Low-light-level channel cross calibration method adopting complete earth shadow area moonlight irradiation

Abstract

The embodiment of the invention provides a satellite load low-light channel cross calibration method adopting complete earth shadow area moonlight irradiation, which comprises the following steps: selecting stable target observation data of a to-be-calibrated satellite low-light instrument and a reference instrument in complete earth shadow area moonlight irradiation, the to-be-calibrated satellite low-light instrument and the reference instrument being imaging radiometers; carrying out low-quality data removal on observation data irradiated by moonlight in a complete earth shadow area of the satellite low-light instrument to be calibrated and the reference instrument; selecting a matching data pair of a satellite low-light instrument to be calibrated and a reference instrument under a cross time matching condition, which allows observation under a multi-moon phase condition, from the observation data from which the low-quality data is removed; calculating lunar radiation based on a lunar phase model, performing spectrum and angle correction on the data of the reference instrument in the matched data pair, and calculating the reference radiance of the to-be-calibrated instrument with the stable target in the complete earth shadow area; and fitting observation data of the satellite low-light instrument to be calibrated in the complete earth shadow area under the multi-moon phase condition in the matched data pair and the calculated reference radiance to obtain a cross calibration coefficient so as to complete the radiation calibration of the satellite low-light instrument. According to the embodiment of the invention, the influence of the solar stray light on calibration can be effectively reduced, and the in-orbit radiation calibration precision of the low-light remote sensor is remarkably improved.

Description

Technical Field

[0001] The invention relates to the field of satellite remote sensing technology and radiation calibration, and in particular to a satellite payload micro-light channel cross-calibration method based on full earth shadow area moonlight illumination observation data. Background Art

[0002] Low-light-level remote sensing can enhance the information collection capability of optical remote sensors under weak light conditions such as nighttime and dusk, and is used in monitoring human activities, tracking urbanization and socio-economic dynamics, assessing environmental pollution, and analyzing light pollution. Accurate radiation calibration is crucial for the quantitative application of low-light-level remote sensor earth observation data. After the satellite is launched, changes in its own characteristics and the surrounding environment will cause sensor aging and reduced sensitivity, which will seriously affect its radiation performance. Therefore, it is necessary to continuously and regularly perform on-orbit radiation calibration of low-light-level remote sensors.

[0003] At present, there are mainly the following methods for operational calibration of satellite-borne low-light imagers: transfer calibration based on pre-launch calibration test data, but this will introduce more uncertainties; field calibration using uniform and stable ice, snow or desert targets after launch, but this type of method requires accurate calculation of the radiation transmission process and is limited by model errors; calibration methods based on ground active light sources can avoid the dependence of stable targets on models, but are expensive and have limited calibration frequency. Cross calibration is a new method with high calibration frequency and low calibration cost, which can effectively reduce the error of the simulation process, but there is little research on this at present. Summary of the invention

[0004] The purpose of the present invention is to provide a cross-calibration method for a low-light channel based on full moonlight illumination in the earth's shadow area, using a stable target fully illuminated by moonlight in the earth's shadow area as a calibration site, and through cross-calibration with a high-precision low-light remote sensor, solve the problem of interference of the satellite low-light channel by solar stray light, thereby improving the on-orbit radiation calibration accuracy of the low-light remote sensor.

[0005] In order to achieve the above object, the present invention proposes a low light channel cross calibration method using full moonlight illumination in the shadow area, comprising the following steps:

[0006] S1, selecting stable target observation data of the satellite low-light detection instrument to be calibrated and the reference instrument illuminated by moonlight in a completely shadowed area, wherein the satellite low-light detection instrument to be calibrated and the reference instrument are imaging radiometers;

[0007] S2, removing low-quality data from the observation data of the stable target completely illuminated by moonlight in the shadow area of ​​the satellite low-light instrument to be calibrated and the reference instrument;

[0008] S3, selecting, from the observation data after removing the low-quality data, matching data pairs under the cross-time matching condition of the satellite low-light instrument to be calibrated and the reference instrument that are allowed to observe under multiple lunar phase conditions;

[0009] S4, calculating the lunar radiation based on the lunar phase model, and performing spectral and angular correction on the data of the reference instrument in the matching data pair, and calculating the reference radiance of the instrument to be calibrated of the completely stable target in the shadow area;

[0010] S5, fitting the observation data of the satellite low-light instrument to be calibrated in the complete earth shadow area under multiple lunar phase conditions in the matching data pair and the calculated reference radiance to obtain a cross-calibration coefficient, and complete the radiation calibration of the satellite low-light instrument.

[0011] In some optional implementations, the solar zenith angle of the observation data selected in step S1 is greater than a first specific value.

[0012] In some optional implementations, the observation data selected in step S1 also satisfies:

[0013] the lunar zenith angle is less than a second specified value; and / or

[0014] The moon phase angle is within a specific range.

[0015] In some optional implementations, step S2 includes:

[0016] Removing non-uniform pixels such as clouds, auroras, lightning, etc. from the observation data of the satellite low-light instrument to be calibrated with target uniformity; and / or

[0017] The data with a certain number of columns of pixels with low spatial resolution at both sides of the scanning edge in the observation data of the satellite low-light instrument to be calibrated are removed.

[0018] In some optional implementations, a threshold is set by the standard deviation and coefficient of variation of a window of size N×N, and stable pixels that meet a preset threshold condition are selected:

[0019]

[0020] Where N is a positive integer, σ and L ave They represent the standard deviation and average value of the radiance of the window pixels respectively, and T is a preset threshold; pixels that do not meet the threshold condition are removed.

[0021] In some optional implementations, step S4 includes:

[0022] Calculate the lunar spectral irradiance at different times and lunar phases according to the lunar irradiance model;

[0023] According to the spectral response functions of the to-be-calibrated satellite low-light instrument and the reference instrument, spectrally correcting the observation data of the reference instrument in the matching data pair;

[0024] The observation data of the reference instrument in the matching data pair is angle-corrected by using the lunar zenith angle of the satellite low-light instrument to be calibrated at the observation time, the lunar zenith angle of the reference instrument at the observation time, and the sun-moon-earth distance correction factor to obtain the reference radiance of the low-light instrument to be calibrated.

[0025] In some optional implementations, step S5 includes:

[0026] For the multi-month matching data pairs of the satellite low-light instrument to be calibrated and the reference instrument in the screened stable target area, the reference radiance values ​​in a certain pixel area relative to each month are averaged, and the averaged reference radiance of the low-light instrument to be calibrated and the actual observation data DN value are linearly fitted through a linear regression analysis method to obtain the radiation calibration coefficient.

[0027] Compared with the prior art, the embodiments of the present invention have the following beneficial effects:

[0028] (1) By selecting a stable target in the complete shadow area without stray interference as the calibration site, the influence of solar stray light on the calibration accuracy is effectively reduced.

[0029] (2) Using the low-light channel of a high-precision reference instrument as a reference benchmark and adopting the cross-calibration method, the on-orbit radiation calibration accuracy of the low-light channel of the instrument to be calibrated can be significantly improved.

[0030] (3) By combining multi-day observation data with the lunar irradiance model, the multi-lunar phase reference radiance of the instrument to be calibrated in the completely shadowed area is obtained, which expands the dynamic range of the radiance of the low-light channel and improves the applicability and robustness of the calibration method. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Other features, objects and advantages of the present application will become more apparent by reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0032] Figure 1 The figure is a flow chart of an embodiment of a low light channel cross calibration technology using full moonlight illumination in the shadow area.

[0033] Figure 2 A schematic diagram of the satellite imaging earth shadow area provided in an embodiment of the present invention.

[0034] Figure 3 This is an image of low-light instrument observation data.

[0035] Figure 4Cross calibration results for low-light instruments. DETAILED DESCRIPTION

[0036] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0037] my country has launched the world's first civil meteorological satellite in a sun-synchronous dawn-dusk orbit, Fengyun-3E (FY-3E). Its medium-resolution spectral imager (MERSI-LL) can obtain low-light images of the Earth scene in the visible spectrum at dawn, dusk and night, and can achieve visible light imaging and quantitative remote sensing applications with a large dynamic span near the dawn-dusk line. Due to its unique orbital characteristics and the high sensitivity of the low-light detector, the observation data of the low-light channel on the night side of the dawn-dusk line is contaminated by stray light, which affects the precise cross-calibration.

[0038] In order to enable the low-light detector to be calibrated on-orbit when it is contaminated by stray light, the present invention proposes a cross-calibration method based on moonlight illumination in the complete earth shadow area. By using the stable observation area of ​​the complete earth shadow area under polar night conditions and combining it with other high-precision low-light remote sensors to carry out cross-calibration technology, the interference of solar stray light can be effectively avoided and the calibration accuracy of the satellite payload low-light channel can be improved.

[0039] like Figure 1 As shown, the embodiment of the present application provides a method for cross-calibration of a low-light channel using full moonlight illumination in the shadow area, comprising the following steps:

[0040] S1, select the stable target observation data of the satellite low-light detection instrument to be calibrated and the reference instrument in the complete moonlight in the shadow area.

[0041] In an embodiment of the present invention, the satellite low-light instrument to be calibrated and the reference instrument are imaging radiometers, wherein the satellite low-light instrument to be calibrated can be, for example, selected as the MERSI-LL low-light channel carried by FY-3E. The reference instrument is also a satellite low-light detector, which has high-precision on-orbit radiation calibration, for example, the daytime / nighttime band channel DNB of the Visible Infrared Imaging Radiometer (VIIRS) carried by the National Oceanic and Atmospheric Administration (NOAA-20) satellite, which can be used as the calibration reference of the FY-3E low-light channel, and the spectral range (500-900nm) and spatial resolution of the VIIRS / DNB low-light channel are similar to those of the MERSI-LL low-light channel of FY-3E, so that the calibration result of the MERSI-LL of FY-3E is not affected by the difference in low-light channels. A completely stable target in the shadow area refers to a target with stable shadow in the night time period, which can be a regional target with a certain range on the surface, and the low-light instrument has uniform and stable radiation brightness within the range of the stable target in the night time period. For example, the Antarctic ice target Dome C (center coordinates 74.5°S, 123°E) can be selected as the target of the completely stable shadow area. The formation and imaging area of ​​the shadow area in this area are shown in the figure below. Figure 2 shown. Figure 2 Middle: (a) Schematic diagram of the shadow area formed based on the position relationship between the sun, the earth and the satellite, and (b) Schematic diagram of the shadow area in the imaging area of ​​the low-light instrument.

[0042] The low-light channel detects weak visible light at night. In some embodiments, the data at night can be selected by limiting the solar zenith angle to ensure that only the moon provides incident radiation and there is no stray light interference. Specifically, the observation data with a solar zenith angle greater than a first specific value can be selected as the observation data without stray light. Optionally, the night moonlight illumination data can be screened by the lunar zenith angle. In some embodiments, the observation data with a lunar zenith angle less than a second specific value can be selected.

[0043] At the same time, the nighttime low-light observation data should meet certain lunar incident radiation, and the incident radiation should be higher than the signal-to-noise ratio of the detector. Because the lunar radiation changes periodically with the lunar phase angle, the lunar phase angle can be used as a constraint parameter. In some embodiments, data with a lunar phase angle within a specific range can be selected.

[0044] According to the longitude and latitude of the stable target Dome C, the time when the two satellites pass through the Dome C site is obtained through orbit prediction, the L1 remote sensing data file of the corresponding time is selected, and the appropriate angle threshold is found through experiments.

[0045] Exemplarily, the screening conditions for the observation data of the twilight channel of the satellite twilight detection instrument to be calibrated and the reference instrument ("two instruments") in the complete shadow area of ​​the ice and snow target are: (1) the solar zenith angle is greater than 118.4° to ensure that there is no solar radiation and stray light, and the observation data is in polar night conditions; (2) the lunar zenith angle is less than 70° to ensure that the lunar radiation can fully illuminate the surface target; (3) the lunar phase angle is between 2° and 65° to ensure that there is sufficient moonlight radiation at night and lunar radiation data under different lunar phases. Through screening, the observation data files of the complete shadow area of ​​MERSILL and VIIRS / DNB passing through the Dome C site are obtained. The pixel closest to the longitude and latitude of Dome C in the file is taken as the central pixel, and the 20×20 observation pixels of the two remote sensors around the central pixel are extracted for subsequent processing.

[0046] S2, remove low-quality data from the observation data after screening stable targets illuminated by moonlight in the complete shadow area of ​​the two instruments.

[0047] In this embodiment, the quality of the observation data of the two instruments can be evaluated according to a pre-set quality evaluation rule, and low-quality data can be removed.

[0048] In some embodiments, the observation data of the satellite low-light instrument to be calibrated can be used to remove non-uniform pixels such as clouds, auroras, lightning, etc. with target uniformity.

[0049] Specifically, because the difference between the surface temperature and cloud temperature in the polar regions is small, the traditional infrared channel threshold method has poor recognition effect in the polar regions, and the uniformity detection method is used to remove non-uniform pixels such as clouds, auroras and lightning. Uniformity detection specifically refers to the detection of the uniformity of pixel radiance, for example, it can be characterized by the deviation of radiance from the average index.

[0050] In some optional implementations, the stable pixels that meet the threshold condition of the following formula (1) can be selected by using the standard deviation and coefficient of variation of the window of size N×N:

[0051]

[0052] Where N is a positive integer, σ and L ave They represent the standard deviation and mean value of the radiance of the window pixels respectively, and T is a preset threshold, for example, set to 0.3; pixels that do not meet the threshold condition of formula (1) are removed.

[0053] For example, within a 20×20 pixel range, the radiance standard deviation (σ) and mean (L ave ), set the threshold Select stable pixels with high uniformity that can be retained. Remove observation data that may be contaminated by stray light to ensure data quality.

[0054] In addition, the spatial resolution of the pixels at the scanning edge is reduced, and its quality is not as good as the pixels at the sub-satellite points. To ensure the calibration accuracy, a certain number of columns (for example, 200 columns) of pixels on both sides of the scanning edge of the observation data of the satellite low-light instrument to be calibrated are removed.

[0055] The image of the Dome-C area of ​​MERSI-LL observation data after removing stray light, clouds, edges and selected areas is shown below. Figure 3 As shown in (a)-(d). Figure 3 Middle: (a), (b), and (c) are low-quality observation data removed due to stray light pollution, cloud pollution, and scanning edge pixels, and (d) is the data left for calibration. The blue box selects the stable target area completely within the shadow area.

[0056] S3, select matching data pairs under the cross-time matching condition that allows observations by the two instruments under multiple lunar phase conditions from the observation data after removing the low-quality data.

[0057] The cross-time matching condition refers to the time difference range allowed for two satellite instruments to observe the same stable target. Due to the difference in the earth observation time between FY-3E and NOAA-20, the stable target has pseudo-invariance of surface and atmospheric characteristics. The atmospheric conditions of Dome C can be regarded as unchanged within a certain time difference threshold. Therefore, the time matching constraint of the cross-satellite can be appropriately relaxed. Combined with the characteristics of satellite orbits, the cross-observation data of the satellites of the two instruments in the Dome C area are selected with a time matching threshold of 30 minutes to obtain matching data pairs. Here, the matching data pair includes the observation data of the two instruments after the low-quality data is removed and the observation time difference of the stable target is within a certain range. The changes in lunar incident radiation caused by the time difference will be corrected by the lunar radiation model in the subsequent steps.

[0058] S4, calculate the lunar radiation based on the lunar phase model, perform spectral and angular corrections on the matching data of the reference instrument, and obtain the reference radiance of the instrument to be calibrated for the fully stabilized target in the shadow area.

[0059] The lunar spectral irradiance at multiple different times and lunar phases can be calculated based on the lunar irradiance model, and then the observation data of the reference instrument in the matching data pair can be spectrally corrected according to the spectral response functions of the two instruments. The observation data of the reference instrument in the matching data pair can be angle-corrected by the lunar zenith angle of the observation time of the satellite low-light instrument to be calibrated, the lunar zenith angle of the observation time of the reference instrument, and the sun-moon-earth distance correction factor to obtain the reference radiance of the low-light instrument to be calibrated.

[0060] Specifically, since the observed radiance units of MERSI-LL and VIIRS / DNB (hereinafter referred to as VIIRS) are different, it is necessary to convert the units to nw / (cm 2 ·sr). Radiance unit conversion formula:

[0061] For MERSI-LL: L' MERSI-LL =L MERSI-LL ×10 5

[0062] For VIIRS: L' VIIRS =L VIIRS ×10 9

[0063] In the embodiment of the present invention, the ROLO lunar radiation model is selected to calculate the lunar radiation intensity under different time and lunar phase conditions.

[0064] The apparent radiance L of a stable target on Earth observed by the low-light channel of a spaceborne imager with only lunar radiation incident on it l for:

[0065]

[0066] Among them, ρ e is the apparent reflectivity of the Earth observation target, θ m is the lunar zenith angle, d m is the Sun-Moon-Earth distance correction factor, E m is the TOA downlink lunar irradiance, which is calculated as:

[0067]

[0068] Where λ represents the wavelength, λ1 represents the starting wavelength of the spectral response, λ2 represents the ending wavelength of the spectral response, SRF(λ) represents the spectral response function of the load, and I m (λ) is the TOA downlink lunar spectral irradiance obtained by the ROLO lunar model at different times. The spectral reflectance of the Dome C target area is relatively flat in the 500-900nm band, with high reflectivity, so the variation of reflectivity with wavelength is not considered.

[0069] In order to fully correct the differences between the reference instrument and the instrument to be calibrated due to the differences in sensor spectral response, atmospheric conditions, and observation geometry, corresponding corrections need to be made. The embodiment of the present application performs cross-calibration based on a stable target and omits atmospheric correction. On the one hand, it can be considered that the observed atmospheric conditions are consistent and the changes are negligible under the time threshold, and on the other hand, it avoids the introduction of deviations due to the use of inaccurate atmospheric parameters.

[0070] The apparent reflectivity of the Dome C stable target has the following relationship with the observation data of the satellite payload:

[0071]

[0072] Among them, ρ DomeC is the apparent reflectivity of the Dome C stabilized target, θ m is the lunar zenith angle, d m is the Sun-Moon-Earth distance correction factor, E m is the TOA downlink lunar irradiance, L l is the apparent radiance of the stable target.

[0073] The surface target reflectivities corresponding to the two satellite payloads VIIRS and MERSI-LL are:

[0074]

[0075] in, is the reflectivity of the surface target observed by VIIRS, θ m-V is the lunar zenith angle at the time of VIIRS observation, d m-V is the Sun-Moon-Earth distance correction factor for VIIRS observation time, E m (λ V ,t V ) is based on the VIIRS observation time t V and the spectral response function for a specific wavelength λ V Calculated TOA downlink lunar irradiance, L VIIRS Observation radiance for VIIRS; is the reflectivity of the surface target observed by MERSI-LL, θ m-M is the lunar zenith angle at the time of MERSI-LL observation, d m-M is the Sun-Moon-Earth distance correction factor for the MERSI-LL observation time, E m (λ M ,t M ) is based on the MERSI-LL observation time t M and the spectral response function for a specific wavelength λ M Calculated TOA downlink lunar irradiance, L MERSI-LL MERSI-LL observed radiance.

[0076] The Dome C ice and snow target has a high reflectivity, stable and uniform reflectivity. It can be approximately regarded as a Lambertian reflector in the southern hemisphere from May to July and in the visible light detection band. According to the same reflectivity observed by VIIRS and MERSI-LL for the same stable target, the radiation reference transfer formula between cross-calibration can be obtained as follows:

[0077]

[0078] Among them, L' MERSI-LL is the MERSI-LL reference radiance calculated based on the VIIRS radiance.

[0079] S5, fit the observation data of the satellite low-light instrument to be calibrated in the complete earth shadow area under multiple lunar phase conditions in the matching data pair and the calculated reference radiance, obtain the cross-calibration coefficient, and complete the radiation calibration of the target satellite low-light instrument.

[0080] The MERSI-LL data is calibrated with VIIRS radiance as the reference. For the multi-month matching data pairs of two instruments in the selected stable target area, the reference radiance values ​​within the stable target range of a certain pixel area (e.g., 20×20 pixel area) of the matching data of each month are averaged. The averaged reference radiance of MERSI-LL and the actual observed data DN value are linearly fitted by linear regression analysis to obtain the radiation calibration coefficient. The linear fitting can be achieved by the regression equation of the following formula (7):

[0081] L' MERSI-LL =a×DN MERSI-LL +b (7)

[0082] Among them, DN MERSI-LL represents the original MERSI-LL DN observations; a and b represent the slope and intercept of the linear fit, respectively.

[0083] Using the above method of the embodiment of the present application, the cross-matching data pairs of the two instruments MERSI-LL and VIIRS / DNB that meet all constraints are obtained, totaling 6 lunar cycles. There is only 1 lunar cycle in 2021, the first year of launch, and 2023 and 2024 each year include 2 to 3 lunar cycles of the complete shadow area of ​​​​Antarctic Dome C; the data of each cycle are distributed 4 to 5 days before and after the full moon on the 15th day of the lunar calendar; the specific ranges corresponding to these 6 lunar cycles are July 19-29, 2021, May 14-21, June 10-19, July 8-18, 2022, May 29-June 9, 2023, and June 27-July 7.

[0084] The calibration is performed using multiple lunar phase data from each lunar cycle. The final calibration results for the 6 lunar cycles are as follows: Figure 4 shown. Figure 4 Middle: (a) to (f) represent different lunar cycles.

[0085] The Pearson correlation coefficient (R 2 ) are all above 0.98. Figure 4(a) Taking July 2021 as an example, the calibration formula is y=0.1119x-5.264, and the calibration coefficients are 0.1119 and -5.264 respectively. Compared with the official business calibration results, the method of the embodiment of the present application achieves a calibration effect that is more consistent with VIIRS, with an average relative deviation of less than 10%, and has a high calibration accuracy. This method can be used for absolute calibration and calibration result verification of the FY3E low-light channel affected by stray light, and can track the stability of the radiation response of subsequent long-term series for the instrument based on a stable target.

Claims

1. A cross-calibration method for low-light channels using full moonlight illumination in the shadow area, characterized in that: The steps include: Step 1, selecting stable target observation data of the satellite low-light instrument to be calibrated and the reference instrument illuminated by moonlight in a completely shadowed area, wherein the satellite low-light instrument to be calibrated and the reference instrument are imaging radiometers; Step 2, removing low-quality data from the observation data of the stable target completely illuminated by moonlight in the shadow area of ​​the satellite low-light instrument to be calibrated and the reference instrument; Step 3, selecting matching data pairs under the cross-time matching constraint condition of the satellite low-light instrument to be calibrated and the reference instrument allowed to observe under multiple lunar phase conditions from the observation data after removing low-quality data; Step 4, calculating the lunar radiation based on the lunar phase model, performing spectral and angular corrections on the data of the reference instrument in the matching data pair, and obtaining the reference radiance of the instrument to be calibrated that is a stable target in the complete shadow area; Step 5, fitting the observation data of the satellite low-light instrument to be calibrated in the complete earth shadow area under multiple lunar phase conditions in the matching data and the calculated reference radiance, obtaining the cross-calibration coefficient, and completing the radiation calibration of the satellite low-light instrument.

2. The method according to claim 1, characterized in that The solar zenith angle of the observation data selected in step 1 is greater than the first specific value.

3. The method according to claim 2, characterized in that The observation data selected in step 1 also satisfies: the lunar zenith angle is less than a second specified value; and / or The moon phase angle is within a specific range.

4. The method according to claim 1, characterized in that: The step 2 comprises: Removing non-uniform pixels such as clouds, auroras, lightning, etc. from the observation data of the satellite low-light instrument to be calibrated with target uniformity; and / or The data with a certain number of columns of pixels with low spatial resolution at both sides of the scanning edge in the observation data of the satellite low-light instrument to be calibrated are removed.

5. The method according to claim 4, characterized in that The non-uniform pixels are removed in the following manner: Through the standard deviation and coefficient of variation of the N×N window, stable pixels that meet the preset threshold conditions are selected. Where N is a positive integer, σ and L ave They represent the standard deviation and mean value of the radiance of the window pixels respectively, and T is the preset threshold; Pixels that do not meet the threshold condition are removed.

6. The method according to claim 1, characterized in that The step 4 comprises: Calculate the lunar spectral irradiance at different times and lunar phases according to the lunar irradiance model; According to the spectral response functions of the to-be-calibrated satellite low-light instrument and the reference instrument, spectrally correcting the observation data of the reference instrument in the matching data pair; The observation data of the reference instrument in the matching data pair is angle-corrected by using the lunar zenith angle of the satellite low-light instrument to be calibrated at the observation time, the lunar zenith angle of the reference instrument at the observation time, and the sun-moon-earth distance correction factor to obtain the reference radiance of the low-light instrument to be calibrated.

7. The method according to claim 1, characterized in that The step 5 comprises: For the multi-month matching data pairs of the satellite low-light instrument to be calibrated and the reference instrument in the screened stable target area, the reference radiance values ​​in a certain pixel area of ​​each lunar phase are averaged, and the averaged reference radiance of the low-light instrument to be calibrated and the actual observation data DN value are linearly fitted through a linear regression analysis method to obtain the radiation calibration coefficient.

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