A method and apparatus for determining the equivalent focal length of low-to-medium resolution optical payloads in orbit

Abstract

This invention discloses a method and apparatus for determining the equivalent focal length of a low-to-medium resolution optical payload in orbit. The method comprises: 1) For each selected target region, selecting an image containing the target region from images captured by a low-to-medium resolution optical payload on a satellite as an observation image, and selecting an image containing the target region from images captured by a high-resolution optical payload on a satellite as a reference image; 2) Calculating the geolocation data of the low-to-medium resolution optical payload for the target region at different focal lengths, generating a geolocation dataset; 3) For each geolocation dataset, generating a simulated image based on the point spread function of the low-to-medium resolution optical payload and the reference image; and pairing the reference image of the target region with the simulated image corresponding to each focal length as an image pair; 4) Calculating the correlation coefficient of the image pairs at different focal lengths; and 5) Fitting the obtained correlation coefficients to obtain the in-orbit focal length of the low-to-medium resolution optical payload.

Description

Technical Field

[0001] This invention relates to a method and apparatus for determining the equivalent focal length of a medium-to-low resolution optical payload in orbit, belonging to the field of medium-to-low resolution remote sensing technology. Background Technology

[0002] As one of the most important characteristic parameters of an optical system, inaccurate focal length measurement can lead to overlap or missed scans between scan lines, and also affect positioning accuracy and image quality. Therefore, it is necessary to accurately measure the focal length of the optical payload before satellite launch. Laboratory techniques for measuring the focal length of visible light cameras are relatively mature, generally employing object-to-image distance methods, autocollimation methods, and photoelectric methods to achieve a focal length measurement with a relative error of 0.138%. In the infrared band, a combination of interferometry, photoelectric autocollimation, and laser tracking can achieve a focal length measurement with a relative error of 0.2%. The pre-launch focal length measurement system for optical payloads is complex. Its components are subjected to various factors during transportation, launch, and on-orbit operation, including vibration, shock, and stress release due to changes in thermal and gravitational environments. This causes elastic deformation of optical elements and mechanical structures, altering the equivalent focal length of the optical payload. Therefore, it is necessary to determine the optical payload's focal length on-orbit to avoid degradation of image geometry and radiometric quality.

[0003] While numerous studies in high-resolution remote sensing discuss methods for determining interior orientation elements, few articles address on-orbit determination of the equivalent focal length of medium- and low-resolution optical payloads. Existing techniques for determining the on-orbit equivalent focal length of high-resolution optical payloads all use geometrically calibrated field images as the research object, employing targets of specific shapes with known geographic coordinates to calculate the equivalent focal length. This calculation heavily relies on the optical payload's resolution capability against the geometrically calibrated field target, making it unsuitable for medium- and low-resolution optical payloads. After nearly 20 years of development, domestically produced land observation satellites have achieved full coverage of high, medium, and low resolutions, with the highest resolution exceeding 0.5 meters. Therefore, a method for determining the equivalent focal length of medium- and low-resolution optical payloads is urgently needed. Summary of the Invention

[0004] To address the problems existing in the prior art, the present invention aims to provide a method and apparatus for determining the equivalent focal length of low-to-medium resolution optical payloads in orbit. The current abundance of high-resolution remote sensing images provides a reference source for designing a method for determining the equivalent focal length of optical payloads in orbit based on simulated images. Furthermore, the equivalent focal length determined in this way can provide reliable prior information for image reconstruction.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for determining the equivalent focal length of a low-to-medium resolution optical payload in orbit, comprising the following steps:

[0007] 1) Construct image pairs for medium- and low-resolution optical payloads and reference payloads: Select images of regions with relatively low cloud cover and high feature contrast from the synchronous observation images of medium- and low-resolution optical payloads and reference payloads (i.e., high-resolution optical payloads) so that the simulation accuracy can be evaluated by cross-correlation; For each selected target region, select the corresponding image from the existing observation images of medium- and low-resolution optical payloads and high-resolution optical payloads on the satellite as the observation image and reference image, forming an image pair. The target region is an image of a region with relatively low cloud cover and high feature contrast.

[0008] 2) Change the focal length of the medium and low resolution optical payload to generate a geolocation dataset of the image: By changing the focal length within a certain range, calculate the geolocation data of the medium and low resolution optical payload at different focal lengths, including the latitude, longitude and altitude of the target area.

[0009] 3) Simulation image generation: Based on the geolocation dataset calculated in step 2) and the point spread function of the medium- and low-resolution optical payload, a simulation image is generated using the reference image selected in step 1).

[0010] 4) Calculate the correlation coefficient between image pairs at different focal lengths: For the selected target area, calculate the correlation coefficient between the optical payload observation image and the simulated image at each focal length;

[0011] 5) Determine the on-orbit focal length: The focal length corresponding to the peak correlation coefficient is obtained by fitting the correlation coefficient calculated in step 4).

[0012] That is, the focal length of a medium-to-low resolution optical payload.

[0013] A preferred method for acquiring the image pair of medium-to-low resolution optical payload and reference payload is as follows:

[0014] Select a reference payload with a spatial resolution that meets the requirements. Generally, the spatial resolution of the reference payload should be at least 1 / 10 that of a low-to-medium resolution optical payload to avoid the impact of positioning errors of the reference payload. For example, if the resolution of a low-to-medium resolution payload is 500m, the resolution of the reference payload should be 30m.

[0015] Select the target region. The selected region needs to have significant spatial features and be uniformly distributed across the entire image to better calculate the correlation coefficient. Regions with a high concentration of lakes can be selected as regions of interest.

[0016] Select a reference payload for synchronous observation. Use orbit prediction to obtain images of the target area observed by the medium- and low-resolution optical payloads and the reference payload at the same time, thus obtaining image pairs of the medium- and low-resolution optical payloads and the reference payload.

[0017] Furthermore, the process of obtaining synchronous observation images of the reference payload using orbit prediction is as follows:

[0018] Acquire orbital data for the low-to-medium resolution optical payload and the reference payload. Simulate the satellite orbits of both payloads using the Satellite Toolkit (STK) to determine the time interval (e.g., 10 minutes) during which they fly over the selected target area. Prepare images for the low-to-medium resolution optical payload and the reference payload based on the time interval determined by the orbital simulation and orbital prediction.

[0019] Further, in step 2), the geolocation data of the remote sensing instrument at different focal lengths are calculated by changing the focal length. Based on past experience, the focal length is changed in steps of 0.2% within ±1%, and the geolocation data is recalculated so that the maximum correlation coefficient can be obtained in subsequent steps.

[0020] Preferably, in step 3), the process of generating the simulation image from the high-resolution reference payload observation image is as follows:

[0021] The selected area image of the medium- and low-resolution optical payload is projected and transformed to be consistent with the high-resolution optical payload image. By calculating the interval between adjacent pixels at the center of the area, the spatial resolution of the medium- and low-resolution optical payload after projection transformation and its ratio coefficient with the spatial resolution of the reference payload are obtained.

[0022] The point spread function of the medium- and low-resolution optical payload is interpolated using the obtained spatial resolution scaling factor, which is then used for down-resolution processing of the high-resolution reference image.

[0023] To reduce computational load, the row and column numbers of the corresponding high-resolution reference image are calculated from the geolocation data of the selected area using the medium- and low-resolution optical payloads.

[0024] Based on the row and column numbers, the corresponding image is cropped from the high-resolution reference image to obtain the cropped reference image;

[0025] By convolving the point spread function (PSF) of the interpolated low-to-medium resolution optical payload with the cropped reference image, a reduced-resolution simulation image is obtained.

[0026] The reduced-resolution simulation image is used to generate a projected image based on the medium-low resolution optical payload imaging model, which is then used as the simulation image.

[0027] Furthermore, the image pair generation process can be implemented in two ways: One is to generate a projected image from the down-resolution simulated image using reverse ray tracing, based on the imaging model of the low-to-medium resolution optical payload. In this case, the projected image of the reference payload and the observed image of the low-to-medium resolution optical payload constitute an image pair. The other is to transform the observed image of the low-to-medium resolution optical payload to the same projection method as the reference image according to the projection model of the reference payload, generating a projected image. In this case, the projected image of the optical payload and the down-resolution simulated image constitute an image pair. The simulated image generated by the first method, based on the imaging model of the low-to-medium resolution optical payload, can also be used to calculate the joint registration amount; the calculation process of the second method is relatively simple. Both have their advantages. The following description is based on the second method.

[0028] Furthermore, prior to step 4), the observed image after optical payload projection and the reduced-resolution simulated image are examined, and image pairs with high water coverage are removed. Because measurements over large water areas typically yield very high spatial correlation, but obtaining the optimal focal length is often random, it is necessary to remove image pairs with high water coverage to ensure a more accurate estimation of the focal length.

[0029] Furthermore, for the selected region, the correlation coefficient of the spatial offset variation between the observed image and the simulated image after the initial focal length optical payload projection is measured, and the spatial offset with the largest spatial correlation coefficient is recorded. Here, the spatial offset corresponding to the largest correlation coefficient is considered as the joint registration amount of the optical payload image relative to the high-resolution reference payload, and the spatial offset of this joint registration amount is applied to the simulated images for each focal length.

[0030] Furthermore, in step 5), the relevant index R is... 2 As an evaluation factor for measuring the quality of the fit, the calculation results for each image pair are tabulated, and the mean and standard deviation of the focal length measurements for each image set are given.

[0031] An apparatus for determining the equivalent focal length of a low-to-medium resolution optical payload in orbit, characterized in that it includes an image pair generation module, a correlation coefficient calculation module, and a focal length determination module.

[0032] The image pair generation module is used to, for each selected target area, select an image containing the target area from images captured by a medium-low resolution optical payload on the satellite as an observation image, and select an image containing the target area from images captured by a high resolution optical payload on the satellite as a reference image; combine the observation image and the reference image corresponding to the target area into an image pair; change the focal length of the medium-low resolution optical payload, calculate the geolocation data of the medium-low resolution optical payload for the target area at different focal lengths, and generate a geolocation dataset for the target area; for each geolocation data in the geolocation dataset, generate a simulated image based on the point spread function of the medium-low resolution optical payload and the reference image of the target area; combine the reference image of the target area and the simulated image corresponding to each focal length into an image pair.

[0033] The correlation coefficient calculation module is used to calculate the correlation coefficient of image pairs under different focal lengths;

[0034] The focal length determination module is used to fit the obtained correlation coefficients to obtain the focal length corresponding to the peak correlation coefficient, which is used as the on-orbit focal length of the medium and low resolution optical payload.

[0035] Furthermore, the image generation module selects an image I containing the target region from the images captured by the high-resolution optical payload on the satellite and processes it to obtain the reference image. The method for processing the image I is as follows: First, the observation image corresponding to the target region is projected and transformed to be the same size as the image I. By calculating the interval between adjacent pixels at the center of the region, the spatial resolution of the low-to-medium resolution optical payload after projection transformation and its ratio coefficient with the spatial resolution of the high-resolution optical payload are obtained. Then, the point spread function of the low-to-medium resolution optical payload is interpolated according to the ratio coefficient to reduce the resolution of the image I. The row and column numbers in the image I are calculated based on the geographic location data of the low-to-medium resolution optical payload in the target region. The corresponding image is cropped from the image I according to the row and column numbers to obtain the reference image.

[0036] Furthermore, the method by which the image generation module generates the simulated image is as follows: the point spread function (PSF) of the interpolated low-to-medium resolution optical payload is convolved with the reference image to obtain a reduced-resolution simulated image; then, according to the imaging model of the low-to-medium resolution optical payload, the reduced-resolution simulated image is projected onto the image obtained by reverse ray tracing, and this image is used as the simulated image.

[0037] Furthermore, the method by which the image generation module generates the simulation image is as follows: the image obtained by projecting the observation image corresponding to the target area according to the projection model of the high-resolution optical payload is used as the simulation image.

[0038] The present invention also provides a server, including a processor and a memory, wherein the processor reads a computer program or instructions from the memory for performing the following operations:

[0039] Construct image pairs between low- and medium-resolution optical payloads and reference payloads: Select regional images with relatively low cloud cover and high feature contrast from the synchronous observation images of low- and medium-resolution optical payloads and reference payloads so that the simulation accuracy can be evaluated by cross-correlation.

[0040] Generating a geolocation dataset for images: By changing the focal length, geolocation data of low-to-medium resolution optical payloads at different focal lengths are calculated;

[0041] Simulation image generation: Based on the geolocation dataset and the system's point spread function calculated above, simulation images are generated using selected reference images;

[0042] Calculate the correlation coefficient of image pairs at different focal lengths: For the selected target area, calculate the correlation coefficient between the observed image after optical load projection and the simulated image at each focal length;

[0043] Determine the on-orbit focal length: The focal length corresponding to the peak correlation coefficient is obtained by fitting the correlation coefficient calculated above, which is the focal length of the optical payload.

[0044] The intended effect of this invention is:

[0045] This invention proposes a general method and apparatus for determining the equivalent focal length of medium- and low-resolution optical payloads in orbit. The method for determining the equivalent focal length of medium- and low-resolution optical payloads in orbit based on simulation images is simple and convenient, and can avoid the dependence of optical payloads on the resolution capability of geometric calibration field targets. It is also applicable to the in-orbit calculation of equivalent focal lengths in the visible light and infrared bands.

[0046] Compared with the prior art, the technical advantages of this application are as follows:

[0047] ① The method of generating simulated images based on high-resolution reference images and determining the on-orbit equivalent focal length of medium and low resolution optical payloads by calculating the peak correlation coefficient through cross-correlation can avoid the dependence of optical payloads on the resolution capability of geometric calibration field targets.

[0048] ② A new and reliable general method for determining the equivalent focal length of low-to-medium resolution optical payloads in orbit is provided;

[0049] ③ A simple, convenient, and highly generalizable algorithm is provided, which is also applicable to on-orbit calculation of equivalent focal length in the visible and infrared bands. Attached Figure Description

[0050] Figure 1A flowchart illustrating the method for determining the equivalent focal length of low-to-medium resolution optical payloads in orbit, as provided in an embodiment of the present invention.

[0051] Figure 2 This is a flowchart of the simulation image generation process of the present invention.

[0052] Figure 3 This is a schematic diagram of the invention based on PSF resampling.

[0053] Figure 4 This is a schematic diagram of the correlation coefficient fitting and peak correlation coefficient of the present invention. Detailed Implementation

[0054] The technical content of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0055] After nearly 20 years of development, China's domestically produced remote sensing satellites have achieved multi-series development in resources, meteorology, and oceanography, with the highest resolution of land observation satellites exceeding 0.5 meters. Remote sensing images from China's high-resolution satellite wide-field-of-view cameras, the US Landsat satellite, and the European Sentinel-2 satellite are available for free download, providing abundant reference images for on-orbit calculation of the equivalent focal length of medium- and low-resolution optical payloads (such as meteorological satellite payloads). Figure 1 As shown, this invention provides a general method for determining the equivalent focal length of a medium-to-low resolution optical payload in orbit. This method generates a simulated image based on a high-resolution reference image, determines the peak correlation coefficient through cross-correlation and least-squares fitting, and obtains the equivalent focal length of the medium-to-low resolution optical payload in orbit. The method includes the following steps:

[0056] Step S1: Obtain image pairs of the low-to-medium resolution optical payload and the reference payload.

[0057] Step S10: On the low-to-medium resolution optical payload observation images, select observation areas with relatively low cloud cover, high feature contrast, and distinct spatial features. In this embodiment of the invention, the Qinghai-Tibet Plateau region (latitude and longitude range of 28°N~33°N, 82°E~92°E), where lakes are abundant, is selected as the ideal target area.

[0058] Step S11: Based on the latitude and longitude of the selected area, read the reference payload image and its map projection model. Generally, the projection methods of remote sensing payload observation data differ, so it is necessary to perform a projection transformation on the medium- and low-resolution optical payload observation images according to the projection method of the reference payload to obtain the projected optical payload image I. proj For example, Landsat and MODIS observation data use UTM and sine curve projection, respectively.

[0059] Step S12: Based on the latitude and longitude of the selected area, locate the reference load image according to the center latitude and longitude and the latitude and longitude range, and save the corresponding image file name to prepare data for the subsequent generation of simulation images.

[0060] Step S2: Change the focal length to generate a geolocation dataset of the image.

[0061] In remote sensing data geolocation, let the view vector of any pixel on the focal plane in the coordinate system of a medium-to-low resolution optical payload be (x... i y i ,-f), its corresponding position in the geocentric rectangular coordinate system is

[0062]

[0063] In the formula, T inst2body T body2orb 、 and T orb2geo These are the transformation matrices from the low-to-medium resolution optical payload coordinate system to the satellite body coordinate system, from the body coordinate system to the orbital coordinate system, and from the orbital coordinate system to the geocentric rectangular coordinate system. It should be noted that the calculation of the above transformation matrices and the algorithms for converting spatial rectangular coordinates to latitude and longitude are relatively mature and will not be elaborated upon here. Therefore, by changing the focal length, the geographic location dataset can be calculated using equation (1).

[0064] Step S3, simulation image generation, the process is as follows: Figure 2 As shown.

[0065] Step S30: Calculate the optical payload image I proj The sampling interval D of adjacent pixels at the center of the region in the east-west and north-south directions. EW and D NS The scaling factor between the spatial resolution of the image and the reference image is obtained, and the scaling factors in the row and column directions are denoted as L. scale and S scale It should be noted that, to ensure that PSF-based resampling is centered on the target location, if L... scale and S scale For even numbers, L needs to be... scale and S scale Add 1 to each.

[0066] Step S31: According to the calculated row and column direction scaling factor L scale and S scale The low-to-medium resolution optical payload PSF is scaled to facilitate subsequent resampling of the high-resolution reference image.

[0067] Step S32: To reduce computational load, the row and column numbers of the corresponding high-resolution image are calculated from the geolocation data of the medium- and low-resolution optical payload in the selected area. The latitude and longitude of each pixel in the medium- and low-resolution optical payload image are then calculated. The row and column numbers (l) are calculated according to the projection method of the high-resolution reference image. ij p ij Generally, the row and column numbers are in decimal form;

[0068] Step S33: On the high-resolution reference image, select the elements with row and column numbers (l) ij p ij Centered on a scale of L scale ×S scale A subset of the image is obtained by resampling each subset based on the scaled PSF to obtain the simulation image I. sim PSF-based resampling, such as Figure 3 As shown.

[0069] Step S4: Calculate the projected image I of the optical load at each focal length based on cross-correlation. proj and simulation image I sim The correlation coefficient.

[0070] To maximize the performance of cross-correlation, the optical payload observation images and simulation images are scaled to the same extent and the data is stored in floating-point format. The optical payload observation images and simulation images are denoted as I0 and I1, respectively. proj and I sim The cross-correlation coefficient between the observed image and the simulated image after projection of the optical payload is calculated using the following formula.

[0071]

[0072] In the formula, x and y represent the row and column numbers of the image, respectively, and M and N are the number of rows and columns of the image, respectively. and These are the average values ​​of the projected image and the simulated image, respectively.

[0073] Step S5: Perform least-squares fitting on each focal length and its corresponding cross-correlation coefficient, and determine the peak correlation coefficient and the on-orbit focal length. The fitting results are as follows: Figure 4 As shown.

[0074] Furthermore, the present invention also provides an on-orbit device for determining the equivalent focal length of low-to-medium resolution optical payloads, including a processor and a memory, and may further include communication components, sensor components, power supply components, multimedia components, and input / output interfaces as needed. The memory, communication components, sensor components, power supply components, multimedia components, and input / output interfaces are all connected to the processor. As previously mentioned, the memory can be static random access memory (SRAM), electrically erasable programmable read-only memory (EEPROM), erasable programmable read-only memory (EPROM), programmable read-only memory (PROM), read-only memory (ROM), magnetic storage, flash memory, etc.; the processor can be a central processing unit (CPU), graphics processing unit (GPU), field-programmable gate array (FPGA), application-specific integrated circuit (ASIC), digital signal processing (DSP) chip, etc. Other communication components, sensor components, power supply components, multimedia components, etc., can all be implemented using common components found in existing smartphones, and will not be specifically described here.

[0075] On the other hand, in the device for determining the equivalent focal length of low-to-medium resolution optical payloads in orbit, the processor reads a computer program or instruction from memory to perform the following operations:

[0076] Acquire image pairs of optical payload and reference payload: Select data with relatively low cloud cover and high feature contrast from the optical payload and reference instrument observation images of the same day so that the simulation accuracy can be evaluated by cross-correlation;

[0077] Generating a geolocation dataset of images: By changing the focal length, calculate the geolocation data of remote sensing instruments at different focal lengths;

[0078] Simulation image generation: Based on the geolocation dataset and the system's point spread function calculated above, simulation images are generated using selected reference images;

[0079] Calculate the correlation coefficient of image pairs at different focal lengths: For the selected target region, calculate the correlation coefficient between the optical payload observation image and the simulated image at each focal length;

[0080] Determine the on-orbit focal length: Using the correlation coefficient and corresponding focal length calculated above, the focal length corresponding to the peak correlation coefficient is obtained through least squares fitting, which is the focal length of the optical payload.

[0081] The method and apparatus for determining the equivalent focal length of a low-to-medium resolution optical payload in orbit, provided in this invention, are based on a geolocation calculation program for the low-to-medium resolution optical payload. They select the starting and ending row and column numbers of an observation area with relatively low cloud cover, high feature contrast, and distinct spatial characteristics. Then, based on the corresponding time and latitude / longitude, they acquire a reference payload observation image and projection model. This ensures, from an absolute geographical perspective, that the selected low-to-medium resolution optical payload image and the reference payload observation image have the same position and grayscale information, thus providing a data source for determining the optimal on-orbit focal length based on relevant calculations. Furthermore, resampling the reference payload image based on the PSF of the low-to-medium resolution optical payload better conforms to the optical imaging mechanism of the low-to-medium resolution optical payload, ensuring, to a certain extent, that the determined on-orbit focal length is more accurate and more accurately reflects the on-orbit status of the low-to-medium resolution optical payload.

[0082] The foregoing has provided a detailed description of a general method and apparatus for determining the equivalent focal length of low-to-medium resolution optical payloads in orbit, as provided by this invention. Any obvious modifications made by those skilled in the art without departing from the essence of this invention will fall within the scope of protection of this invention's patent rights.

Claims

1. A method for determining the equivalent focal length of a low-to-medium resolution optical payload in orbit, comprising the following steps: 1) For each selected target area, select an image containing the target area from the images captured by the medium- and low-resolution optical payloads on the satellite as the observation image, and select an image containing the target area from the images captured by the high-resolution optical payloads on the satellite as the reference image; and form an image pair between the observation image and the reference image corresponding to the target area. 2) Change the focal length of the medium-low resolution optical payload, calculate the geolocation data of the medium-low resolution optical payload for the target area under different focal lengths, and generate the geolocation dataset of the target area; 3) For each geolocation data in the geolocation dataset, generate a simulated image based on the point spread function of the medium-low resolution optical payload and the reference image of the target area; and take the reference image of the target area and the simulated image corresponding to each focal length as an image pair; 4) Calculate the correlation coefficient of image pairs at different focal lengths; 5) The focal length corresponding to the peak correlation coefficient is obtained by fitting the obtained correlation coefficients, and is used as the on-orbit focal length of the medium and low resolution optical payload.

2. The method according to claim 1, characterized in that, An image I containing the target region is selected from images captured by a high-resolution optical payload on a satellite and processed to obtain the reference image. The processing method for image I is as follows: first, the observation image corresponding to the target region is projected and transformed to be the same size as image I; by calculating the interval between adjacent pixels at the center of the region, the spatial resolution of the low-to-medium resolution optical payload after projection transformation and its ratio coefficient with the spatial resolution of the high-resolution optical payload are obtained; then, the point spread function of the low-to-medium resolution optical payload is interpolated according to the ratio coefficient to reduce the resolution of image I. The row and column numbers in image I are calculated based on the geolocation data of the target area using a medium- and low-resolution optical payload; the corresponding image is then cropped from image I based on these row and column numbers to obtain the reference image.

3. The method according to claim 2, characterized in that, The method for generating the simulation image is as follows: the point spread function (PSF) of the interpolated low-to-medium resolution optical payload is convolved with the reference image to obtain a reduced-resolution simulation image; then, according to the imaging model of the low-to-medium resolution optical payload, the reduced-resolution simulation image is projected onto the image obtained by reverse ray tracing, and this image is used as the simulation image.

4. The method according to claim 1 or 2, characterized in that, The method for generating the simulation image is as follows: the image obtained by projecting the observation image corresponding to the target area according to the projection model of the high-resolution optical payload is used as the simulation image.

5. The method according to claim 1, characterized in that, The spatial resolution of the high-resolution optical payload is more than 1 / 10 of the spatial resolution of the medium-low resolution optical payload; the target region is a region with high feature contrast.

6. The method according to claim 1, characterized in that, The focal length is changed in 0.2% steps within ±1% of the focal length of the medium-low resolution optical payload.

7. The method according to claim 1, characterized in that, Before step 4), remove image pairs with a high water coverage ratio.

8. The method according to claim 1, characterized in that, The correlation coefficient of the corresponding image pair is obtained by measuring the spatial offset change between the two images in the image pair.

9. A device for determining the equivalent focal length of a low-to-medium resolution optical payload in orbit, characterized in that, It includes an image pair generation module, a correlation coefficient calculation module, and a focal length determination module; The image pair generation module is used to, for each selected target area, select an image containing the target area from images captured by a medium-low resolution optical payload on the satellite as an observation image, and select an image containing the target area from images captured by a high resolution optical payload on the satellite as a reference image; combine the observation image and the reference image corresponding to the target area into an image pair; change the focal length of the medium-low resolution optical payload, calculate the geolocation data of the medium-low resolution optical payload for the target area at different focal lengths, and generate a geolocation dataset for the target area; for each geolocation data in the geolocation dataset, generate a simulated image based on the point spread function of the medium-low resolution optical payload and the reference image of the target area; combine the reference image of the target area and the simulated image corresponding to each focal length into an image pair. The correlation coefficient calculation module is used to calculate the correlation coefficient of image pairs under different focal lengths; The focal length determination module is used to fit the obtained correlation coefficients to obtain the focal length corresponding to the peak correlation coefficient, which is used as the on-orbit focal length of the medium and low resolution optical payload.

10. A server, characterized in that, It includes a memory and a processor, the memory storing a computer program configured to be executed by the processor, the computer program including instructions for performing each step of the method of any one of claims 1 to 8.

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