Visible light space camera multi-dimensional on-orbit imaging mode construction method

Abstract

The invention discloses a multi-dimensional in-orbit imaging mode construction method for a visible light space camera. The method comprises the following steps: firstly, acquiring calibration absolute responsivity and a response linear interval of the visible light space camera; then acquiring global ground reflectivity and a global atmosphere profile database, and performing simulation calculation on visible light entrance pupil equivalent radiance of the visible light space camera from multiple dimensions; constructing a multi-dimensional on-orbit imaging mode of the visible light space camera according to the entrance pupil equivalent spectral radiance database of the visible light space camera and the calibration absolute responsivity of the visible light space camera, so that the visible light space camera works in the optimal response linear interval, and obtaining an initial value of the multi-dimensional on-orbit imaging mode of the visible light space camera; and finally, after the visible light space camera enters the orbit, optimizing the multi-dimensional in-orbit imaging mode of the visible light space camera according to the response characteristics of the visible light space camera at the initial in-orbit stage. According to the construction method, the proper imaging mode can be set, so that the image radiation quality of the visible light space camera is optimal, and the optimal detection capability of the visible light space camera is exerted.

Description

Technical Field

[0001] The present invention relates to a method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera, and in particular to a method for constructing an on-orbit imaging mode of a visible light space camera by using an absolute response function, a solar altitude angle, and a global ground reflectivity database, and belongs to the technical field of on-orbit application of visible light space cameras. Background Art

[0002] Visible light space cameras can realize ground and air imaging in the visible spectrum, and are widely used in many fields such as forestry monitoring, agricultural surveys, marine observations, disaster prevention, topographic observations, military target reconnaissance, land surveys, and agricultural quantitative remote sensing. In the process of developing space cameras, the camera's imaging capabilities can be ensured through the design of large-aperture and long-focal-length systems, the optimization of high-performance (high quantum efficiency, high signal-to-noise ratio, low dark current, and low readout noise) detectors, and high-precision assembly and integration technology. The appropriate imaging mode setting is the key to bringing out the camera's imaging capabilities. Therefore, in order to maximize the detection capabilities of visible light space cameras, it is necessary to plan and design the on-orbit imaging mode table of visible light space cameras.

[0003] The links of the visible light space camera imaging link are: solar spectrum reflected by ground objects, atmospheric transmission, optical system, detector and electronic system, data compression and storage, data transmission, data reception and processing, etc. Ground objects are the scenery on the surface of the earth. Sunlight is reflected by ground objects and emits visible light radiation energy upward. It is transmitted by atmospheric radiation to the satellite and imaged by the visible light space camera to obtain the image corresponding to the ground object. In addition, part of the radiation received by the visible light space camera comes from atmospheric radiation. This part of radiation is the radiation that directly reaches the camera sensor after being scattered by various components and aerosol particles of solar radiation during atmospheric transmission. Since this part of radiation is direct current, it has nothing to do with the reflectivity of ground objects and can be obtained by simulating the ground object atmospheric transmission software. When the atmospheric radiation is ignored, the image grayscale output by the visible light space camera is the product of the transmittance of the optical system and filter and the visible detector response and the visible light radiation energy of the ground object. That is, the radiation response equation of the visible light space camera is shown in formula (1).

[0004]

[0005] Where: DN is the digital output value of the image; E(H) is the radiant illumination of the sun reaching the ground when the solar altitude angle is H; ρ is the reflectivity of the ground object; τ atm is the atmospheric transmittance; D is the aperture of the visible light space camera; f is the focal length of the visible light space camera; τ opt is the transmittance of the optical system; τ fil is the filter transmittance; R det is the absolute responsivity of the visible detector; βe is the gain of the electronics system; t int is the integration time, DN dark is the background response of the visible light space camera, is the radiance of the ground object with reflectivity ρ, R cam is the absolute responsivity of the visible light space camera.

[0006] From formula (1), we can know that the radiation response of the visible light space camera mainly depends on factors such as the solar altitude angle, ground reflectivity, atmospheric transmittance and absolute responsiveness of the visible light space camera at the time of shooting. In 2015, the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences proposed a method for establishing the on-orbit imaging mode of a visible light space camera. First, the radiant brightness before the camera entrance pupil is calculated using atmospheric radiation simulation software, and the camera's radiation calibration data is analyzed, including the responsiveness, radiant brightness range, and integration time relationship of each mode; then, the integration time relationship between the camera's pendulum imaging and level flight imaging is analyzed to obtain the radiant brightness range of each imaging mode at each side swing angle. Finally, based on the radiant brightness range of the imaging mode and the radiant brightness range of the scene at each solar high angle, a two-dimensional imaging mode table under the combination of solar high angle and camera side swing angle is established (Optical Precision Engineering, 2015, (07): 1852-1858). This method focuses on the influence of solar altitude angle and satellite side swing on imaging performance, while setting the reflectivity of the ground object to a fixed value for design. The imaging performance of the visible light space camera is greatly limited and cannot achieve the optimal imaging performance. Summary of the invention

[0007] In view of this, in order to ensure that the most appropriate imaging mode is adopted by the visible light space camera during on-orbit imaging so as to bring into play the best performance of the camera, the present invention proposes a method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera.

[0008] The technical solutions adopted by the present invention to achieve the above-mentioned purpose are as follows.

[0009] The present invention provides a method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera, comprising the following steps:

[0010] The first step is to obtain the calibration absolute responsivity and response linear range of the visible light space camera;

[0011] The second step is to obtain the global ground reflectivity and global atmospheric profile database, simulate and calculate the visible light entrance pupil equivalent radiance of the visible light space camera from multiple dimensions, and establish the visible light entrance pupil equivalent radiance database of the visible light space camera;

[0012] The third step is to construct a multi-dimensional on-orbit imaging mode of the visible light space camera based on the entrance pupil equivalent spectral radiance database of the visible light space camera and the calibrated absolute responsivity of the visible light space camera, so that the visible light space camera works in the optimal response linear interval and obtains the initial value of the multi-dimensional on-orbit imaging mode of the visible light space camera;

[0013] The fourth step is to optimize the multi-dimensional on-orbit imaging mode of the visible light space camera based on its initial response characteristics after it enters orbit.

[0014] Preferably, in the first step, a visible light radiation calibration system is used to obtain a calibration absolute responsivity and a response linear range of the visible light space camera through a visible light radiation calibration test;

[0015] The visible light radiation calibration system consists of a visible light calibration cabin, a visible light space camera, an integrating sphere light source, a camera controller, a camera quick-view system and a spectral radiometer; the visible light calibration cabin provides calibration environment conditions for the radiation calibration test of the visible light space camera, the visible light space camera, the integrating sphere light source and the spectral radiometer are placed in the visible light calibration cabin, the integrating sphere light source provides the visible light space camera with uniform Lambertian light of any radiation brightness within a dynamic range, the spectral radiometer is used for collecting radiation brightness data, the camera controller and the camera quick-view system are both connected to the visible light space camera, the camera controller sets suitable imaging parameters for the visible light space camera, and the camera quick-view system displays and stores image data.

[0016] More preferably, the method of the visible light radiation calibration test is:

[0017] The camera controller sets imaging parameters for the visible light space camera, sets the integrating sphere light source to a suitable radiation brightness so that the visible light space camera response reaches 90% of the saturation value, and after the light source is stable, the camera quick-view system saves the image, while the spectral radiometer collects the radiation brightness output by the integrating sphere in real time;

[0018] The brightness level of the integrating sphere light source is changed in sequence so that the response of the visible light space camera reaches 80%, 70%, 60%, 50%, 40%, 30%, 20%, and 10% of the saturation value. After the light source is stable, the camera quick-view system saves the image, and the spectral radiometer collects the radiation brightness output by the integrating sphere in real time.

[0019] Finally, the integrating sphere light source is turned off, the camera quick-view system saves the dark field image, and the spectroradiometer collects the dark field radiation brightness in real time;

[0020] The response of the visible light space camera at different brightness levels is calculated as DN. Combined with the radiation brightness L at different brightness levels obtained by the spectroradiometer, the radiation calibration coefficients k and b are obtained by fitting using the least squares method.

[0021] DN=k*L+b (2).

[0022] Preferably, in the second step, the dimensions are four: longitude dimension, latitude dimension, month dimension and solar altitude angle.

[0023] More preferably, in the second step, the granularity of the longitude dimension is 5 degrees, with a total of 72 intervals; the granularity of the latitude dimension is 5 degrees, with a total of 36 intervals; the granularity of the month dimension is 1 month, with a total of 12 intervals; the effective range of the solar altitude angle is 0°~90°, the granularity is 10°, with a total of 9 intervals.

[0024] Preferably, in the second step, the global object emissivity and the global atmospheric profile database are substituted into the atmospheric radiation transmission software to calculate the entrance pupil equivalent spectral radiance of the visible light space camera.

[0025] Preferably, in the fourth step, the initial period is within one week of entering orbit.

[0026] Preferably, in the fourth step, after the visible light space camera enters orbit, the response DN values ​​and corresponding multiple dimensional information of typical objects of the visible light space camera at the initial stage of orbit are counted;

[0027] The entrance pupil equivalent spectral radiance of the visible light space camera is calculated based on multiple dimensional information, and the response DN value of typical objects is calculated as the theoretical expected value based on the calibrated absolute responsivity.

[0028] The deviation between the statistical value and the theoretical expected value is compared. If the deviation is within a reasonable range, there is no need to adjust the imaging mode. Otherwise, the imaging mode is updated based on the deviation and the calibration absolute responsivity calculation.

[0029] More preferably, the reasonable range is within 20%.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera of the present invention can set a suitable imaging mode to optimize the image radiation quality of the visible light space camera, so as to exert the optimal detection capability of the visible light space camera. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the specific implementation modes of the present invention, the drawings required for use in the specific implementation modes will be briefly introduced below. Obviously, the drawings described below are only some specific implementation modes of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0033] Figure 1 The present invention is a flowchart of a method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera.

[0034] Figure 2 It is a structural schematic diagram of the visible light space camera calibration system of the present invention;

[0035] In the figure: 1. Visible light calibration cabin, 2. Visible light space camera, 3. Integrating sphere light source, 4. Camera controller, 5. Camera quick view system, 6. Spectroradiometer. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below with reference to the accompanying drawings.

[0037] like Figure 1 As shown, the method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera of the present invention comprises the following steps:

[0038] In the first step, a visible light radiation calibration system is used to obtain the calibration absolute responsivity and response linear range of the visible light space camera 2 through a visible light radiation calibration test. Figure 2 As shown, the visible light radiation calibration system consists of a visible light calibration cabin 1, a visible light space camera 2, an integrating sphere light source 3, a camera controller 4, a camera quick view system 5, and a spectral radiometer 6. The visible light calibration cabin 1 provides calibration environment conditions for the radiation calibration test of the visible light space camera 2. The visible light space camera 2, the integrating sphere light source 3, and the spectral radiometer 6 are placed in the visible light calibration cabin 1. The integrating sphere light source 3 provides the visible light space camera 2 with uniform Lambertian light of any radiance within the dynamic range, and the spectral radiometer 6 is used to collect radiance data. The camera controller 4 and the camera quick view system 5 are both connected to the visible light space camera 2. The camera controller 4 sets suitable imaging parameters for the visible light space camera 2, and the camera quick view system 5 displays and stores image data.

[0039] The method for the visible light radiation calibration test is:

[0040] The camera controller 4 sets imaging parameters for the visible light space camera 2, and sets the integrating sphere light source 3 to a suitable radiation brightness so that the visible light space camera 2 response reaches 90% of the saturation value. After the light source is stable, the camera quick-view system 5 saves the image, and the spectral radiometer 6 collects the radiation brightness output by the integrating sphere in real time.

[0041] The brightness level of the integrating sphere light source 3 is changed in sequence so that the response of the visible light space camera 2 reaches 80%, 70%, 60%, 50%, 40%, 30%, 20%, and 10% of the saturation value. After the light source is stable, the camera quick-view system 5 saves the image, and the spectral radiometer 6 collects the radiation brightness output by the integrating sphere in real time.

[0042] Finally, the integrating sphere light source 3 is turned off, the camera quick-view system 5 saves the dark field image, and the spectroradiometer 6 collects the dark field radiation brightness in real time;

[0043] The response of the visible light space camera 2 at different brightness levels is DN, and the radiation brightness L at different brightness levels obtained by the spectroradiometer 6 is combined to obtain the radiation calibration coefficients k and b using the least squares method;

[0044] DN=k*L+b (2).

[0045] The second step is to obtain the global ground reflectivity and global atmospheric profile database, simulate and calculate the visible light entrance pupil equivalent radiance of the visible light space camera 2 from multiple dimensions, and establish the visible light entrance pupil equivalent radiance database of the visible light space camera 2; the preferred dimensions are longitude dimension, latitude dimension, month dimension and solar altitude angle. Define the granularity of the four dimensions according to the needs, such as the granularity of the longitude dimension is 5 degrees, with a total of 72 intervals; the granularity of the latitude dimension is 5 degrees, with a total of 36 intervals; the granularity of the month dimension is 1 month, with a total of 12 intervals; the effective interval of the solar altitude angle is 0°~90°, the granularity is 10°, with a total of 9 intervals. Substitute the global ground object emissivity and the global atmospheric profile database into the atmospheric radiation transmission software to calculate the entrance pupil equivalent spectral radiance of the visible light space camera 2. The first-level lookup table of the entrance pupil equivalent spectral radiance database and the second-level lookup table 1 of the entrance pupil equivalent spectral radiance are shown in Table 1 and Table 2 respectively.

[0046] Table 1. Primary lookup table of entrance pupil equivalent spectral radiance database

[0047]

[0048]

[0049] Table 2 Secondary lookup table 1 of entrance pupil equivalent spectral radiance database (month: mon = 1, solar altitude angle: 0°<H≤10°)

[0050]

[0051] The third step is to construct the multi-dimensional on-orbit imaging mode of the visible light space camera 2 according to the entrance pupil equivalent radiance database of the visible light space camera 2 and the calibrated absolute response of the visible light space camera 2, so that the visible light space camera 2 works in the optimal response linear interval, and obtain the initial value of the multi-dimensional on-orbit imaging mode of the visible light space camera 2; for example, taking four dimensions (longitude dimension, latitude dimension, month dimension and solar altitude angle) as an example, the imaging parameter lookup table is divided into two levels of lookup tables, and the imaging parameter first level lookup table and the imaging parameter second level lookup table 1 are shown in Table 3 and Table 4 respectively. The granularity of the four dimensions is as follows: the granularity of the longitude dimension is 5 degrees, with a total of 72 intervals; the granularity of the latitude dimension is 5 degrees, with a total of 36 intervals; the granularity of the month dimension is 1 month, with a total of 12 intervals; the effective interval of the solar altitude angle is 0°~90°, the granularity is 10°, with a total of 9 intervals.

[0052] Table 3 Imaging parameter first level lookup table

[0053]

[0054] Table 4 Imaging parameter secondary lookup table 1 (month: mon = 1, solar altitude angle: 0° < H ≤ 10°)

[0055]

[0056] Step 4: After the visible light space camera 2 is put into orbit, the response DN values ​​of typical objects and the corresponding longitude dimension, latitude dimension, month dimension and solar altitude angle dimension of the visible light space camera 2 in the early stage of orbit (preliminarily within one week) are counted;

[0057] The entrance pupil equivalent spectral radiance of the visible light space camera 2 is calculated according to the information of the longitude dimension, the latitude dimension, the month dimension and the solar altitude angle dimension, and the response DN value of the typical ground object is calculated as the theoretical expected value according to the calibrated absolute responsivity;

[0058] Compare the deviation between the statistical value and the theoretical expected value. If the deviation is within 20%, there is no need to adjust the imaging mode. Otherwise, the imaging mode is updated based on the deviation and the calibration absolute response calculation. If necessary, multiple optimization adjustments are performed.

Claims

1. A method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera, characterized in that: The following steps are involved: The first step is to obtain the calibration absolute responsivity and response linear range of the visible light space camera; The second step is to obtain the global ground reflectivity and global atmospheric profile database, simulate and calculate the visible light entrance pupil equivalent radiance of the visible light space camera from multiple dimensions, and establish the visible light entrance pupil equivalent radiance database of the visible light space camera; The third step is to construct a multi-dimensional on-orbit imaging mode of the visible light space camera based on the entrance pupil equivalent spectral radiance database of the visible light space camera and the calibrated absolute responsivity of the visible light space camera, so that the visible light space camera works in the optimal response linear interval and obtains the initial value of the multi-dimensional on-orbit imaging mode of the visible light space camera; The fourth step is to optimize the multi-dimensional on-orbit imaging mode of the visible light space camera based on its initial response characteristics after it enters orbit.

2. The method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera according to claim 1, characterized in that: In the first step, a visible light radiation calibration system is used to obtain the calibration absolute responsivity and response linear range of the visible light space camera through a visible light radiation calibration test; The visible light radiation calibration system consists of a visible light calibration cabin, a visible light space camera, an integrating sphere light source, a camera controller, a camera quick-view system and a spectral radiometer; the visible light calibration cabin provides calibration environment conditions for the radiation calibration test of the visible light space camera, the visible light space camera, the integrating sphere light source and the spectral radiometer are placed in the visible light calibration cabin, the integrating sphere light source provides the visible light space camera with uniform Lambertian light of any radiation brightness within a dynamic range, the spectral radiometer is used for collecting radiation brightness data, the camera controller and the camera quick-view system are both connected to the visible light space camera, the camera controller sets suitable imaging parameters for the visible light space camera, and the camera quick-view system displays and stores image data.

3. The method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera according to claim 2, characterized in that: The method of the visible light radiation calibration test is: The camera controller sets imaging parameters for the visible light space camera, sets the integrating sphere light source to a suitable radiation brightness so that the visible light space camera response reaches 90% of the saturation value, and after the light source is stable, the camera quick-view system saves the image, while the spectral radiometer collects the radiation brightness output by the integrating sphere in real time; The brightness level of the integrating sphere light source is changed in sequence so that the response of the visible light space camera reaches 80%, 70%, 60%, 50%, 40%, 30%, 20%, and 10% of the saturation value. After the light source is stable, the camera quick-view system saves the image, and the spectral radiometer collects the radiation brightness output by the integrating sphere in real time. Finally, the integrating sphere light source is turned off, the camera quick-view system saves the dark field image, and the spectroradiometer collects the dark field radiation brightness in real time; The response of the visible light space camera at different brightness levels is calculated as DN. Combined with the radiation brightness L at different brightness levels obtained by the spectroradiometer, the radiation calibration coefficients k and b are obtained by fitting using the least squares method. DN=k*L+b (2).

4. The method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera according to claim 1, characterized in that: In the second step, the dimensions are four: longitude dimension, latitude dimension, month dimension and solar altitude angle.

5. The method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera according to claim 4, characterized in that: In the second step, the granularity of the longitude dimension is 5 degrees, with a total of 72 intervals; the granularity of the latitude dimension is 5 degrees, with a total of 36 intervals; the granularity of the month dimension is 1 month, with a total of 12 intervals; the effective range of the solar altitude angle is 0°~90°, the granularity is 10°, with a total of 9 intervals.

6. The method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera according to claim 1, characterized in that: In the second step, the global ground object emissivity and the global atmospheric profile database are substituted into the atmospheric radiation transfer software to calculate the entrance pupil equivalent spectral radiance of the visible light space camera.

7. The method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera according to claim 1, characterized in that: In the fourth step, the initial period is within one week of entering orbit.

8. The method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera according to claim 1, characterized in that: In the fourth step, after the visible light space camera enters orbit, the response DN values ​​and corresponding multi-dimensional information of typical objects of the visible light space camera in the early stage of orbit are counted; The entrance pupil equivalent spectral radiance of the visible light space camera is calculated based on multiple dimensional information, and the response DN value of typical objects is calculated as the theoretical expected value based on the calibrated absolute responsivity. The deviation between the statistical value and the theoretical expected value is compared. If the deviation is within a reasonable range, there is no need to adjust the imaging mode. Otherwise, the imaging mode is updated based on the deviation and the calibration absolute responsivity calculation.

9. The method for constructing a multi-dimensional on-orbit imaging mode of a visible light space camera according to claim 8, characterized in that: The reasonable range is within 20%.

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