A method and system for inverting the rotation angle of a spaceborne L-band microwave radiometer

Abstract

The application discloses a kind of methods and systems for calculating the rotation angle of spaceborne L-band microwave radiometer, method includes: obtaining the satellite orbit parameters and the brightness temperature data of ocean time of spaceborne L-band microwave radiometer;Simulate horizontal polarization, vertical polarization and cross-polarization radiation brightness temperature data;The stokes radiation brightness temperature data obtained by simulation is converted into the brightness temperature data under antenna coordinate system using rotation matrix;Using joint cost function, the above-mentioned simulation and conversion are carried out using nonlinear iteration until the rotation angle change is less than the first set threshold and the sea surface salinity change is less than the second set threshold, solve the rotation angle and sea surface salinity;According to the rotation angle, recalculate the rotation matrix;According to the new rotation matrix, the obtained brightness temperature data is converted into corresponding horizontal polarization TH, vertical polarization TV, cross-polarization T3 and T4, complete the accurate conversion of spaceborne L-band microwave radiometer brightness temperature.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of microwave remote sensing, and in particular to a method and system for calculating a rotation angle of a spaceborne L-band microwave radiometer. BACKGROUND

[0002] In spaceborne microwave remote sensing observation, Faraday rotation angle, geometric rotation angle and attitude angle, etc. will cause polarization rotation. When the electromagnetic wave signal of L-band (near 1.4 GHz) passes through the ionosphere, due to the action of the earth's magnetic field and the free electrons in the ionosphere, Faraday rotation phenomenon occurs, that is, the polarization direction of the electromagnetic wave is deflected. The polarization rotation caused by the rotation angle will cause the radiation signal of a certain polarization (such as horizontal polarization H) of the ground surface radiation to become a mixed signal of horizontal and vertical polarization (V) when reaching the satellite sensor. For remote sensing inversion algorithms that rely on polarization information (such as sea surface salinity field, sea surface wind field and sea surface temperature field, etc.), if the influence of the rotation angle cannot be accurately calculated, significant errors will be introduced.

[0003] At present, the calculation of the rotation angle mainly relies on two methods:

[0004] 1. Calculate the Faraday rotation angle plus the geometric rotation angle and the real-time attitude angle of the satellite based on the external ionospheric model: use the ionospheric total electron content (TEC) data provided by the International GNSS Service (IGS) and the geomagnetic field model to calculate the Faraday rotation angle. However, the ionospheric TEC has the characteristics of spatio-temporal dynamic change, and the model accuracy is limited, especially when the solar activity is intense or the regional ionosphere is uneven, the error is larger.

[0005] 2. Inversion algorithm of rotation angle based on cross-polarization T3: SMO S, Aquarius and SMAP L-band satellite missions all use the cross-polarization radiometer to measure the ratio (T3 / Q) of the cross-polarization and the second Stokes parameter (Q). The Faraday rotation angle is inverted by the arctangent relationship. In the range of partial incidence angle, the T3 brightness temperature is very small, and in the area where TV and TH are close (such as vegetation coverage), the error of the rotation angle calculated by this method is larger.

[0006] The existing technology has the following problems:

[0007] (1) The accuracy of the external ionospheric model is limited: the resolution and real-time performance of global TEC data are insufficient, which is difficult to meet the high-precision remote sensing requirements.

[0008] (2) The measurement based on T3 is affected by noise: the T3 calibration accuracy is poor, and it is not suitable for the case where TV and TH are approximately equal.

[0009] ​(3) The first method cannot correct errors introduced by channel crosstalk, limited beam width, and geometric rotation angle, etc.

[0010] Therefore, there is an urgent need for a high-precision and self-adaptive rotation angle correction method that can combine polarimetric observation data of radiometers and auxiliary information to reduce dependence on external models and data. SUMMARY

[0011] The present application aims to overcome the defects of the prior art and provides a method for calculating the rotation angle of a spaceborne L-band microwave radiometer, and a system for calculating the rotation angle of a spaceborne L-band microwave radiometer.

[0012] Therefore, the present application provides a method for calculating the rotation angle of a spaceborne L-band microwave radiometer, which comprises the following steps:

[0013] Step 1: Obtain satellite orbit parameters of the spaceborne L-band microwave radiometer;

[0014] Step 2: Obtain brightness temperature data of the spaceborne L-band microwave radiometer when passing over the ocean;

[0015] Step 3: Simulate horizontal polarization, vertical polarization, and cross-polarization radiance brightness temperature data according to the brightness temperature data and the satellite orbit parameters;

[0016] Step 4: Convert the Stokes radiance brightness temperature data obtained in Step 3 to brightness temperature data in the antenna coordinate system using a rotation matrix;

[0017] Step 5: Use a joint cost function to perform Steps 3 and 4 using nonlinear iteration until the rotation angle change is less than a first set threshold and the sea surface salinity change is less than a second set threshold, to solve the rotation angle and the sea surface salinity;

[0018] Step 6: Recalculate the rotation matrix according to the rotation angle;

[0019] Step 7: Convert the brightness temperature data obtained in Step 2 to corresponding horizontal polarization TH, vertical polarization TV, cross-polarization T3, and radiance brightness temperature T4 using the rotation matrix obtained in Step 6, thereby accurately converting the brightness temperature of the spaceborne L-band microwave radiometer.

[0020] Preferably, the rotation angle includes a Faraday rotation angle, a geometric rotation angle, an attitude angle, and an angle of polarization rotation caused by the hardware of the load itself.

[0021] Preferably, the satellite orbit parameters obtained in Step 1 include real-time position and attitude of the satellite.

[0022] Preferably, the brightness temperature data obtained in step 2 includes: vertical-satellite-orbit-direction-earth-scanning brightness temperature data TX, along-satellite-orbit-direction brightness temperature data TY, cross-polarization brightness temperature data TXY and T4 in antenna coordinates.

[0023] Preferably, the step 3 includes:

[0024] According to the real-time position of the satellite, the atmospheric profile and surface data corresponding to the real-time position are obtained, and horizontal-polarization radiation brightness temperature data TH_sim, vertical-polarization radiation brightness temperature data TV_sim, cross-polarization radiation brightness temperature data T3_sim and T4_sim are obtained through simulation according to an atmospheric transmission model and a land surface radiation model.

[0025] Preferably, the step 4 includes:

[0026] Four simulated Stokes radiation brightness temperature parameters I, Q, U, V , and the conversion relationship with horizontal polarization TH, vertical polarization TV, cross-polarization T3 and T4 is:

[0027]

[0028]

[0029]

[0030]

[0031] Similarly, the four observed Stokes radiation brightness temperature parameters , , , , and the conversion relationship with vertical-satellite-orbit-direction-earth-scanning brightness temperature data TX, along-satellite-orbit-direction brightness temperature data TY, cross-polarization brightness temperature data TXY and T4 in antenna coordinates is:

[0032]

[0033]

[0034]

[0035]

[0036] The polarization rotation matrix between the antenna-received brightness temperature and the incident brightness temperature is:

[0037] After the polarization rotation matrix, the change relationship of the four Stokes brightness temperature parameters is:

[0038]

[0039] wherein, is the rotation angle.

[0040] Preferably, the combined cost function of step 5 is:

[0041]

[0042] wherein, p represents V polarization or H polarization, is the measured brightness temperature of the L / C / K band radiometer, is the calculated rotated matrix antenna port surface simulated brightness temperature of the L / C / K band, is the uncertainty of the measured brightness temperature of the different band radiometer, i represents the polarization corresponding frequency band, N represents the total number of frequency bands and polarization channels.

[0043] Preferably, the step 6 comprises:

[0044] According to the polarization rotation angle obtained by iterative inversion The recalculated polarization rotation matrix is:

[0045] .

[0046] Preferably, the step 7 comprises:

[0047] According to the polarization rotation matrix obtained in step 6, the change relationship between the four simulated Stokes radiation brightness temperature parameters and the four observed Stokes radiation brightness temperature parameters , , , is:

[0048]

[0049] According to the following formula:

[0050]

[0051]

[0052]

[0053]

[0054] The horizontal polarization TH, the vertical polarization TV, the cross-polarization brightness temperature T3 and T4 are obtained, and thus the accurate conversion of the spaceborne L-band microwave radiometer brightness temperature is completed.

[0055] In another aspect, the present application provides a system for calculating the rotation angle of a spaceborne L-band microwave radiometer, comprising:

[0056] an orbit parameter acquisition module for acquiring satellite orbit parameters of the spaceborne L-band microwave radiometer;

[0057] a brightness temperature data acquisition module for acquiring brightness temperature data of the spaceborne L-band microwave radiometer when passing over the ocean;

[0058] a simulation module for simulating horizontally polarized, vertically polarized and cross-polarized radiation brightness temperature data based on the brightness temperature data and the satellite orbit parameters;

[0059] a conversion module for converting the simulated Stokes radiation brightness temperature data into brightness temperature data in the antenna coordinate system using a rotation matrix;

[0060] an iterative execution module for using a joint cost function to iteratively execute the simulation module and the conversion module until the rotation angle change is less than a first set threshold and the sea surface salinity change is less than a second set threshold, to solve the rotation angle and the sea surface salinity;

[0061] a rotation matrix calculation module for recalculating the rotation matrix based on the rotation angle; and

[0062] a result solving module for converting the brightness temperature data acquired by the brightness temperature data acquisition module into corresponding horizontally polarized TH, vertically polarized TV, cross-polarized T3 and T4 based on the recalculated rotation matrix, thereby completing accurate conversion of the spaceborne L-band microwave radiometer brightness temperature.

[0063] Compared with the prior art, the present application has the following advantages:

[0064] 1. The present application proposes a real-time correction scheme for Faraday rotation angle and geometric rotation angle based on joint inversion of rotation angle, sea surface salinity and other parameters for the first time.

[0065] 2. The present application can effectively improve the accuracy and reliability of data products such as sea surface salinity field, sea surface wind field and sea surface temperature field of the spaceborne L-band microwave radiometer. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 is a flowchart of the method for calculating the rotation angle of a spaceborne L-band microwave radiometer. DETAILED DESCRIPTION

[0067] The present application is a method for calculating the rotation angle based on joint inversion, which is suitable for L-band microwave radiometers and is used for the inversion of geophysical parameters such as sea surface salinity field, sea surface wind field and sea surface temperature field to improve the inversion accuracy.

[0068] The method comprises:

[0069] Step 1: obtaining satellite orbit parameters of a spaceborne L-band microwave radiometer;

[0070] Step 2: obtaining brightness temperature data of the spaceborne L-band microwave radiometer when passing over the ocean;

[0071] Step 3: simulating horizontal polarization, vertical polarization and cross-polarization radiation brightness temperature data according to the brightness temperature data and the satellite orbit parameters;

[0072] Step 4: converting the Stokes radiation brightness temperature data obtained in step 3 into brightness temperature data in an antenna coordinate system by using a rotation matrix;

[0073] Step 5: performing steps 3 and 4 by using a joint cost function and a nonlinear iteration until a rotation angle change is less than a first set threshold and a sea surface salinity change is less than a second set threshold, so as to solve the rotation angle and the sea surface salinity;

[0074] Step 6: recalculating the rotation matrix according to the rotation angle;

[0075] Step 7: converting the brightness temperature data obtained in step 2 into corresponding horizontal polarization TH, vertical polarization TV, cross-polarization T3 and T4 by using the rotation matrix obtained in step 6, so as to complete accurate conversion of the spaceborne L-band microwave radiometer brightness temperature.

[0076] The technical solutions of the present application will be described in detail below in combination with the drawings and embodiments.

[0077] Embodiment 1

[0078] Embodiment 1 of the present application proposes a method for inversely calculating a rotation angle of a spaceborne L-band microwave radiometer, wherein the rotation angle comprises a Faraday rotation angle, a geometric rotation angle, an attitude angle and an angle caused by polarization rotation of the load itself hardware.

[0079] comprising the following steps:

[0080] Step S1: satellite orbit parameter acquisition: real-time position and attitude of the satellite;

[0081] Step S2: obtaining two-dimensional observation brightness temperature data of TX and TY in two vertical directions and TXY cross-polarization brightness temperature data of a spaceborne L-band microwave radiometer in an antenna coordinate system; TX is vertical-satellite-orbit-flight-direction earth scanning brightness temperature data, and TY is brightness temperature data along the satellite orbit flight direction;

[0082] Step S3: According to the real-time position of the satellite, the atmospheric profile and surface data corresponding to the real-time position are obtained, and horizontal polarization radiation brightness temperature data TH_sim, vertical polarization radiation brightness temperature data TV_sim, cross-polarization radiation brightness temperature data T3_sim and T4_sim are simulated and obtained according to an atmospheric transmission model (such as mpm93) and a land surface radiation model (such as a sea surface fastem6 model or a land surface empirical model);

[0083] Step S4: The simulated horizontal TH_sim and vertical TV_sim polarization radiation brightness temperature and cross-polarization radiation brightness temperature data T3_sim and T4_sim are converted into TX_sim, TY_sim, TXY_sim and T4 brightness temperature data in the antenna coordinates by using a rotation matrix Stokes parameter conversion;

[0084] Specifically, the four simulated Stokes radiation brightness temperature parameters I, Q, U, V are converted into the horizontal polarization TH, the vertical polarization TV, the cross-polarization T3 and T4 as follows:

[0085]

[0086]

[0087]

[0088]

[0089] Similarly, the four observed Stokes radiation brightness temperature parameters , , , are converted into the vertical satellite orbit flight direction earth scanning brightness temperature data TX, the brightness temperature data TY along the satellite orbit flight direction, the cross-polarization brightness temperature data TXY and T4 in the antenna coordinates as follows:

[0090]

[0091]

[0092]

[0093]

[0094] The polarization rotation matrix between the antenna receiving brightness temperature and the incident brightness temperature is:

[0095] After the polarization rotation matrix, the change relationship of the four Stokes brightness temperature parameters is:

[0096]

[0097] wherein, is the total angle of polarization rotation caused by Faraday rotation angle, geometric rotation angle and attitude angle,

[0098] Step S5: using the joint cost function, solving the rotation angle, sea surface salinity and other parameters by using the Levenberg-Marquardt nonlinear iterative steps S3 and S4 until the rotation angle change is less than the first set threshold, in one embodiment, 0.01 degrees, and the sea surface salinity change is less than the second set threshold, in one embodiment, 0.01 psu;

[0099] Joint cost function is:

[0100]

[0101] wherein, p represents V polarization or H polarization, is the measured brightness temperature of the L / C / K band radiometer, is the calculated L / C / K band antenna port simulation brightness temperature after rotation matrix, is the uncertainty of the measured brightness temperature of the different band radiometer, i represents the polarization corresponding to the frequency band, N represents the total number of frequency bands and polarization channels;

[0102] Step S6: using the inversion obtained rotation angle Recalculate the rotation matrix:

[0103]

[0104] Step S7: using the newly generated rotation matrix to convert TX, TY, TXY and T4 into corresponding horizontal polarization TH, vertical polarization TV, cross polarization T3 and T4. Specifically:

[0105] According to the polarization rotation matrix obtained in step 6, the change relationship between the four simulated Stokes radiation brightness temperature parameters and the four observed Stokes radiation brightness temperature parameters , , , is:

[0106]

[0107] According to the following formula:

[0108]

[0109]

[0110]

[0111]

[0112] The horizontal polarization TH, vertical polarization TV, and cross-polarization brightness temperatures T3 and T4 are obtained, thus completing the accurate conversion of the brightness temperature of the spaceborne L-band microwave radiometer.

[0113] Example 2

[0114] Embodiment 2 of the present invention provides a system for inverting and calculating the rotation angle of a spaceborne L-band microwave radiometer, based on the method of Embodiment 1. The system includes:

[0115] The orbit parameter acquisition module is used to acquire the satellite orbit parameters of the onboard L-band microwave radiometer;

[0116] Brightness temperature data acquisition module is used to acquire brightness temperature data of the spaceborne L-band microwave radiometer when it passes over the ocean;

[0117] The simulation module is used to simulate the brightness temperature data of horizontal polarization, vertical polarization, and cross polarization radiation brightness temperature based on brightness temperature data and satellite orbit parameters.

[0118] The conversion module is used to convert the Stokes radiation brightness temperature data obtained from the simulation into brightness temperature data in the antenna coordinate system using a rotation matrix.

[0119] The iterative execution module is used to employ a joint cost function and utilize nonlinear iterative execution of the simulation module and the transformation module until the rotation angle change is less than a first set threshold and the sea surface salinity change is less than a second set threshold, thereby solving for the rotation angle and sea surface salinity.

[0120] The rotation matrix calculation module is used to recalculate the rotation matrix based on the rotation angle; and

[0121] The result solving module is used to convert the brightness temperature data obtained by the brightness temperature data acquisition module into the corresponding horizontal polarization TH, vertical polarization TV, cross polarization T3 and T4 according to the recalculated rotation matrix, thereby completing the accurate conversion of the brightness temperature of the spaceborne L-band microwave radiometer.

[0122] It is worth noting that in the embodiments of the above system, the modules included are divided according to functional logic, but are not limited to the above division, as long as the corresponding functions can be achieved; in addition, the specific names of each functional module are only for easy differentiation and are not used to limit the scope of protection of the present invention.

[0123] Experiments have shown that the method of this invention is effective:

[0124] 1. Reduce reliance on external ionospheric models to avoid errors introduced by insufficient accuracy of TEC (total electron content) data or limited spatiotemporal resolution.

[0125] 2. Improve the measurement accuracy of rotation angle, especially its applicability in complex terrestrial scenarios (such as vegetation cover and surface heterogeneity), and overcome the limitations of the traditional T3 / Q method under non-uniform surface conditions.

[0126] 3. Reduce the impact of radiometer noise and cross-polarization coupling, and achieve more robust rotation angle estimation through multi-Stokes parameter joint inversion and error compensation.

[0127] This invention is applicable to the inversion of geophysical parameters such as sea surface salinity field, sea surface wind field, and sea surface temperature field from L-band microwave remote sensing satellites, and can significantly improve the accuracy and reliability of data products.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for inverting and calculating the rotation angle of a spaceborne L-band microwave radiometer, comprising: Step 1: Obtain the satellite orbit parameters of the spaceborne L-band microwave radiometer; Step 2: Acquire brightness temperature data of the spaceborne L-band microwave radiometer as it passes over the ocean; Step 3: Based on the brightness temperature data and satellite orbit parameters, simulate the brightness temperature data of horizontal polarization, vertical polarization, and cross polarization radiation; Step 4: Four simulated Stokes radiation brightness temperature parameters I, Q, U, V The conversion relationship between the horizontal polarization TH, vertical polarization TV, and cross-polarization brightness temperature data T3 and T4 is as follows: ； ； ； ； Four observed Stokes radiation brightness temperature parameters , , , The conversion relationships between Earth scan brightness temperature data TX (perpendicular to the satellite orbital flight direction), brightness temperature data TY (along the satellite orbital flight direction), cross-polarized brightness temperature data TXY, and T4 in the antenna coordinate system are as follows: ； ； ； ； The polarization rotation matrix between the received brightness temperature and the incident brightness temperature is: ； After polarization rotation, the relationship between the four Stokes brightness temperature parameters is as follows: ； in, The rotation angle; Step 5: Employ the joint cost function Steps 3 and 4 are executed using nonlinear iteration until the change in rotation angle is less than a first set threshold and the change in sea surface salinity is less than a second set threshold, thus solving for the rotation angle and sea surface salinity; the joint cost function for: ； in, p This represents V-polarization or H-polarization. It measures the brightness temperature using an L-band radiometer. It is the calculated simulated brightness temperature of the antenna aperture after the L-band is rotated by a matrix. Uncertainty in measuring brightness temperature with an L-band radiometer. i Indicates the frequency band corresponding to polarization. N Indicates the total number of frequency bands and polarization channels; Step 6: Recalculate the rotation matrix based on the rotation angle; Step 7: Based on the rotation matrix obtained in Step 6, convert the brightness temperature data obtained in Step 2 into the corresponding horizontal polarization brightness temperature data TH, vertical polarization brightness temperature data TV, and cross-polarization brightness temperature data T3 and T4, thereby completing the accurate conversion of the brightness temperature of the spaceborne L-band microwave radiometer.

2. The method for inverting and calculating the rotation angle of a spaceborne L-band microwave radiometer according to claim 1, characterized in that, The rotation angle includes: Faraday rotation angle, geometric rotation angle, attitude angle, and angle of polarization rotation caused by the load's own hardware.

3. The method for inverting and calculating the rotation angle of a spaceborne L-band microwave radiometer according to claim 1, characterized in that, The satellite orbital parameters obtained in step 1 include the satellite's real-time position and attitude.

4. The method for inverting and calculating the rotation angle of a spaceborne L-band microwave radiometer according to claim 1, characterized in that, The brightness temperature data obtained in step 2 includes: Earth scan brightness temperature data TX in the vertical direction of satellite orbit flight under antenna coordinates, brightness temperature data TY along the direction of satellite orbit flight, cross-polarization brightness temperature data TXY and T4.

5. The method for inverting and calculating the rotation angle of a spaceborne L-band microwave radiometer according to claim 1, characterized in that, Step 6 includes: The polarization rotation angle obtained from iterative inversion The recalculated polarization rotation matrix is: 。 6. The method for inverting and calculating the rotation angle of a spaceborne L-band microwave radiometer according to claim 1, characterized in that, Step 7 includes: Based on the polarization rotation matrix obtained in step 6, four simulated Stokes radiation brightness temperature parameters and four observed Stokes radiation brightness temperature parameters are then obtained. , , , The relationship of change is as follows: 。 7. A system for inverting and calculating the rotation angle of a spaceborne L-band microwave radiometer, characterized in that, include: The orbit parameter acquisition module is used to acquire the satellite orbit parameters of the onboard L-band microwave radiometer; Brightness temperature data acquisition module is used to acquire brightness temperature data of the spaceborne L-band microwave radiometer when it passes over the ocean; The simulation module is used to simulate the brightness temperature data of horizontal polarization, vertical polarization, and cross polarization radiation brightness temperature based on brightness temperature data and satellite orbit parameters. The conversion module is used to perform conversions: Four simulated Stokes radiation brightness temperature parameters I, Q, U, V The conversion relationship between the horizontal polarization TH, vertical polarization TV, and cross-polarization brightness temperature data T3 and T4 is as follows: ； ； ； ； Four observed Stokes radiation brightness temperature parameters , , , The conversion relationships between Earth scan brightness temperature data TX (perpendicular to the satellite orbital flight direction), brightness temperature data TY (along the satellite orbital flight direction), cross-polarized brightness temperature data TXY, and T4 in the antenna coordinate system are as follows: ； ； ； ； The polarization rotation matrix between the received brightness temperature and the incident brightness temperature is: ； After polarization rotation, the relationship between the four Stokes brightness temperature parameters is as follows: ； in, The rotation angle; Iterative execution module, used to employ the joint cost function The simulation and transformation modules are executed using nonlinear iterative processes until the change in rotation angle is less than a first set threshold and the change in sea surface salinity is less than a second set threshold, thus solving for the rotation angle and sea surface salinity; the joint cost function for: ； in, p This represents V-polarization or H-polarization. It measures the brightness temperature using an L-band radiometer. It is the calculated simulated brightness temperature of the antenna aperture after the L-band is rotated by a matrix. Uncertainty in measuring brightness temperature with an L-band radiometer. i Indicates the frequency band corresponding to polarization. N Indicates the total number of frequency bands and polarization channels; The rotation matrix calculation module is used to recalculate the rotation matrix based on the rotation angle; and The result solving module is used to convert the brightness temperature data obtained by the brightness temperature data acquisition module into the corresponding horizontal polarization brightness temperature data TH, vertical polarization brightness temperature data TV, and cross polarization brightness temperature data T3 and T4 based on the recalculated rotation matrix, thereby completing the accurate conversion of the brightness temperature of the spaceborne L-band microwave radiometer.

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