Joint correction method and system for spaceborne multispectral imager

Abstract

The application discloses a kind of joint correction method and system of spaceborne multispectral imager, it is related to spectral imaging instrument calibration technical field, including obtaining the current ambient temperature of spaceborne multispectral imager and the measured center effective signal of observed image;Temperature effect correction coefficient look-up table is searched based on current ambient temperature or current ambient temperature is substituted into temperature effect fitting function, determine the temperature effect correction coefficient corresponding to each spectral channel under current ambient temperature;Based on measured center effective signal, non-linear effect correction coefficient look-up table is searched or measured center effective signal is substituted into non-linear fitting function, and the non-linear effect correction coefficient corresponding to each spectral channel under measured center effective signal is acquired;Based on the temperature effect correction coefficient and non-linear effect correction coefficient of each spectral channel, the absolute responsivity under target temperature and target brightness condition obtained by laboratory calibration is jointly corrected.

Description

Technical Field

[0001] This invention relates to the field of spectral imager calibration technology, specifically to a joint calibration method and system for a spaceborne multispectral imager. Background Technology

[0002] Before being deployed into orbit, spaceborne multispectral imagers require absolute radiometric calibration in a laboratory to establish a mapping relationship between radiation energy and the instrument's output signal, i.e., to obtain the absolute responsivity of the spaceborne multispectral imager. However, due to the complexity of the detection system, many non-ideal factors cause the mapping relationship to change with environmental or radiation energy levels. These include temperature effects on optical and photoelectric detection devices, and nonlinear responses of the detector itself due to various complex factors. This renders the absolute responsivity at a specific temperature or radiation energy level in the laboratory inapplicable, severely impacting the quantitative application of data. Existing technologies mainly suffer from the following problems:

[0003] (1) Currently, common corrections are limited to nonlinear response effect correction or temperature effect correction. Correcting only one aspect means that the radiation energy calculation error is still affected by another aspect. For example, the patent application document CN114279566A proposes to calculate the radiance correction value of a certain state of a standard light source from the known initial responsivity and the initial calibration light source luminance dataset. The object of its correction is radiance rather than the correction of absolute responsivity by combining nonlinearity and temperature. In addition, the nonlinear calibration method introduced in the literature "Research on Key Technologies of Multi-point Nonlinear Calibration of Infrared Spectroscopic Imager, Wang Yanheng, Master's Thesis of University of Chinese Academy of Sciences" uses a better method to divide several linear regions, and then uses a linear method to perform two-point radiometric calibration to obtain the calibration coefficient of each linear region. The process is cumbersome.

[0004] (2) The test object is only the detector, that is, the nonlinear effect correction coefficient or temperature effect correction coefficient is not obtained when the product is in its complete state. In addition to the nonlinearity and temperature effect of the detector itself, other components of the product, such as the structure and the coating of the optical lens, also have temperature effects, which will also cause changes in the absolute radiometric calibration parameters and affect the level of data quantification; (3) The vacuum thermal simulation environment was not provided. The high and low temperature chamber can only simulate the thermal environment, which is significantly different from the heat propagation mode under vacuum conditions. The vacuum thermal environment is more complex and therefore cannot reflect the real temperature effect.

[0005] Therefore, in order to accurately calculate the radiation energy, the spaceborne multispectral imager needs to simultaneously correct for nonlinearity and temperature effects. This requires creating conditions in the laboratory to simulate the vacuum thermal environment and radiation energy level under on-orbit conditions and to calibrate the correlation coefficients. Summary of the Invention

[0006] The technical problem to be solved by this invention is how to perform joint correction of nonlinearity and temperature effect of spaceborne multispectral imagers, thereby significantly improving the quantification level of spaceborne multispectral imager data.

[0007] The present invention solves the above-mentioned technical problems through the following technical means: A joint calibration method for a spaceborne multispectral imager is proposed, the method comprising: Acquire the current ambient temperature of the spaceborne multispectral imager and the measured center effective signal of the observed image; Based on the current ambient temperature, find the temperature effect correction coefficient reference table or substitute the current ambient temperature into the temperature effect fitting function to determine the temperature effect correction coefficient corresponding to each spectral channel at the current ambient temperature. Based on the measured center effective signal, find the nonlinear effect correction coefficient reference table or substitute the measured center effective signal into the nonlinear fitting function to obtain the nonlinear effect correction coefficients corresponding to each spectral channel under the measured center effective signal. Based on the temperature effect correction coefficient and nonlinear effect correction coefficient of each spectral channel, the absolute responsivity under the target temperature and target brightness conditions obtained by laboratory calibration is jointly corrected. Among them, the temperature effect correction coefficient comparison table, the nonlinear effect correction coefficient comparison table, the temperature effect fitting function, and the nonlinear fitting function were all obtained by radiometric calibration of the spaceborne multispectral imager in a simulated on-orbit vacuum thermal environment.

[0008] Furthermore, before jointly correcting the absolute responsivity under the target temperature and target brightness conditions obtained from laboratory calibration based on the temperature effect correction coefficients and nonlinear effect correction coefficients of each spectral channel, the method further includes: In a simulated on-orbit vacuum thermal environment, the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions was measured at the set brightness of the light source. Based on the center effective signal A of each spectral channel and the corresponding equivalent brightness A, the temperature effect fitting function and the temperature effect correction coefficient comparison table of each spectral channel at any temperature were determined. The equivalent brightness A was obtained by integrating the light source spectral brightness and the relative spectral response of each spectral channel under different temperature conditions. In a simulated on-orbit vacuum thermal environment, the center effective signal B of each spectral channel of the spaceborne multispectral imager at the target temperature under different brightness levels of the light source was measured. Based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel under different brightness levels of the light source, a nonlinear fitting function and a table of nonlinear effect correction coefficients for each spectral channel under arbitrary center effective signal were determined. The equivalent brightness B was obtained by integrating the spectral brightness and the relative spectral response of each spectral channel under each brightness level of the light source.

[0009] Furthermore, in the simulated on-orbit vacuum thermal environment, the effective center signal A of each spectral channel of the spaceborne multispectral imager is measured under different temperature conditions at the set brightness of the light source. Based on the effective center signal A of each spectral channel and the corresponding equivalent brightness A, a temperature effect fitting function and a table of temperature effect correction coefficients for each spectral channel at any temperature are determined, including: In a simulated on-orbit vacuum thermal environment, the ambient temperature of the spaceborne multispectral imager was adjusted and stabilized under different temperature conditions. Several background images and several light source images and spectral brightness of the light source were acquired under different temperature conditions. Based on several light source images and several background images, the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions is calculated; The spectral brightness and the relative spectral response of each spectral channel are integrated separately to obtain the equivalent brightness A of each spectral channel under different temperature conditions. Based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel, a temperature effect fitting function is determined, and a table of temperature effect correction coefficients for each spectral channel at any temperature is provided.

[0010] Furthermore, the calculation of the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions, based on several light source images and several background images, includes: Averaging is performed on several light source images and several background images to obtain the average light source image and the average background image under different temperature conditions. Subtract the corresponding background mean image from the mean image of the light source under different operating temperatures to obtain the effective signal image under different temperature conditions. The mean value of the central region of the effective signal image under different temperature conditions is taken to obtain the central effective signal A of each spectral channel under different temperature conditions.

[0011] Furthermore, the table for calculating the temperature effect correction coefficients for each spectral channel at any temperature based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel includes: Divide the center effective signal A of each spectral channel by its corresponding equivalent brightness A to obtain the single-point responsivity of each spectral channel under different temperature conditions. Specify a temperature condition as the target temperature, divide the single-point responsivity of each spectral channel under all temperature conditions by the single-point responsivity of each spectral channel at the target temperature, and obtain the normalized single-point responsivity of each spectral channel under different temperature conditions. The normalized single-point responsivity of each spectral channel under different temperature conditions is fitted to the temperature conditions of the spaceborne multispectral imager to obtain the normalized responsivity function of each spectral channel under any temperature condition, which is used as the temperature effect fitting function. The ratio of the normalized response of each spectral channel under any temperature condition to the normalized response of each spectral channel at the target temperature on the temperature effect fitting function of each spectral channel is used as the temperature effect correction coefficient of each spectral channel, forming a temperature effect correction coefficient reference table.

[0012] Furthermore, the step of substituting the current ambient temperature into the temperature effect fitting function to determine the temperature effect correction coefficients corresponding to each spectral channel at the current ambient temperature includes: Substituting the current ambient temperature into the temperature effect fitting function, we obtain the normalized response of each spectral channel at the current ambient temperature. The ratio between the normalized response of each spectral channel at the current ambient temperature and the normalized response of each spectral channel at the target temperature is used as the temperature effect correction coefficient for each spectral channel at the current ambient temperature.

[0013] Furthermore, in the simulated on-orbit vacuum thermal environment, the effective center signal B of each spectral channel of the spaceborne multispectral imager at different brightness levels under the target temperature is measured. Based on the effective center signal B and the corresponding equivalent brightness B of each spectral channel, a nonlinear fitting function and a comparison table of nonlinear effect correction coefficients for each spectral channel under arbitrary effective center signals are determined, including: In the simulated on-orbit vacuum thermal environment, the ambient temperature of the spaceborne multispectral imager was stabilized at the target temperature, and different brightness levels of the light source were set according to the maximum and minimum apparent radiance values ​​on orbit. At the target temperature, acquire several background images as well as several light source images and spectral brightness at different brightness levels; Based on several background images and several light source images corresponding to different brightness levels, the center effective signal B of each spectral channel of the spaceborne multispectral imager is calculated under different brightness levels. The spectral brightness of different brightness levels and the relative spectral response of each spectral channel are integrated to obtain the equivalent brightness B of each spectral channel at different brightness levels. Based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel under different brightness levels, a comparison table of nonlinear fitting functions and nonlinear effect correction coefficients for each spectral channel under arbitrary center effective signals is determined.

[0014] Furthermore, the upper limit of the different brightness levels of the light source is greater than the maximum apparent radiance of the spaceborne multispectral imager in orbit and ensures that the spaceborne multispectral imager is not saturated, while the lower limit of the different brightness levels of the light source is less than the minimum apparent radiance of the spaceborne multispectral imager in orbit.

[0015] Furthermore, the calculation of the center effective signal B of each spectral channel of the spaceborne multispectral imager at different brightness levels, based on several background images and several light source images corresponding to different brightness levels, includes: Averaging is performed on several background images and several light source images corresponding to different brightness levels to obtain the background mean image and the light source mean image at different brightness levels. Subtract the corresponding background mean image from the light source mean image at different brightness levels to obtain the effective signal image at different brightness levels; The mean value of the central region of the effective signal image at different brightness levels is taken to obtain the central effective signal B of each spectral channel at different brightness levels.

[0016] Furthermore, the table comparing the nonlinear fitting function and the nonlinear effect correction coefficients for each spectral channel under arbitrary center effective signals, based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel at different brightness levels, includes: Divide the center effective signal B of each spectral channel by its corresponding equivalent brightness B to obtain the single-point responsivity of each spectral channel at different temperature levels. Assign a brightness level to each spectral channel as the target brightness of that spectral channel, and divide the single-point responsivity of each spectral channel at the target temperature by the single-point responsivity of each spectral channel at the target brightness to obtain the normalized single-point responsivity of each spectral channel at different brightness levels. The center effective signal B of each spectral channel at different brightness levels is fitted with the normalized single-point responsivity of each spectral channel at different brightness levels to obtain the normalized responsivity function of each spectral channel under any center effective signal B, which is used as a nonlinear fitting function. The ratio between the normalized single-point responsivity of any central effective signal B on the nonlinear fitting function of each spectral channel and the normalized single-point responsivity of each spectral channel under the target brightness is used as the nonlinear effect correction coefficient of each spectral channel under any central effective signal, forming a nonlinear effect correction coefficient reference table.

[0017] Furthermore, the measured center effective signal is substituted into the nonlinear fitting function to obtain the nonlinear effect correction coefficients corresponding to the measured center effective signal, including: Substituting the measured center effective signal into the nonlinear fitting function, the normalized single-point responsivity of the measured center effective signal of each spectral channel is obtained. The ratio between the normalized single-point responsivity of the measured center effective signal of each spectral channel and the normalized single-point responsivity of each spectral channel under the target brightness is used as the nonlinear effect correction coefficient for each spectral channel.

[0018] Furthermore, the temperature effect correction coefficient and nonlinear effect correction coefficient based on each spectral channel are used to jointly correct the absolute responsivity under the target temperature and target brightness conditions obtained from laboratory calibration. The formula is expressed as follows:

[0019] In the formula, For the corrected first Absolute response of each spectral channel For the target temperature and target brightness conditions during laboratory calibration, the first The absolute response of each spectral channel For the first Each spectral channel is at the current ambient temperature. The corresponding temperature effect correction factor, For the first Nonlinear effect correction coefficients for each spectral channel.

[0020] Furthermore, this invention also proposes a joint calibration system for a spaceborne multispectral imager, the system comprising: The acquisition module is used to acquire the current ambient temperature of the spaceborne multispectral imager and the measured center effective signal of the observed image; The temperature effect correction coefficient determination module is used to look up the temperature effect correction coefficient lookup table based on the current ambient temperature or to substitute the current ambient temperature into the temperature effect fitting function to determine the temperature effect correction coefficient corresponding to each spectral channel at the current ambient temperature. The nonlinear effect correction coefficient determination module is used to look up the nonlinear effect correction coefficient lookup table based on the measured center effective signal or to substitute the measured center effective signal into the nonlinear fitting function to obtain the nonlinear effect correction coefficient corresponding to each spectral channel under the measured center effective signal. The joint calibration module is used to jointly correct the absolute responsivity under the target temperature and target brightness conditions obtained from laboratory calibration, based on the temperature effect correction coefficient and nonlinear effect correction coefficient of each spectral channel. Among them, the temperature effect correction coefficient comparison table, the nonlinear effect correction coefficient comparison table, the temperature effect fitting function, and the nonlinear fitting function were all obtained by radiometric calibration of the spaceborne multispectral imager in a simulated on-orbit vacuum thermal environment using a calibration device.

[0021] Furthermore, the calibration device includes a light source, a vacuum chamber, and a spectral radiance measuring instrument; An optical window is set on the side wall of the vacuum tank, and the center of the optical window is directly opposite the center of the light source. The spaceborne multispectral imager is arranged in a movable manner in the cavity of the vacuum tank so that the optical axis of the spectral channel to be measured is perpendicular to the optical window by moving the spaceborne multispectral imager. The spectral radiance meter is positioned outside the field of view of the optical window and pointed towards the center of the light source.

[0022] Furthermore, a device platform is provided at the bottom of the cavity of the vacuum tank, and a vacuum translation stage is provided on the device platform, with the translation direction of the vacuum translation stage being parallel to the optical window; The spaceborne multispectral imager is positioned on a vacuum translation stage to move along with the stage.

[0023] Furthermore, the system also includes: The first radiation calibration module is used to measure the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions and at a set brightness using a calibration device, and to determine the temperature effect fitting function and the temperature effect correction coefficient comparison table of each spectral channel at any temperature based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel. The second radiation calibration module is used to measure the center effective signal B of each spectral channel of the spaceborne multispectral imager at different brightness levels under the target temperature using a calibration device, and to determine the nonlinear fitting function and the nonlinear effect correction coefficient comparison table of each spectral channel under arbitrary center effective signal based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel. Among them, the equivalent brightness A is obtained by integrating the spectral brightness under different temperature conditions with the relative spectral response of each spectral channel; the equivalent brightness B is obtained by integrating the spectral brightness at each brightness level with the relative spectral response of each spectral channel.

[0024] Furthermore, the first radiation calibration module includes: The first calibration parameter acquisition unit is used to set the brightness of the light source and adjust the temperature of the vacuum tank to stabilize the ambient temperature of the spaceborne multispectral imager under different temperature conditions. By moving the spaceborne multispectral imager so that the optical axes of each spectral channel are perpendicular to the optical window in sequence, several images of the light source are acquired at the set brightness. The spectral radiance of the light source is acquired using a spectral radiance meter. Then, the optical window is blocked, and several background images of the spaceborne multispectral imager are acquired. The center effective signal A calculation unit is used to calculate the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions, based on several light source images and several background images. The equivalent brightness A calculation unit is used to integrate the spectral brightness of the light source with the relative spectral response of each spectral channel to obtain the equivalent brightness A of each spectral channel under different temperature conditions. The temperature effect correction coefficient calculation unit is used to determine the temperature effect fitting function and the temperature effect correction coefficient comparison table for each spectral channel at any temperature based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel.

[0025] Furthermore, the second radiation calibration module includes: The second calibration parameter acquisition unit is used to adjust the temperature of the vacuum tank to stabilize the ambient temperature of the spaceborne multispectral imager at the target temperature, and to set different brightness levels of the light source. By moving the spaceborne multispectral imager so that the optical axes of each spectral channel are perpendicular to the optical window in sequence, several images of the light source at different brightness levels are acquired. The spectral radiance meter is used to acquire the spectral brightness of the light source at different brightness levels. Then, the optical window is blocked, and several background images of the spaceborne multispectral imager are acquired. The center effective signal B calculation unit is used to calculate the center effective signal B of each spectral channel of the spaceborne multispectral imager under different brightness levels, based on several background images and several light source images corresponding to different brightness levels. The equivalent brightness B calculation unit is used to integrate the spectral brightness of the light source at different brightness levels with the relative spectral response of each spectral channel to obtain the equivalent brightness B of each spectral channel at different brightness levels. The nonlinear effect correction coefficient calculation unit is used to determine the nonlinear fitting function and the nonlinear effect correction coefficient comparison table for each spectral channel under any center effective signal based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel under different brightness levels.

[0026] Furthermore, the joint correction module is used to jointly correct the absolute responsivity under the target temperature and target brightness conditions obtained from laboratory calibration, based on the temperature effect correction coefficient and nonlinear effect correction coefficient of each spectral channel. The formula is expressed as follows:

[0027] In the formula, For the corrected first Absolute response of each spectral channel For the target temperature and target brightness conditions during laboratory calibration, the first The absolute response of each spectral channel For the first Each spectral channel is at the current ambient temperature. The corresponding temperature effect correction factor, For the first Nonlinear effect correction coefficients for each spectral channel.

[0028] The advantages of this invention are: (1) This invention obtains the temperature effect correction coefficient and nonlinear effect correction coefficient by calibrating the complete product state of the spaceborne multispectral imager in a simulated on-orbit vacuum thermal environment in advance. Based on the current ambient temperature of the detector and the measured center effective signal of the observed image during actual use of the spaceborne multispectral imager, the corresponding temperature effect correction coefficient and nonlinear effect correction coefficient are obtained. The absolute responsivity of each spectral channel of the spaceborne multispectral imager calibrated in the laboratory is jointly corrected under the target temperature and target brightness conditions, eliminating the problem of imperfect correction caused by the traditional calibration method that only corrects one of the nonlinear effect or temperature effect.

[0029] (2) This invention calibrates the entire product of the spaceborne multispectral imager in a vacuum thermal simulation environment, eliminating the defects of traditional calibration methods that cannot perform nonlinear and temperature effect calibration under the actual application conditions of the product (including the integrity conditions of the product itself and the thermal environment conditions), which leads to errors in the calibration coefficients. This effectively improves the effectiveness and calibration effect of the calibration coefficients, thereby significantly improving the data quantification level of the spaceborne multispectral imager.

[0030] (3) The vacuum thermal environment test of the spaceborne multispectral imager in this invention can be carried out simultaneously with the absolute and relative radiation calibration, which effectively reduces the cost of manpower and material resources.

[0031] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0032] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0033] Figure 1 This is a flowchart illustrating a joint calibration method for a spaceborne multispectral imager according to an embodiment of the present invention. Figure 2 This is a schematic diagram illustrating the principle of obtaining nonlinear and temperature effect correction coefficients and joint correction by a spaceborne multispectral imager in one embodiment of the present invention. Figure 3 This is a schematic diagram illustrating the improvement in absolute response consistency of data from a spaceborne multispectral imager after joint correction for nonlinearity and temperature effects in one embodiment of the present invention. Figure 4 This is a schematic diagram illustrating the improvement of nonlinearity in different spectral channels after nonlinearity and temperature effect joint correction of data from a spaceborne multispectral imager in one embodiment of the present invention. Figure 5 This is a schematic diagram of the structure of a joint correction system for a spaceborne multispectral imager proposed in one embodiment of the present invention; Figure 6 This is a schematic diagram of the calibration device in one embodiment of the present invention; In the picture: 1-Large-aperture Lambertian light source, 2-Vacuum tank, 3-Spectral radiance measuring instrument, 4-Spaceborne multispectral imager, 21-Optical window, 22-Equipment platform, 23-Vacuum translation stage. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] like Figure 1 As shown, the first embodiment of the present invention proposes a joint calibration method for a spaceborne multispectral imager, the method comprising the following steps: S10. Obtain the current ambient temperature of the spaceborne multispectral imager and the measured center effective signal of the observed image; S20. Based on the current ambient temperature, find the temperature effect correction coefficient reference table or substitute the current ambient temperature into the temperature effect fitting function to determine the temperature effect correction coefficient corresponding to each spectral channel at the current ambient temperature. S30. Based on the measured center effective signal, find the nonlinear effect correction coefficient reference table or substitute the measured center effective signal into the nonlinear fitting function to obtain the nonlinear effect correction coefficients corresponding to each spectral channel under the measured center effective signal. S40. Based on the temperature effect correction coefficient and nonlinear effect correction coefficient of each spectral channel, the absolute responsivity under the target temperature and target brightness conditions obtained by laboratory calibration is jointly corrected. Among them, the temperature effect correction coefficient comparison table, the nonlinear effect correction coefficient comparison table, the temperature effect fitting function, and the nonlinear fitting function were all obtained by radiometric calibration of the spaceborne multispectral imager in a simulated on-orbit vacuum thermal environment.

[0036] This embodiment calibrates the entire product state of the spaceborne multispectral imager in a simulated on-orbit vacuum thermal environment by obtaining temperature effect correction coefficients and nonlinear effect correction coefficients in advance. This effectively improves the validity and correction effect of the calibration results, thereby greatly enhancing the quantification level of the spaceborne multispectral imager data. Furthermore, based on the current ambient temperature of the spaceborne multispectral imager and the measured effective signal of the observed image center during actual use, the corresponding temperature effect correction coefficients and nonlinear effect correction coefficients are obtained. This allows for joint correction of the absolute responsivity of each spectral channel of the spaceborne multispectral imager under target temperature and target brightness conditions, eliminating the problem of incomplete correction caused by traditional calibration methods that only correct one of the nonlinearity or temperature effects.

[0037] Furthermore, this embodiment considers the varying computational resource conditions in practical applications. When computational resources are limited, a lookup table is used to obtain the temperature effect correction coefficient corresponding to a temperature close to the current ambient temperature and the nonlinear effect correction coefficient corresponding to a center effective signal close to the measured center effective signal, thereby achieving joint correction of the measured center effective signal. When computational resources are sufficient, to ensure the accuracy of the joint correction, the corresponding temperature effect correction coefficient can be calculated in real time based on the current ambient temperature, and the corresponding nonlinear effect correction coefficient can be calculated in real time based on the measured center effective signal, thereby achieving joint correction of the measured center effective signal.

[0038] As a further preferred technical solution, the process for pre-determining the temperature effect correction coefficient lookup table and the temperature effect fitting function is as follows: S1. Under simulated on-orbit vacuum thermal environment, the center effective signal A of each spectral channel of the spaceborne multispectral imager is measured under different temperature conditions at the set brightness of the light source. Based on the center effective signal A of each spectral channel and the corresponding equivalent brightness A, the temperature effect fitting function and the temperature effect correction coefficient comparison table of each spectral channel at any temperature are determined. The equivalent brightness A is obtained by integrating the spectral brightness and the relative spectral response of each spectral channel under different temperature conditions.

[0039] A more detailed process for determining the temperature effect correction coefficient table and the temperature effect fitting function includes the following steps: S11. In the simulated on-orbit vacuum thermal environment, the ambient temperature of the spaceborne multispectral imager is adjusted and stabilized under different temperature conditions. Several background images of the spaceborne multispectral imager are acquired under different temperature conditions, as well as several images of the light source and the spectral brightness of the light source at a set brightness. The set brightness of the light source is greater than the typical value of the apparent radiance of the spaceborne multispectral imager on orbit. S12. Based on several light source images and several background images, calculate the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions. S13. Integrate the spectral brightness with the relative spectral response of each spectral channel to obtain the equivalent brightness A of each spectral channel under different temperature conditions. Specifically, to acquire the spectral brightness With the After integrating the relative spectral responses of the spectral channels, the result of the spaceborne multispectral imager at the ... Temperature conditions Next Equivalent brightness of each spectral channel For example, the formula is expressed as:

[0040] in, Spectral channels The relative spectral response, ~ Spectral channels Spectral range.

[0041] It should be noted that the relative spectral response is an inherent, known parameter of the product, determined by the filter transmittance and detector quantum efficiency curves of each channel of the multispectral imager; spectral brightness... The spectral brightness of the light source can be obtained by using a spectral radiance meter.

[0042] S14. Determine the temperature effect fitting function and the temperature effect correction coefficient comparison table for each spectral channel at any temperature based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel.

[0043] As a further preferred technical solution, step S12: Based on several light source images and several background images, calculate the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions, specifically including the following steps: S121. Averaging several light source images and several background images respectively, to obtain the average light source image and the average background image under different temperature conditions; S122. Subtract the corresponding background mean image from the light source mean image at different operating temperatures to obtain the effective signal image under different temperature conditions. Specifically, in this embodiment, the effective signal image is obtained by subtracting the collected background image from the average image of the light source.

[0044] S123. Take the average value of the central region of the effective signal image under different temperature conditions to obtain the central effective signal A of each spectral channel under different temperature conditions.

[0045] Specifically, this embodiment calculates the effective signal images corresponding to each spectral channel under each temperature condition, and then averages the central regions of the effective signal images corresponding to each spectral channel to obtain the central effective signal set A of the spaceborne multispectral imager. ,in, This indicates that the spaceborne multispectral imager is at the... Temperature conditions Next The center effective signal A of each spectral channel.

[0046] As a further preferred technical solution, step S14: determining the temperature effect fitting function and the temperature effect correction coefficient comparison table for each spectral channel at any temperature based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel, specifically includes the following steps: S141. Divide the center effective signal A of each spectral channel by its corresponding equivalent brightness A to obtain the single-point responsivity of each spectral channel under different temperature conditions. For example, in this embodiment, the first The center effective signal value of each spectral channel and the corresponding equivalent brightness After division, the detector of the spaceborne multispectral imager is obtained at the th... Temperature conditions Next Single-point responsivity of each spectral channel .

[0047] S142. Specify a temperature condition as the target temperature, divide the single-point responsivity of each spectral channel under all temperature conditions by the single-point responsivity of each spectral channel under the target temperature, and obtain the normalized single-point responsivity of each spectral channel under different temperature conditions. For example, specifying the first Temperature conditions As the target temperature, and for all temperature conditions, the [missing information] is [missing information]. The single-point responsivity of the spectral channel divided by the _th_ spectral channel Temperature conditions Next The single-point responsivity of the spectral channel is obtained, thus yielding the ... The normalized single-point responsivity of each spectral channel was obtained, and the normalized single-point responsivity of each spectral channel under different temperature conditions was obtained in the same way.

[0048] It should be noted that in this embodiment, the target temperature can be arbitrarily selected within the ambient temperature range of the spaceborne multispectral imager, and then the response temperature effect correction is performed relative to this target temperature condition.

[0049] Preferably, in this embodiment, the typical on-orbit temperature is selected as the target temperature.

[0050] S143. The normalized single-point responsivity of each spectral channel under different temperature conditions is fitted to the temperature conditions of the spaceborne multispectral imager to obtain the normalized responsivity function of each spectral channel under any temperature condition as the temperature effect fitting function. For example, under all temperature conditions, the first Normalized single-point responsivity of each spectral channel and that of a spaceborne multispectral imager Fitting the temperature condition, we obtain the... Taking the normalized responsivity function of each spectral channel under arbitrary temperature conditions as an example, with temperature as the x-axis and the normalized single-point responsivity at each temperature as the y-axis, a polynomial fitting can be performed to obtain the normalized responsivity function as follows: ..,in, , Equations are the fitting coefficients. This is the temperature value. This is the normalized single-point response.

[0051] It should be understood that the normalized response function obtained by fitting will be different for different types of multispectral imagers. For example, the fitted curve may be segmented. If a certain segment of the curve oscillates significantly, a higher-order polynomial can be used, while a certain segment of the curve is relatively smooth, a lower-order polynomial can be used. Therefore, this embodiment does not specifically limit the form of the normalized response function obtained by fitting.

[0052] It should be noted that this embodiment focuses on the detector temperature of a spaceborne multispectral imager, which is a hardware parameter of the spaceborne multispectral imager itself to be calibrated. The detector responsivity varies at different temperatures, and this temperature effect is unrelated to the light source.

[0053] S144. The ratio between the normalized response of each spectral channel under any temperature condition and the normalized response of each spectral channel at the target temperature on the temperature effect fitting function of each spectral channel is used as the temperature effect correction coefficient of each spectral channel, forming a temperature effect correction coefficient reference table.

[0054] For example, applying the normalized response function to any temperature condition Next The normalized response of the spectral channel and the Temperature conditions Next The ratio of the normalized responsivity of the spectral channels is used as the ... Temperature effect correction coefficient for each spectral channel By analogy, the temperature effect correction coefficients for each spectral channel at any temperature can be obtained, thus constructing a temperature effect correction coefficient reference table.

[0055] It should be noted that the method for obtaining the temperature effect correction coefficient of the spaceborne multispectral imager can be carried out simultaneously with the vacuum thermal test of the spaceborne multispectral imager, effectively reducing the cost of manpower and materials.

[0056] As a further preferred technical solution, the process of constructing the nonlinear fitting function or nonlinear effect correction coefficient lookup table is as follows: S2. Under the simulated on-orbit vacuum thermal environment, the center effective signal B of each spectral channel of the spaceborne multispectral imager at different brightness levels is measured at the target temperature. Based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel, a nonlinear fitting function and a table of nonlinear effect correction coefficients for each spectral channel under arbitrary center effective signal are determined. The equivalent brightness B is obtained by integrating the spectral brightness and the relative spectral response of each spectral channel at each brightness level.

[0057] In detail, the process of determining the nonlinear fitting function and the nonlinear effect correction coefficient lookup table includes: S21. In the simulated on-orbit vacuum thermal environment, adjust the ambient temperature of the spaceborne multispectral imager to stabilize at the target temperature, and set different brightness levels of the light source according to the maximum and minimum values ​​of the apparent radiance on orbit. It should be noted that in this embodiment, the upper limit of the light source's brightness is set to be greater than the maximum apparent radiance of the spaceborne multispectral imager in orbit; the lower limit of the light source's brightness is set to be less than the minimum apparent radiance of the spaceborne multispectral imager in orbit, and the brightness of each spectral channel is set between the lower and upper limits of the light source's brightness. Level brightness ,in, The first light source The spectral channel of the _th _ Level of brightness.

[0058] S22. At the target temperature, acquire several background images and several light source images and spectral brightness at different brightness levels; It should be noted that in this embodiment, the different spectral channels of the spaceborne multispectral imager can be aligned with the light source by moving the spaceborne multispectral imager to acquire several images of the light source at different brightness levels under the set temperature conditions, as well as the spectral brightness of the light source at different brightness levels. After blocking the light source, several background images can be acquired using the spaceborne multispectral imager.

[0059] S23. Based on several background images and several light source images corresponding to different brightness levels, calculate the center effective signal B of each spectral channel of the spaceborne multispectral imager under different brightness levels. S24. Integrate the spectral brightness of different brightness levels with the relative spectral response of each spectral channel to obtain the equivalent brightness B of each spectral channel at different brightness levels. S25. Based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel under different brightness levels, determine the nonlinear fitting function and the nonlinear effect correction coefficient comparison table for each spectral channel under arbitrary center effective signal.

[0060] As a further preferred technical solution, step S23: Based on several background images and several light source images corresponding to different brightness levels, calculate the center effective signal B of each spectral channel of the spaceborne multispectral imager at different brightness levels, specifically including the following steps: S231. Averaging is performed on several background images and several light source images corresponding to different brightness levels to obtain the background mean image and the light source mean image under different brightness levels. S232. Subtract the corresponding background mean image from the light source mean image at different brightness levels to obtain the effective signal image at different brightness levels. S233. Take the average value of the central region of the effective signal image under different brightness levels to obtain the central effective signal B of each spectral channel under different brightness levels.

[0061] Specifically, this embodiment calculates the effective signal images corresponding to each spectral channel at each brightness level, and then averages the central regions of the effective signal images corresponding to each spectral channel to obtain the central effective signal set B of the spaceborne multispectral imager. ,in, The first [item] of the spaceborne multispectral imager The spectral channel in the first... The center effective signal B at the brightness level.

[0062] As a further preferred technical solution, step S25: Based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel under different brightness levels, determine the nonlinear fitting function and the nonlinear effect correction coefficient comparison table for each spectral channel under arbitrary center effective signal, specifically including the following steps: S251. Divide the center effective signal B of each spectral channel by its corresponding equivalent brightness B to obtain the single-point responsivity of each spectral channel at different temperature levels. Specifically, in this embodiment, the collected data will be used to... The spectral brightness of the first level of brightness and the second level of brightness The spectral channel in the first... After integrating the relative spectral response of the brightness level, the result of the satellite-borne multispectral imager at the [missing value] level is obtained. The first level of brightness The equivalent brightness B of each spectral channel Then the first The spectral channel in the first... Center effective signal B at level brightness With the corresponding equivalent brightness B After division, the detector of the spaceborne multispectral imager is obtained at the th... The first level of brightness Single-point responsivity of each spectral channel By analogy, the single-point responsivity of each spectral channel at different brightness levels can be obtained.

[0063] S252. Assign a brightness level to each spectral channel as the target brightness of that spectral channel, and divide the single-point responsivity of each spectral channel at the target temperature by the single-point responsivity of each spectral channel at the target brightness to obtain the normalized single-point responsivity of each spectral channel at different brightness levels. For example, specifying the first Level 1 brightness is used as the target brightness, and the level 2 brightness is set as the target brightness. Single-point responsivity of each spectral channel and the first under all brightness levels The single-point responsivity of each spectral channel divided by Thus, the first value at all brightness levels is obtained. Normalized single-point responsivity of each spectral channel.

[0064] It should be noted that in this embodiment, the target brightness can be arbitrarily selected between the maximum and minimum apparent radiance values ​​of the on-orbit multispectral imager, and then a nonlinear response effect correction relative to this target brightness is performed.

[0065] S253. Fit the center effective signal B of each spectral channel under different brightness levels with the normalized single-point responsivity of each spectral channel under different brightness levels to obtain the normalized responsivity function of each spectral channel under any center effective signal B as a nonlinear fitting function. For example, under all brightness levels, the first The center effective signal value of the spectral channel and the luminance level at all brightness levels The normalized single-point responsivity of the nth spectral channel is fitted to obtain the nth spectral channel. The normalized response function of each spectral channel under an effective signal at an arbitrary center.

[0066] It should be noted that the specific order of the polynomial chosen during fitting depends on the state of the actual normalized response curve. If it is linear, a first-order polynomial is used; if it fluctuates significantly, a second-order, third-order, or Nth-order polynomial can be used, etc.

[0067] In this specific embodiment, the temperature effect is fitted using a first-order polynomial, i.e., linear fitting; while the nonlinear effect is fitted piecewise, with a ninth-order polynomial used for values ​​below 4400DN and a cubic polynomial used for values ​​above 4400DN.

[0068] It should be noted that in this embodiment, the normalized response function is obtained by directly fitting the nonlinear curve, which is a more intuitive and efficient fitting method.

[0069] S254. The ratio between the normalized single-point responsivity of each spectral channel under any central effective signal B on the nonlinear fitting function of each spectral channel and the normalized single-point responsivity of each spectral channel under the target brightness is used as the nonlinear effect correction coefficient of each spectral channel under any central effective signal, forming a nonlinear effect correction coefficient reference table.

[0070] Specifically, in this embodiment, any central effective signal on the normalized response function is used. Next The normalized response of the spectral channel and the The first level of brightness Single-point responsivity of each spectral channel The ratio as the first Each spectral channel has an effective signal at the center. Nonlinear effect correction coefficient By analogy, the nonlinear effect correction coefficients for each spectral channel under an effective signal at any center are obtained, thus forming a nonlinear effect correction coefficient reference table.

[0071] It should be noted that, as Figure 2 As shown, the acquisition processes for temperature effect correction coefficient and nonlinear effect correction coefficient are, in a general sense, based on the same principle: in the acquisition process of nonlinear effect correction coefficient, the temperature remains constant, and only the brightness of the product changes; in the acquisition process of temperature effect correction coefficient, only the product temperature changes, while the target brightness remains constant.

[0072] As a further preferred technical solution, step S30: the joint correction formula for the absolute responsivity under the target temperature and target brightness conditions obtained from laboratory calibration, based on the temperature effect correction coefficient and nonlinear effect correction coefficient of each spectral channel, is expressed as follows:

[0073] In the formula, For the corrected first Absolute response of each spectral channel The absolute responsivity under target temperature and target brightness conditions during laboratory calibration refers to the single-point responsivity of each spectral channel at the target temperature and target brightness. For the first Each spectral channel is at the current ambient temperature. The corresponding temperature effect correction factor, For the first Nonlinear effect correction coefficients for each spectral channel.

[0074] It should be noted that, when computational resources are sufficient, the measured center effective signal can be substituted into the nonlinear fitting function to obtain the normalized single-point responsivity of each spectral channel under the measured center effective signal. The ratio between the normalized single-point responsivity of each spectral channel under the measured center effective signal and the normalized single-point responsivity of each spectral channel under the target brightness is used as the nonlinear effect correction coefficient for each spectral channel under the measured center effective signal. Similarly, the current ambient temperature can be substituted into the temperature effect fitting function to obtain the normalized responsivity of each spectral channel under the current ambient temperature. The ratio between the normalized responsivity of each spectral channel under the current ambient temperature and the normalized responsivity of each spectral channel under the target temperature is used as the temperature effect correction coefficient for each spectral channel under the current ambient temperature.

[0075] It should be noted that in this embodiment, the center effective signal and radiance, etc., output by the multispectral imager are obtained in advance as objective measured results. Then, the corresponding change law between the responsivity and the measured results of the center effective signal and temperature is obtained by using the fitting polynomial. Finally, the correction coefficients of the responsivity (nonlinear effect correction coefficient and temperature effect correction coefficient) are obtained according to the actual usage conditions, and they jointly participate in the responsivity correction under the current state, thereby realizing the responsivity correction under different center effective signals and temperatures. Among them, the joint correction effect of nonlinearity and temperature effect is as follows: Figure 3 , Figure 4 As shown, the consistency of the response after joint correction is significantly improved. At the same time, the nonlinearity of the data in each spectral channel is significantly improved, decreasing from 1.5% to within 0.3%.

[0076] In addition, such as Figure 5As shown, the second embodiment of the present invention proposes a joint calibration system for a spaceborne multispectral imager, the system comprising: The acquisition module 10 is used to acquire the current ambient temperature of the spaceborne multispectral imager and the measured center effective signal of the observed image; The temperature effect correction coefficient determination module 20 is used to look up the temperature effect correction coefficient lookup table based on the current ambient temperature or substitute the current ambient temperature into the temperature effect fitting function to determine the temperature effect correction coefficient corresponding to each spectral channel at the current ambient temperature. The nonlinear effect correction coefficient determination module 30 is used to look up the nonlinear effect correction coefficient lookup table based on the measured center effective signal or to substitute the measured center effective signal into the nonlinear fitting function to obtain the nonlinear effect correction coefficient corresponding to each spectral channel under the measured center effective signal. The joint correction module 40 is used to jointly correct the absolute responsivity under the target temperature and target brightness conditions obtained by laboratory calibration based on the temperature effect correction coefficient and nonlinear effect correction coefficient of each spectral channel. Among them, the temperature effect correction coefficient comparison table, the nonlinear effect correction coefficient comparison table, the temperature effect fitting function, and the nonlinear fitting function were all obtained by radiometric calibration of the spaceborne multispectral imager in a simulated on-orbit vacuum thermal environment using a calibration device.

[0077] As a further preferred technical solution, such as Figure 6 As shown, the calibration device includes a large-aperture Lambertian light source 1, a vacuum chamber 2, and a spectral radiance measuring instrument 3; An optical window 21 is provided on the side wall of the vacuum tank 2, and the center of the optical window 21 is directly facing the center of the large-aperture Lambertian light source 1. The spaceborne multispectral imager 4 is arranged in a movable manner inside the cavity of the vacuum tank 2 so that the optical axis of the spectral channel to be measured is perpendicular to the optical window and directly facing the center of the optical window 21 by moving the spaceborne multispectral imager 4. The spectral radiance meter 3 is arranged outside the field of view of the optical window 21 and points to the center of the large-aperture Lambertian light source 1.

[0078] Specifically, the bottom of the cavity of the vacuum tank 2 is provided with an equipment platform 22, and a vacuum translation stage 23 is provided on the equipment platform 22. The translation direction of the vacuum translation stage 23 is parallel to the optical window 21. The spaceborne multispectral imager 4 is arranged on the vacuum translation stage 23 to follow the vacuum translation stage 23 to move.

[0079] Among them, the optical window 21 of the vacuum tank 2 has high transmittance over a wide spectral range, such as full-spectrum fused silica. Its transmittance is obtained by collecting the spectral brightness of the large-aperture Lambertian light source 1 inside and outside the vacuum tank using a spectral radiance measuring instrument, and then excluding the spectral brightness outside the vacuum tank from the spectral brightness inside the vacuum tank.

[0080] Furthermore, the large-aperture Lambertian light source 1 can have optical glass installed at the opening and the light source cavity can be filled with nitrogen to expel air from the cavity and avoid energy interference in the oxygen absorption band.

[0081] As a further preferred technical solution, the system also includes: The first radiation calibration module is used to measure the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions and at a set brightness using a calibration device, and to determine the temperature effect fitting function and the temperature effect correction coefficient comparison table of each spectral channel at any temperature based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel. The second radiation calibration module is used to measure the center effective signal B of each spectral channel of the spaceborne multispectral imager at different brightness levels under the target temperature using a calibration device, and to determine the nonlinear fitting function and the nonlinear effect correction coefficient comparison table of each spectral channel under arbitrary center effective signal based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel. Among them, the equivalent brightness A is obtained by integrating the spectral brightness under different temperature conditions with the relative spectral response of each spectral channel; the equivalent brightness B is obtained by integrating the spectral brightness at each brightness level with the relative spectral response of each spectral channel.

[0082] As a further preferred technical solution, the first radiation calibration module includes: The first calibration parameter acquisition unit is used to set the brightness of the light source and adjust the temperature of the vacuum tank to stabilize the ambient temperature of the spaceborne multispectral imager under different temperature conditions. By moving the spaceborne multispectral imager so that the optical axes of each spectral channel are perpendicular to the optical window in sequence, several images of the light source are acquired at the set brightness. The spectral radiance of the light source is acquired using a spectral radiance meter. Then, the optical window is blocked, and several background images of the spaceborne multispectral imager are acquired. The center effective signal A calculation unit is used to calculate the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions, based on several light source images and several background images. The equivalent brightness A calculation unit is used to integrate the spectral brightness with the relative spectral response of each spectral channel to obtain the equivalent brightness A of each spectral channel under different temperature conditions. The temperature effect correction coefficient calculation unit is used to determine the temperature effect fitting function and the temperature effect correction coefficient comparison table for each spectral channel at any temperature based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel.

[0083] As a further preferred technical solution, the central effective signal A calculation unit specifically includes: The first image acquisition subunit is used to average several light source images and several background images to obtain the average light source image and the average background image under different temperature conditions. The first effective signal image calculation subunit is used to subtract the corresponding background average image from the light source average image under different operating temperatures to obtain the effective signal image under different temperature conditions. The center effective signal A calculation subunit is used to average the central region of the effective signal image under different temperature conditions to obtain the center effective signal A of each spectral channel under different temperature conditions.

[0084] As a further preferred technical solution, the temperature effect correction coefficient calculation unit specifically includes: The first single-point responsivity calculation subunit is used to divide the center effective signal A of each spectral channel by its corresponding equivalent brightness A to obtain the single-point responsivity of each spectral channel under different temperature conditions. The first normalization subunit is used to specify a temperature condition as the target temperature, and divide the single-point responsivity of each spectral channel under all temperature conditions by the single-point responsivity of each spectral channel at the target temperature to obtain the normalized single-point responsivity of each spectral channel under different temperature conditions. The first fitting subunit is used to fit the normalized single-point responsivity of each spectral channel under different temperature conditions with the temperature conditions of the spaceborne multispectral imager, so as to obtain the normalized responsivity function of each spectral channel under any temperature condition as the temperature effect fitting function. The temperature effect correction coefficient calculation subunit is used to take the ratio between the normalized response of each spectral channel under any temperature condition on the temperature effect fitting function of each spectral channel and the normalized response of each spectral channel at the target temperature as the temperature effect correction coefficient of each spectral channel, and form a temperature effect correction coefficient reference table.

[0085] The detailed process for obtaining the temperature effect correction coefficient is as follows: Step 1a, Parameter settings: Based on the ambient temperature range of the detector in the spaceborne multispectral imager, N temperature conditions of the spaceborne multispectral imager are set, and n is initialized to 1. Set the vacuum state and temperature of vacuum tank 2 to stabilize the ambient temperature of the spaceborne multispectral imager at the nth temperature condition Tn. The brightness of the large-aperture Lambertian light source 1 is set to be greater than the typical on-orbit apparent radiance value of the spaceborne multispectral imager; Configure the parameters of the spectral radiance meter 3, including integration time, averaging times, calibration parameter file, etc. After setting the integration time of the spaceborne multispectral imager, all equipment was preheated to a stable state.

[0086] Step 2a, Information Collection: The spaceborne multispectral imager continuously acquires m images of the large-aperture Lambertian light source 1 under the nth temperature condition Tn; at the same time, the spectral radiance meter 3 acquires the spectral radiance of the large-aperture Lambertian light source 1. After blocking the optical window 21, several background images were continuously acquired from the spaceborne multispectral imager.

[0087] Step 3a, Calculation of temperature effect correction coefficient: Step 3a-1: Average the background images and light source images to obtain the mean background image and the mean light source image under the nth temperature condition Tn. Step 3a-2: Subtract the background mean image from the light source mean image to obtain the effective signal image of the spaceborne multispectral imager under the nth temperature condition Tn. Step 3a-3: After averaging the central regions of the effective signal images, the central effective signal set of the spaceborne multispectral imager is obtained. ,in, This indicates that the spaceborne multispectral imager is at the... Temperature conditions Next The center effective signal value of each spectral channel; Steps 3a-4: Collect the spectral brightness of 3 using the spectral radiance meter. With the After integrating the relative spectral responses of the spectral channels, the result of the spaceborne multispectral imager at the ... Temperature conditions Next Equivalent brightness of each spectral channel ; Step 3a-5, Center effective signal value of the k-th spectral channel With equivalent brightness After division, the detector of the spaceborne multispectral imager is obtained at the th... Temperature conditions Next Single-point responsivity of each spectral channel , Step 4a, Assign to Then, return to step 2a and execute sequentially until... So far, this yields the first value under all temperature conditions. Single-point responsivity of each spectral channel, Step 5a, specify the first Temperature conditions And under all temperature conditions, the first The single-point responsivity of the spectral channel divided by the _th_ spectral channel Temperature conditions Next The single-point responsivity of the spectral channel is obtained, thus yielding the ... Normalized single-point responsivity of the spectral channel; under all temperature conditions, the Normalized single-point responsivity of each spectral channel and that of a spaceborne multispectral imager Fitting the temperature condition, we obtain the... The normalized response function of each spectral channel under arbitrary temperature conditions is used as the temperature effect fitting function.

[0088] Step 6a: Apply the normalized response function to any temperature condition Next The normalized response of the spectral channel and the Temperature conditions Next The ratio of the normalized responsivity of the spectral channels is used as the ... Temperature effect correction coefficient for each spectral channel By analogy, the temperature effect correction coefficients for each spectral channel at different temperatures can be obtained.

[0089] As a further preferred technical solution, the second radiation calibration module specifically includes: The second calibration parameter acquisition unit is used to adjust the temperature of the vacuum tank to stabilize the ambient temperature of the spaceborne multispectral imager at the target temperature, and to set different brightness levels of the light source. By moving the spaceborne multispectral imager so that the optical axes of each spectral channel are perpendicular to the optical window in sequence, several images of the light source at different brightness levels are acquired. The spectral radiance meter is used to acquire the spectral brightness of the light source at different brightness levels. Then, the optical window is blocked, and several background images of the spaceborne multispectral imager are acquired. The center effective signal B calculation unit is used to calculate the center effective signal B of each spectral channel of the spaceborne multispectral imager under different brightness levels, based on several background images and several light source images corresponding to different brightness levels. The equivalent brightness B calculation unit is used to integrate the spectral brightness of different brightness levels with the relative spectral response of each spectral channel to obtain the equivalent brightness B of each spectral channel at different brightness levels. The nonlinear effect correction coefficient calculation unit is used to determine the nonlinear fitting function and the nonlinear effect correction coefficient comparison table for each spectral channel under any center effective signal based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel under different brightness levels.

[0090] As a further preferred technical solution, the central effective signal B calculation unit specifically includes: The second image acquisition subunit is used to average several background images and several light source images corresponding to different brightness levels to obtain the background mean image and the light source mean image under different brightness levels. The second effective signal image calculation subunit is used to subtract the corresponding background mean image from the light source mean image at different brightness levels to obtain the effective signal image at different brightness levels. The center effective signal B calculation subunit is used to average the central region of the effective signal image at different brightness levels to obtain the center effective signal B of each spectral channel at different brightness levels.

[0091] As a further preferred technical solution, the nonlinear effect correction coefficient calculation unit specifically includes: The second single-point responsivity calculation subunit is used to divide the center effective signal B of each spectral channel by its corresponding equivalent brightness B to obtain the single-point responsivity of each spectral channel at different temperature levels. The second normalization subunit is used to assign a brightness level to each spectral channel as the target brightness of that spectral channel. The single-point responsivity of each spectral channel at the target temperature is divided by the single-point responsivity of each spectral channel at the target brightness to obtain the normalized single-point responsivity of each spectral channel at different brightness levels. The second fitting subunit is used to fit the center effective signal B of each spectral channel under different brightness levels with the normalized single-point responsivity of each spectral channel under different brightness levels, and obtain the normalized responsivity function of each spectral channel under any center effective signal B as a nonlinear fitting function. The nonlinear effect correction coefficient calculation subunit is used to take the ratio between the normalized single-point responsivity of any center effective signal B on the nonlinear fitting function of each spectral channel and the normalized single-point responsivity of each spectral channel under the target brightness as the nonlinear effect correction coefficient of each spectral channel under any center effective signal, and form a nonlinear effect correction coefficient reference table.

[0092] In detail, the process of obtaining the nonlinear effect correction coefficients includes: Step 1b, Parameter Settings: Set the vacuum state and temperature of vacuum tank 2 to stabilize the ambient temperature of the spaceborne multispectral imager at the target temperature; The upper limit of the brightness of the large-aperture Lambertian light source 1 is set to be greater than the maximum apparent radiance of the spaceborne multispectral imager in orbit; the lower limit of the brightness of the large-aperture Lambertian light source 1 is set to be less than the minimum apparent radiance of the spaceborne multispectral imager in orbit. Set the parameters of the spectral radiance meter 3; After setting the integration time of the spaceborne multispectral imager, all equipment was preheated to a stable state. A first value is set between the lower and upper limits of the brightness of the large-aperture Lambertian light source 1. Each spectral channel Level brightness ,in, The first large-aperture Lambertian light source represents the... The spectral channel of the _th _ Level brightness and initialize .

[0093] Step 2b, Information Collection: The spaceborne multispectral imager continuously acquired data from a large-aperture Lambertian light source 1 under set temperature conditions. Several light source images at various brightness levels; simultaneously, the spectral radiance meter 3 acquires images of the large-aperture Lambertian light source 1 at the [missing information - likely a specific brightness level]. Spectral brightness of the level brightness; Then, after blocking the optical window 21, several background images of the spaceborne multispectral imager were continuously acquired.

[0094] Step 3b: Calculation of nonlinear effect correction coefficients: Step 3b-1: Average the background images and the light source images to obtain the mean background image and the mean image. Image of the average light source at a given brightness level; Step 3b-2: After subtracting the background mean image from the light source mean image, the result of the spaceborne multispectral imager at the [missing value] is obtained. Effective signal image at level brightness; Step 3b-3: After averaging the central regions of the effective signal images, the central effective signal set of the spaceborne multispectral imager is obtained. ,in, The first [item] of the spaceborne multispectral imager The spectral channel in the first... The effective center signal value at a certain brightness level; Step 3b-4: Collect data from the spectral radiance meter. The spectral brightness of the first level of brightness and the second level of brightness After integrating the relative spectral responses of the spectral channels, the result of the spaceborne multispectral imager at the ... The first level of brightness Equivalent brightness of each spectral channel ; Step 3b-5, the The spectral channel in the first... Center effective signal value at level brightness With equivalent brightness After division, the detector of the spaceborne multispectral imager is obtained at the th... The first level of brightness Single-point responsivity of each spectral channel .

[0095] Step 4b, Assign to Then, return to step 2b and execute sequentially until... So far, this yields the th for all brightness levels. Single-point responsivity of each spectral channel.

[0096] Step 5b, specify the first The first level of brightness Single-point responsivity of each spectral channel and at all brightness levels The single-point responsivity of each spectral channel divided by Thus, the first value at all brightness levels is obtained. Normalized single-point responsivity of each spectral channel; the first spectral channel at all brightness levels. The center effective signal value of the spectral channel and the luminance level at all brightness levels The normalized single-point responsivity of the nth spectral channel is fitted to obtain the nth spectral channel. The normalized response function of each spectral channel under an effective signal at an arbitrary center is used as a nonlinear fitting function.

[0097] Step 6b: Find any one of the central valid signals on the normalized response function. Next The normalized response of the spectral channel and the The first level of brightness Single-point responsivity of each spectral channel The ratio as the first Each spectral channel has an effective signal at the center. Nonlinear effect correction coefficient By analogy, the nonlinear effect correction coefficients corresponding to the effective signal at any center of each spectral channel under different brightness levels can be obtained.

[0098] As a further preferred technical solution, the joint correction module 40 is used to perform joint correction on the absolute responsivity under the target temperature and target brightness conditions obtained from laboratory calibration, based on the temperature effect correction coefficient and nonlinear effect correction coefficient of each spectral channel. The formula is expressed as follows:

[0099] In the formula, For the corrected first Absolute response of each spectral channel For the target temperature and target brightness conditions during laboratory calibration, the first The absolute response of each spectral channel For the first Each spectral channel is at the current ambient temperature. The corresponding temperature effect correction factor, For the first Nonlinear effect correction coefficients for each spectral channel.

[0100] It should be noted that other embodiments or specific implementation methods of the joint correction system of the spaceborne multispectral imager described in this invention can refer to the above-described method embodiments, and will not be repeated here.

[0101] It should be noted that the computer-readable medium disclosed in this embodiment may be a computer-readable signal medium or a computer-readable storage medium, or any combination thereof. A computer-readable storage medium may be, for example,—but not limited to—an electrical, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any combination thereof. More specific examples of a computer-readable storage medium may include, but are not limited to: an electrical connection having one or more wires, a portable computer disk, a hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fiber, and portable compact disk read-only memory (CD-ROM). ROM, optical storage devices, magnetic storage devices, or any suitable combination thereof. In this disclosure, a computer-readable storage medium can be any tangible medium containing or storing a program that can be used by or in connection with an instruction execution system, apparatus, or device. In this disclosure, a computer-readable signal medium can include a data signal propagated in baseband or as part of a carrier wave, carrying computer-readable program code. Such propagated data signals can take various forms, including but not limited to electromagnetic signals, optical signals, or any suitable combination thereof. A computer-readable signal medium can also be any computer-readable medium other than a computer-readable storage medium, which can send, propagate, or transmit a program for use by or in connection with an instruction execution system, apparatus, or device. The program code contained on the computer-readable medium can be transmitted using any suitable medium, including but not limited to: wires, optical fibers, RF (radio frequency), etc., or any suitable combination thereof.

[0102] The aforementioned computer-readable medium may be included in the aforementioned electronic device; or it may exist independently and not assembled into the electronic device. The aforementioned computer-readable medium carries one or more programs, which, when executed by the electronic device, cause the electronic device to perform the methods of the above embodiments.

[0103] Computer program code for performing the operations of this disclosure can be written in one or more programming languages ​​or a combination thereof, including object-oriented programming languages ​​such as Java, Smalltalk, and C++, and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The program code can be executed entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server.

[0104] In cases involving remote computers, the remote computer can be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN), or it can be connected to an external computer (e.g., via the Internet using an Internet service provider).

[0105] It should be understood that various parts of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented using any one or a combination of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0106] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0107] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" or "several" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0108] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A joint calibration method for a spaceborne multispectral imager, characterized in that, include: Acquire the current ambient temperature of the spaceborne multispectral imager and the measured center effective signal of the observed image; Based on the current ambient temperature, find the temperature effect correction coefficient reference table or substitute the current ambient temperature into the temperature effect fitting function to determine the temperature effect correction coefficient corresponding to each spectral channel at the current ambient temperature. Based on the measured center effective signal, find the nonlinear effect correction coefficient reference table or substitute the measured center effective signal into the nonlinear fitting function to obtain the nonlinear effect correction coefficients corresponding to each spectral channel under the measured center effective signal. Based on the temperature effect correction coefficient and nonlinear effect correction coefficient of each spectral channel, the absolute responsivity under the target temperature and target brightness conditions obtained by laboratory calibration is jointly corrected. Among them, the temperature effect correction coefficient comparison table, the nonlinear effect correction coefficient comparison table, the temperature effect fitting function, and the nonlinear fitting function were all obtained by radiometric calibration of the spaceborne multispectral imager in a simulated on-orbit vacuum thermal environment.

2. The joint calibration method for a spaceborne multispectral imager as described in claim 1, characterized in that, Before jointly correcting the absolute responsivity under the target temperature and target brightness conditions obtained from laboratory calibration based on the temperature effect correction coefficients and nonlinear effect correction coefficients of each spectral channel, the method further includes: In a simulated on-orbit vacuum thermal environment, the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions was measured at a set brightness. Based on the center effective signal A of each spectral channel and the corresponding equivalent brightness A, the temperature effect fitting function and the temperature effect correction coefficient comparison table of each spectral channel at any temperature were determined. The equivalent brightness A was obtained by integrating the spectral brightness and the relative spectral response of each spectral channel under different temperature conditions. In a simulated on-orbit vacuum thermal environment, the center effective signal B of each spectral channel of the spaceborne multispectral imager at different brightness levels is measured at the target temperature. Based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel, a nonlinear fitting function and a table of nonlinear effect correction coefficients for each spectral channel under arbitrary center effective signal are determined. The equivalent brightness B is obtained by integrating the spectral brightness and the relative spectral response of each spectral channel at each brightness level.

3. The joint calibration method for a spaceborne multispectral imager as described in claim 2, characterized in that, The method involves measuring the center effective signal A of each spectral channel of a spaceborne multispectral imager at a set brightness under different temperature conditions in a simulated on-orbit vacuum thermal environment. Based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel, a temperature effect fitting function and a table of temperature effect correction coefficients for each spectral channel at any temperature are determined, including: In a simulated on-orbit vacuum thermal environment, the ambient temperature of the spaceborne multispectral imager was adjusted and stabilized under different temperature conditions. Several background images and several light source images and spectral brightness of the light source were acquired under different temperature conditions. Based on several light source images and several background images, the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions is calculated; The spectral brightness and the relative spectral response of each spectral channel are integrated separately to obtain the equivalent brightness A of each spectral channel under different temperature conditions. Based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel, a temperature effect fitting function is determined, and a table of temperature effect correction coefficients for each spectral channel at any temperature is provided.

4. The joint calibration method for a spaceborne multispectral imager as described in claim 3, characterized in that, The calculation of the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions, based on several light source images and several background images, includes: Averaging is performed on several light source images and several background images to obtain the average light source image and the average background image under different temperature conditions. Subtract the corresponding background mean image from the mean image of the light source under different operating temperatures to obtain the effective signal image under different temperature conditions. The mean value of the central region of the effective signal image under different temperature conditions is taken to obtain the central effective signal A of each spectral channel under different temperature conditions.

5. The joint calibration method for a spaceborne multispectral imager as described in claim 3, characterized in that, The table showing the temperature effect fitting function determined based on the center effective signal A and corresponding equivalent brightness A of each spectral channel, and the temperature effect correction coefficients for each spectral channel at any temperature, includes: Divide the center effective signal A of each spectral channel by its corresponding equivalent brightness A to obtain the single-point responsivity of each spectral channel under different temperature conditions. Specify a temperature condition as the target temperature, divide the single-point responsivity of each spectral channel under all temperature conditions by the single-point responsivity of each spectral channel at the target temperature, and obtain the normalized single-point responsivity of each spectral channel under different temperature conditions. The normalized single-point responsivity of each spectral channel under different temperature conditions is fitted to the temperature conditions of the spaceborne multispectral imager to obtain the normalized responsivity function of each spectral channel under any temperature condition, which is used as the temperature effect fitting function. The ratio of the normalized response of each spectral channel under any temperature condition to the normalized response of each spectral channel at the target temperature on the temperature effect fitting function of each spectral channel is used as the temperature effect correction coefficient of each spectral channel, forming a temperature effect correction coefficient reference table.

6. The joint calibration method for a spaceborne multispectral imager as described in claim 5, characterized in that, The step of substituting the current ambient temperature into the temperature effect fitting function to determine the temperature effect correction coefficients corresponding to each spectral channel at the current ambient temperature includes: Substituting the current ambient temperature into the temperature effect fitting function, we obtain the normalized response of each spectral channel at the current ambient temperature. The ratio between the normalized response of each spectral channel at the current ambient temperature and the normalized response of each spectral channel at the target temperature is used as the temperature effect correction coefficient for each spectral channel at the current ambient temperature.

7. The joint calibration method for a spaceborne multispectral imager as described in claim 2, characterized in that, The method involves measuring the center effective signal B of each spectral channel of a spaceborne multispectral imager at different brightness levels under a simulated on-orbit vacuum thermal environment at a target temperature. Based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel, a nonlinear fitting function and a comparison table of nonlinear effect correction coefficients for each spectral channel under arbitrary center effective signals are determined, including: In the simulated on-orbit vacuum thermal environment, the ambient temperature of the spaceborne multispectral imager was stabilized at the target temperature, and different brightness levels of the light source were set according to the maximum and minimum apparent radiance values ​​on orbit. At the target temperature, acquire several background images as well as several light source images and spectral brightness at different brightness levels; Based on several background images and several light source images corresponding to different brightness levels, the center effective signal B of each spectral channel of the spaceborne multispectral imager is calculated under different brightness levels. The spectral brightness of different brightness levels and the relative spectral response of each spectral channel are integrated to obtain the equivalent brightness B of each spectral channel at different brightness levels. Based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel under different brightness levels, a comparison table of nonlinear fitting functions and nonlinear effect correction coefficients for each spectral channel under arbitrary center effective signals is determined.

8. The joint calibration method for a spaceborne multispectral imager as described in claim 7, characterized in that, The upper limit of the different brightness levels of the light source is greater than the maximum apparent radiance of the on-orbit multispectral imager, and the lower limit of the different brightness levels of the light source is less than the minimum apparent radiance of the on-orbit multispectral imager.

9. The joint calibration method for a spaceborne multispectral imager as described in claim 7, characterized in that, The calculation of the center effective signal B of each spectral channel of the spaceborne multispectral imager at different brightness levels, based on several background images and several light source images corresponding to different brightness levels, includes: Averaging is performed on several background images and several light source images corresponding to different brightness levels to obtain the background mean image and the light source mean image at different brightness levels. Subtract the corresponding background mean image from the light source mean image at different brightness levels to obtain the effective signal image at different brightness levels; The mean value of the central region of the effective signal image at different brightness levels is taken to obtain the central effective signal B of each spectral channel at different brightness levels.

10. The joint calibration method for a spaceborne multispectral imager as described in claim 7, characterized in that, The table comparing the nonlinear fitting function and the nonlinear effect correction coefficients for each spectral channel under arbitrary center effective signals, based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel at different brightness levels, includes: Divide the center effective signal B of each spectral channel by its corresponding equivalent brightness B to obtain the single-point responsivity of each spectral channel at different temperature levels. Assign a brightness level to each spectral channel as the target brightness of that spectral channel, and divide the single-point responsivity of each spectral channel at the target temperature by the single-point responsivity of each spectral channel at the target brightness to obtain the normalized single-point responsivity of each spectral channel at different brightness levels. The center effective signal B of each spectral channel at different brightness levels is fitted with the normalized single-point responsivity of each spectral channel at different brightness levels to obtain the normalized responsivity function of each spectral channel under any center effective signal B, which is used as a nonlinear fitting function. The ratio between the normalized single-point responsivity of any central effective signal B on the nonlinear fitting function of each spectral channel and the normalized single-point responsivity of each spectral channel under the target brightness is used as the nonlinear effect correction coefficient of each spectral channel under any central effective signal, forming a nonlinear effect correction coefficient reference table.

11. The joint calibration method for a spaceborne multispectral imager as described in claim 7, characterized in that, Substituting the measured center effective signal into the nonlinear fitting function, the nonlinear effect correction coefficients corresponding to each spectral channel under the measured center effective signal are obtained, including: Substituting the measured center effective signal into the nonlinear fitting function, the normalized single-point responsivity of the measured center effective signal of each spectral channel is obtained. The ratio between the normalized single-point responsivity of the measured center effective signal of each spectral channel and the normalized single-point responsivity of each spectral channel under the target brightness is used as the nonlinear effect correction coefficient for each spectral channel.

12. The joint calibration method for a spaceborne multispectral imager as described in claim 1, characterized in that, The temperature effect correction coefficient and nonlinear effect correction coefficient based on each spectral channel are used to jointly correct the absolute responsivity under the target temperature and target brightness conditions obtained from laboratory calibration. The formula is expressed as follows: In the formula, For the corrected first Absolute response of each spectral channel For the target temperature and target brightness conditions during laboratory calibration, the first The absolute response of each spectral channel For the first Each spectral channel is at the current ambient temperature. The corresponding temperature effect correction factor, For the first Nonlinear effect correction coefficients for each spectral channel.

13. A joint calibration system for a spaceborne multispectral imager, characterized in that, include: The acquisition module is used to acquire the current ambient temperature of the spaceborne multispectral imager and the measured center effective signal of the observed image; The temperature effect correction coefficient determination module is used to look up the temperature effect correction coefficient lookup table based on the current ambient temperature or to substitute the current ambient temperature into the temperature effect fitting function to determine the temperature effect correction coefficient corresponding to each spectral channel at the current ambient temperature. The nonlinear effect correction coefficient determination module is used to look up the nonlinear effect correction coefficient lookup table based on the measured center effective signal or to substitute the measured center effective signal into the nonlinear fitting function to obtain the nonlinear effect correction coefficient corresponding to each spectral channel under the measured center effective signal. The joint calibration module is used to jointly correct the absolute responsivity under the target temperature and target brightness conditions obtained from laboratory calibration, based on the temperature effect correction coefficient and nonlinear effect correction coefficient of each spectral channel. Among them, the temperature effect correction coefficient comparison table, the nonlinear effect correction coefficient comparison table, the temperature effect fitting function, and the nonlinear fitting function were all obtained by radiometric calibration of the spaceborne multispectral imager in a simulated on-orbit vacuum thermal environment using a calibration device.

14. The joint calibration system for a spaceborne multispectral imager as described in claim 13, characterized in that, The calibration device includes a light source, a vacuum chamber, and a spectral radiance measuring instrument; An optical window is set on the side wall of the vacuum tank, and the center of the optical window is directly opposite the center of the light source. The spaceborne multispectral imager is arranged in a movable manner in the cavity of the vacuum tank so that the optical axis of the spectral channel to be measured is perpendicular to the optical window by moving the spaceborne multispectral imager. The spectral radiance meter is positioned outside the field of view of the optical window and pointed towards the center of the light source.

15. The joint calibration system for a spaceborne multispectral imager as described in claim 14, characterized in that, The vacuum tank has an equipment platform at the bottom of its cavity, and a vacuum translation stage is set on the equipment platform. The translation direction of the vacuum translation stage is parallel to the optical window. The spaceborne multispectral imager is positioned on a vacuum translation stage to move along with the stage.

16. The joint calibration system for a spaceborne multispectral imager as described in claim 14, characterized in that, The system also includes: The first radiation calibration module is used to measure the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions and at a set brightness using a calibration device, and to determine the temperature effect fitting function and the temperature effect correction coefficient comparison table of each spectral channel at any temperature based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel. The second radiation calibration module is used to measure the center effective signal B of each spectral channel of the spaceborne multispectral imager at different brightness levels under the target temperature using a calibration device, and to determine the nonlinear fitting function and the nonlinear effect correction coefficient comparison table of each spectral channel under arbitrary center effective signal based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel. Among them, the equivalent brightness A is obtained by integrating the spectral brightness under different temperature conditions with the relative spectral response of each spectral channel; the equivalent brightness B is obtained by integrating the spectral brightness at each brightness level with the relative spectral response of each spectral channel.

17. The joint calibration system for a spaceborne multispectral imager as described in claim 16, characterized in that, The first radiation calibration module includes: The first calibration parameter acquisition unit is used to set the brightness of the light source and adjust the temperature of the vacuum tank to stabilize the ambient temperature of the spaceborne multispectral imager under different temperature conditions. By moving the spaceborne multispectral imager so that the optical axes of each spectral channel are perpendicular to the optical window in sequence, several images of the light source are acquired at the set brightness. The spectral radiance of the light source is acquired using a spectral radiance meter. Then, the optical window is blocked, and several background images of the spaceborne multispectral imager are acquired. The center effective signal A calculation unit is used to calculate the center effective signal A of each spectral channel of the spaceborne multispectral imager under different temperature conditions, based on several light source images and several background images. The equivalent brightness A calculation unit is used to integrate the spectral brightness of the light source with the relative spectral response of each spectral channel to obtain the equivalent brightness A of each spectral channel under different temperature conditions. The temperature effect correction coefficient calculation unit is used to determine the temperature effect fitting function and the temperature effect correction coefficient comparison table for each spectral channel at any temperature based on the center effective signal A and the corresponding equivalent brightness A of each spectral channel.

18. The joint calibration system for a spaceborne multispectral imager as described in claim 16, characterized in that, The second radiation calibration module includes: The second calibration parameter acquisition unit is used to adjust the temperature of the vacuum tank to stabilize the ambient temperature of the spaceborne multispectral imager at the target temperature, and to set different brightness levels of the light source. By moving the spaceborne multispectral imager so that the optical axes of each spectral channel are perpendicular to the optical window in sequence, several images of the light source at different brightness levels are acquired. The spectral radiance meter is used to acquire the spectral brightness of the light source at different brightness levels. Then, the optical window is blocked, and several background images of the spaceborne multispectral imager are acquired. The center effective signal B calculation unit is used to calculate the center effective signal B of each spectral channel of the spaceborne multispectral imager under different brightness levels, based on several background images and several light source images corresponding to different brightness levels. The equivalent brightness B calculation unit is used to integrate the spectral brightness of the light source at different brightness levels with the relative spectral response of each spectral channel to obtain the equivalent brightness B of each spectral channel at different brightness levels. The nonlinear effect correction coefficient calculation unit is used to determine the nonlinear fitting function and the nonlinear effect correction coefficient comparison table for each spectral channel under any center effective signal based on the center effective signal B and the corresponding equivalent brightness B of each spectral channel under different brightness levels.

19. The joint calibration system for a spaceborne multispectral imager as described in claim 13, characterized in that, The joint correction module is used to jointly correct the absolute responsivity under the target temperature and target brightness conditions obtained from laboratory calibration, based on the temperature effect correction coefficient and nonlinear effect correction coefficient of each spectral channel. The formula is expressed as follows: In the formula, For the corrected first Absolute response of each spectral channel For the kth digit under the target temperature and target brightness conditions during laboratory calibration The absolute response of each spectral channel For the first Each spectral channel is at the current ambient temperature. The corresponding temperature effect correction factor, For the first Nonlinear effect correction coefficients for each spectral channel.

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