection light from different angles of the diffuse reflection plate in a time-sharing manner, effectively eliminating the on-orbit attenuation factors of the diffuse reflection plate and the spectral radiometer, and realizing the spectral radiance traceability of the diffuse reflection light. The schematic diagram of the radiation calibration process is shown in the figure below. Figure 3 shown.

[0053] During the radiation calibration process, it is assumed that the solar radiance does not change with time, which serves as the benchmark for radiation tracing (the actual solar radiance in the visible light band varies at the order of 0.1% during the solar activity cycle and can be ignored).

[0054] Calibration attitude and coordinate system constraints: In order to improve the traceability accuracy of absolute radiation, the satellite attitude, diffuse reflector expansion angle and spectroradiometer optical axis angle of the radiation calibration process and the solar diffuse reflector radiance collection process are reasonably selected to meet the following constraints: When designing the optical path, the angle between the diffuse reflector radiance monitoring optical path and the camera imaging optical path should be minimized while meeting the stray light requirements. ; During the monitoring of the radiance of the diffuse reflector, the expansion angle of the diffuse reflector is The expansion angle of the radiometric calibration (To ensure the diffuse reflection plate covers the full aperture of the space camera) , the entire star has additional maneuvers , (In this embodiment, Take 10°) to ensure that the incident and outgoing angles of sunlight in the process of monitoring the radiance of the diffuse reflector and the process of calibrating the radiation of the space camera are consistent, such as Figure 4 shown.

Claims

1. A method for on-orbit absolute radiometric calibration of an optical satellite with full field of view and full dynamic range, characterized in that: The method comprises the following steps: S1: The satellite maneuvers to radiation calibration attitude 1, sets the exposure time and rotation angle of the spectroradiometer in this attitude, and continuously collects the spectral radiance of direct sunlight; S2, expand the diffuse reflection plate to the angle , in this posture, the exposure time of the spectroradiometer and the rotation angle of the diffuse reflection plate are set to continuously collect the spectral radiance of the solar diffuse reflection light and the angle data of the diffuse reflection plate; S3, the satellite maneuvers to the radiation calibration attitude 2, and unfolds the diffuse reflector to the angle , set the space camera exposure time and the diffuse reflection plate rotation angle in this posture, and continuously collect space camera image data and diffuse reflection plate angle data; S4. Based on the data collected in steps S1-S3, the absolute calibration coefficients of the full field of view and full dynamic range of the space camera are calculated to complete the absolute radiation calibration of the optical satellite on orbit.

2. The full-field-of-view full-dynamic-range on-orbit absolute radiometric calibration method according to claim 1, characterized in that: The radiation calibration posture 1 is specifically: the satellite is maneuvered so that the sun vector is located in the XOZ plane of the space camera and forms an angle of 10° with the +X axis.

3. The full-field-of-view full-dynamic-range on-orbit absolute radiometric calibration method according to claim 2, characterized in that: In the radiation calibration posture 1, the exposure time of the spectroradiometer is 0.1ms, and the rotation angle of the spectroradiometer is the angle between the optical axis of the spectroradiometer and the +X axis of the XOZ plane of the space camera, which rotates uniformly from 90° to 5°.

4. The full-field-of-view full-dynamic-range on-orbit absolute radiometric calibration method according to claim 3, characterized in that: Expand the diffuse reflector to the angle When the exposure time of the spectroradiometer is 10ms, the diffuse reflection plate rotates at a constant speed of 1° / s until it is -90° with respect to the +X axis. Greater than .

5. The full-field-of-view full-dynamic-range on-orbit absolute radiometric calibration method according to claim 4, characterized in that: The radiometric calibration posture 2 is specifically as follows: the entire satellite is maneuvered so that the sun vector is located in the XOZ plane of the camera and forms an angle of 0° with the +X axis.

6. The full-field-of-view full-dynamic-range on-orbit absolute radiometric calibration method according to claim 5, characterized in that: In the radiation calibration attitude 2, the exposure time of the space camera is the nominal line hour, and the rotation angle of the diffuse reflection plate is that the diffuse reflection plate is retracted at a uniform speed of 1° / s to -90° with the +X axis of the XOZ plane of the space camera.

7. The full-field-of-view full-dynamic-range on-orbit absolute radiometric calibration method according to claim 6, characterized in that: The spectral radiance of direct sunlight is Obtain, among which, Indicates the spectroradiometer The spectrum is under direct sunlight The response value at the moment, and Spectroradiometer The start and end wavelengths of the spectrum band, represents the solar radiance, Indicates the exposure time of the spectroradiometer. represents the spectral response function of the spectroradiometer, Indicates the wavelength of light.

8. The full-field-of-view full-dynamic-range on-orbit absolute radiometric calibration method according to claim 7, characterized in that: The spectral radiance of the sun's diffuse reflected light is Obtain, among which, Indicates the spectroradiometer The spectrum is in the diffuse sunlight The response value at the moment, Indicates the exposure time of the spectroradiometer. represents the spectral reflectance function of the diffuse reflector, Represents the angle between the sun vector and the diffuse reflector.

9. The full-field-of-view full-dynamic-range on-orbit absolute radiometric calibration method according to claim 8, characterized in that: The spectral response value of the space camera is obtained by Obtain, among which, Indicates the space camera The spectrum is The response value at the moment, and Represents the space camera The start and end wavelengths of the spectrum band, Represents the spectral response characteristic function of the space camera.

10. The full-field-of-view full-dynamic-range on-orbit absolute radiometric calibration method according to claim 9, characterized in that: Optical satellite on-orbit absolute radiometric calibration passed Conduct, among which Indicates the The solar radiance of the spectral channel is a known quantity, represents the spectral resolution of the spectroradiometer, which is a known quantity. Indicates the diffuse reflector The reflectance of the spectral channel is a measured quantity, Represents the absolute calibration coefficient of the mth spectral segment of the space camera, which is the quantity to be solved. is a known quantity, so the space camera can be The absolute calibration coefficient of the spectrum segment is solved and the transformation is performed as needed. The absolute calibration coefficients of all spectral bands of the space camera can be calculated from the numerical value, that is, the absolute calibration coefficients of the full field of view and full dynamic range of the space camera.

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