Satellite-borne SAR azimuth two-way antenna pattern on-orbit test method

Abstract

The invention discloses an on-orbit test method for a satellite-borne SAR (Synthetic Aperture Radar) azimuth two-way antenna pattern, and belongs to the cross technical field of spaceflight and microwave remote sensing. The method comprises the following steps: acquiring the coordinate of each calibration device in a calibration field under an earth-fixed system and the position of a satellite in an imaging period under the earth-fixed system so as to solve the distance between the satellite and the central position of the calibration field; determining rectangular window slice data corresponding to each calibration device in each single-view complex image; doppler envelope extraction is carried out; the satellite speed in the imaging time period is determined by using the distance between the satellite and the center position of the calibration field; carrying out SAR azimuth direction two-way antenna pattern inversion; and averaging the satellite-borne SAR azimuth two-way antenna directional diagrams corresponding to all the calibration devices. According to the invention, high-efficiency and high-precision satellite-borne SAR two-way antenna pattern in-orbit measurement can be realized, and important support is provided for rapid tuning and imaging quality improvement after satellite injection.

Description

Technical Field

[0001] The present invention belongs to the cross - technical field of aerospace and microwave remote sensing, and particularly relates to an in - orbit test method for the azimuth - two - way antenna pattern of spaceborne SAR. Background Technique

[0002] The SAR satellite is an active remote - sensing device that can work all - day and all - weather to obtain high - resolution ground - scene SAR images. Affected by the changes in the satellite - ground state, after the satellite is launched into orbit, there are certain changes between the SAR antenna pattern and the ground - measured and modeled antenna pattern. It is necessary to conduct in - orbit tests on the antenna pattern based on the external calibration method, and complete model optimization by combining the analysis of in - orbit internal calibration data, providing important support for radiometric calibration and high - precision imaging processing.

[0003] The existing range - direction Figure 1 Generally, the in - orbit measurement is completed based on calibration fields with uniform backscattering such as the Amazon rainforest. For the azimuth - direction Figure 1 Generally, the transmitting and receiving antenna patterns are measured respectively by deploying active calibration equipment on the ground. This method has two deficiencies:

[0004] First, affected by the inconsistency between the transmitting and receiving gains, it is difficult to accurately invert the two - way antenna pattern.

[0005] Second, affected by satellite revisit, it is difficult to quickly complete the measurement of the transmitting and receiving antenna patterns at the same wave position. Summary of the Invention

[0006] The purpose of the present invention is to provide an in - orbit test method for the azimuth - two - way antenna pattern of spaceborne SAR, which can achieve efficient and high - precision in - orbit measurement of the two - way antenna pattern of spaceborne SAR, providing important support for the rapid optimization after the satellite is launched into orbit and the improvement of imaging quality.

[0007] To achieve the above - mentioned purpose, the present invention is implemented by adopting the following technical solutions:

[0008] An in - orbit test method for the azimuth - two - way antenna pattern of spaceborne SAR, the in - orbit test method for the azimuth - two - way antenna pattern of spaceborne SAR includes:

[0009] Step S1: Obtain the coordinates of each calibration device in the geocentric coordinate system and the position of the satellite in the geocentric coordinate system during the imaging period, so as to obtain the distance between the satellite and the center position of the calibration field;

[0010] Step S2: Use the coordinates of each calibration device in the geocentric coordinate system to determine the rectangular window slice data corresponding to each calibration device in each single - look complex image;

[0011] Step S3: Use the rectangular window sliced data to perform Doppler envelope extraction to determine the Doppler envelope curve corresponding to each calibration device;

[0012] Step S4: Use the distance between the satellite and the center position of the calibration field to determine the satellite velocity during the imaging period;

[0013] Step S5: Use the satellite velocity during the imaging period and the Doppler envelope curve corresponding to each calibration device to perform SAR azimuth two-way antenna pattern inversion to determine the spaceborne SAR azimuth two-way antenna pattern corresponding to each calibration device;

[0014] Step S6: Perform averaging processing on the spaceborne SAR azimuth two-way antenna patterns corresponding to each calibration device.

[0015] Further, in the step S1, the specific process of obtaining the coordinates of each calibration device in the geocentric coordinate system in the calibration field includes:

[0016] Step S11: Select multiple calibration points in the calibration field and deploy a calibration device at each calibration point;

[0017] Step S12: According to the satellite access time, SAR incident angle and side view angle, adjust the strongest scattering direction of each calibration device to the SAR beam center direction;

[0018] Step S13: Measure the scattering center position of each adjusted calibration device to determine the coordinates of each calibration device in the geocentric coordinate system.

[0019] Further, in the step S2, the specific process of determining the rectangular window sliced data corresponding to each calibration device in each single-look complex image includes:

[0020] Step S21: Use the calibration field echo data transmitted by the satellite and satellite parameters to perform imaging processing to obtain a single-look complex image;

[0021] Step S22: Use the coordinates of each calibration device in the geocentric coordinate system to determine the positions of each calibration device in each single-look complex image;

[0022] Step S23: Take each calibration device as the center and extract the rectangular window sliced data corresponding to each calibration device in the single-look complex image.

[0023] Further, in the step S3, the specific process of the Doppler envelope extraction includes:

[0024] Step S31: Perform range frequency domain transformation, frequency domain zero-padding and range time domain transformation on the rectangular window sliced data corresponding to each calibration device in sequence;

[0025] Step S32: Perform zero-padding in the azimuth time domain and Doppler domain transformation on the rectangular window slice data after range time domain transformation in sequence.

[0026] Step S33: Calculate the average power of the rectangular window slice data after Doppler domain transformation in the range direction to form a Doppler envelope curve.

[0027] Furthermore, the specific implementation process of the said Step S4 includes:

[0028] Step S41: Solve the Doppler history using the distance between the satellite and the center position of the calibration field.

[0029] Step S42: Take the derivative of the said Doppler history in the azimuth time domain.

[0030] Step S43: Perform linear fitting on the derivative result and then perform averaging to obtain the satellite velocity in the geocentric coordinate system during the satellite imaging period.

[0031] Furthermore, in the said Step S5, the specific process of azimuth two-way antenna pattern inversion includes:

[0032] Step S51: According to the corresponding relationship between the azimuth angle and the Doppler frequency, use the satellite velocity and the Doppler envelope curve of each calibration device during the imaging period to perform mapping to obtain the azimuth two-way antenna pattern corresponding to each calibration device.

[0033] Step S52: Perform equal-angle interval interpolation fitting on the azimuth two-way antenna pattern corresponding to each calibration device to obtain the spaceborne SAR azimuth two-way antenna pattern corresponding to each calibration device.

[0034] Furthermore, in the said Step S51, the corresponding relationship between the azimuth angle and the Doppler frequency is:

[0035]

[0036] where f η is the Doppler frequency; λ is the center wavelength of the radar payload; V sat is the satellite velocity during the imaging period; θ d is the azimuth angle corresponding to the Doppler frequency f η corresponding.

[0037] Furthermore, in the said Step S6, averaging is performed according to the following formula;

[0038]

[0039] where represents the spaceborne SAR azimuth two-way antenna pattern after averaging processing; It represents the azimuth two-way antenna pattern corresponding to the nth calibration device after equal-angle interval interpolation fitting; n = 1, 2,..., N, where N is the number of calibration devices deployed in the calibration field.

[0040] In summary, the technical solution of the present invention has the following technical effects:

[0041] The present invention uses the coordinates of each calibration device in the geocentric coordinate system to determine the rectangular window slice data corresponding to each calibration device in each single-look complex image, improving the signal-to-clutter ratio; through the rectangular window slice data, Doppler envelope extraction is performed to determine the Doppler envelope curve corresponding to each calibration device; the distance between the satellite and the center position of the calibration field is used to determine the satellite velocity during the imaging period; based on the corresponding relationship between the azimuth angle and the Doppler frequency, using the satellite velocity during the imaging period and the Doppler envelope curve corresponding to each calibration device, after SAR azimuth two-way antenna pattern inversion and then averaging processing, the accuracy of the SAR azimuth two-way antenna pattern is improved; the present invention can achieve efficient and high-precision on-orbit measurement of the satellite-borne SAR two-way antenna pattern, providing important support for the rapid optimization after the satellite is launched into orbit and the improvement of imaging quality. Description of the Drawings

[0042] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0043] Figure 1 It is a schematic flowchart of the on-orbit test method for the satellite-borne SAR azimuth two-way antenna pattern according to the embodiment of the present invention;

[0044] Figure 2 It is a schematic diagram of the on-orbit test principle of the satellite-borne SAR azimuth two-way antenna pattern according to the embodiment of the present invention. Detailed Embodiments

[0045] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0046] This embodiment provides an on-orbit test method for the satellite-borne SAR azimuth two-way antenna pattern, referring to Figure 1 andFigure 2 , the method for on-orbit testing of the azimuth two-way antenna pattern of the spaceborne SAR includes:

[0047] Step S1: Obtain the coordinates of each calibration device in the geodetic coordinate system within the calibration field and the position of the satellite in the geodetic coordinate system during the imaging period, so as to calculate the distance between the satellite and the center position of the calibration field.

[0048] In the calibration field, select N calibration points, and deploy a calibration device (such as a trihedral corner reflector) at each calibration point. It is required that the calibration field should have a weak radar backscattering coefficient, the signal-to-clutter ratio of the corner reflector to the background should not be less than 30 dB, the undulating terrain, artificial buildings, etc. within the calibration field will not cover and block the calibration points and their surroundings in the SAR image, and there will be no mutual covering and blocking between the calibration points in the SAR image. To reduce the influence of the deployment error of the calibration device and the difference in its own scattering characteristics, generally take the number of calibration points N≥5.

[0049] Formulate the work plan of the satellite SAR payload, and according to the satellite access time, SAR incident angle and side view angle, make corresponding adjustments to the pointing of the corner reflector so that the strongest scattering direction of the corner reflector coincides with the pointing of the SAR beam center. After the equipment pointing adjustment is completed, make a high-precision measurement of the scattering center position of the corner reflector to obtain the coordinates of the corner reflector in the geodetic coordinate system. The specific process of obtaining the coordinates of each calibration device in the geodetic coordinate system within the calibration field includes:

[0050] Step S11: Select multiple calibration points from within the calibration field, and deploy a calibration device at each calibration point;

[0051] Step S12: According to the satellite access time, SAR incident angle and side view angle, adjust the strongest scattering direction of each calibration device to the pointing of the SAR beam center;

[0052] Step S13: Measure the scattering center position of each adjusted calibration device to determine the coordinates of each calibration device in the geodetic coordinate system.

[0053] Step S2: Use the coordinates of each calibration device in the geodetic coordinate system to determine the rectangular window slice data corresponding to each calibration device in each single-look complex image.

[0054] After the satellite obtains the calibration field data, it downlinks the echo data and data such as the satellite position and speed. The ground completes the imaging processing to obtain the single-look complex image data. In the single-look complex image, with each corner reflector as the center, select a rectangular window slice data of L A1 ×L R1 resolution cells. Generally, take L R1 =16, L A1 =32.

[0055] The specific process of determining the rectangular window slice data corresponding to each calibration device in each single-look complex image in this embodiment includes:

[0056] Step S21: Use the calibration field echo data transmitted by the satellite and the satellite parameters to perform imaging processing to obtain a single-look complex image;

[0057] Step S22: Use the coordinates of each calibration device in the geocentric coordinate system to determine the positions of each calibration device in each single-look complex image;

[0058] Step S23: With each calibration device as the center, extract the rectangular window slice data corresponding to each calibration device in the single-look complex image.

[0059] Step S3: Use the rectangular window slice data to perform Doppler envelope extraction to determine the Doppler envelope curve corresponding to each calibration device.

[0060] , Change the slice data to the range frequency domain, zero-pad in the frequency domain and then transform to the range time domain to complete range interpolation. The interpolation factor is generally taken as L R2 = 16. Zero-pad in the azimuth direction to complete Doppler domain interpolation. The interpolation factor is generally taken as L A2 = 32 - 64. Transform the data to the range-Doppler domain, and obtain the average power in the range direction to get the ground clutter Doppler envelope curve A(f η ), where f η is the Doppler frequency.

[0061] The specific process of Doppler envelope extraction in this embodiment includes:

[0062] Step S31: Perform range frequency domain transformation, frequency domain zero-padding, and range time domain transformation on the rectangular window slice data corresponding to each calibration device in sequence;

[0063] Step S32: Perform azimuth time domain zero-padding and Doppler domain transformation on the rectangular window slice data after range time domain transformation in sequence;

[0064] Step S33: Calculate the average power in the range direction of the rectangular window slice data after Doppler domain transformation to form a Doppler envelope curve.

[0065] Step S4: Use the distance between the satellite and the center position of the calibration field to determine the satellite velocity during the imaging period.

[0066] According to the position relationship between the satellite and the ground object, solve the Doppler history R(η), perform derivative, linear fitting, and averaging processing in the azimuth time domain to obtain the satellite velocity V sat (i.e., the satellite velocity during the imaging period), where η represents the azimuth time. The specific implementation process of this step includes:

[0067] Step S41: Solve the Doppler history using the distance between the satellite and the center position of the calibration field.

[0068] Step S42: Differentiate the Doppler history in the azimuth time domain.

[0069] Step S43: Perform linear fitting on the differentiation result and then average it to obtain the satellite velocity in the geocentric coordinate system during the satellite imaging period.

[0070] Step S5: Use the satellite velocity during the imaging period and the Doppler envelope curve corresponding to each calibration device to perform SAR azimuth two-way antenna pattern inversion to determine the SAR azimuth two-way antenna pattern corresponding to each calibration device.

[0071] Invert from the Doppler envelope curve to obtain the on-board azimuth two-way antenna pattern corresponding to each calibration device. Perform equal-angle interval interpolation fitting in the angle domain to obtain the pattern curve. The specific process of the azimuth two-way antenna pattern inversion in this embodiment includes:

[0072] Step S51: According to the correspondence between the azimuth angle and the Doppler frequency, use the satellite velocity during the imaging period and the Doppler envelope curve corresponding to each calibration device to perform mapping to obtain the azimuth two-way antenna pattern corresponding to each calibration device.

[0073] The correspondence between the azimuth angle and the Doppler frequency in this embodiment is:

[0074]

[0075] where f η is the Doppler frequency; λ is the center wavelength of the radar payload; V sat is the satellite velocity during the imaging period; θ d is the azimuth angle corresponding to the Doppler frequency f η .

[0076] Step S52: Perform equal-angle interval interpolation fitting on the azimuth two-way antenna pattern corresponding to each calibration device to obtain the on-board SAR azimuth two-way antenna pattern corresponding to each calibration device.

[0077] Perform the above operations on each corner reflector, and a total of N two-way antenna pattern data can be obtained.

[0078] Step S6: Average the on-board SAR azimuth two-way antenna patterns corresponding to each calibration device.

[0079] For the N two-way antenna pattern data Perform averaging to further improve the estimation accuracy and obtain the azimuth two-way antenna pattern below this wave position.

[0080] This embodiment performs averaging according to the following formula:

[0081]

[0082] Where, represents the azimuth two-way antenna pattern of the spaceborne SAR after averaging; represents the azimuth two-way antenna pattern corresponding to the nth calibration device after interpolation fitting at equal angular intervals; n = 1, 2,..., N, and N is the number of calibration devices arranged in the calibration field.

[0083] This embodiment uses the coordinates of each calibration device in the earth-fixed coordinate system to determine the rectangular window slice data corresponding to each calibration device in each single-look complex image, improving the signal-to-clutter ratio; through the rectangular window slice data, Doppler envelope extraction is performed to determine the Doppler envelope curve corresponding to each calibration device; the distance between the satellite and the center position of the calibration field is used to determine the satellite speed during the imaging period; based on the correspondence between the azimuth angle and the Doppler frequency, using the satellite speed during the imaging period and the Doppler envelope curve corresponding to each calibration device, after inverting the SAR azimuth two-way antenna pattern, averaging is performed to improve the accuracy of the SAR azimuth two-way antenna pattern; this embodiment can achieve efficient and high-precision on-orbit measurement of the spaceborne SAR two-way antenna pattern, providing important support for the rapid optimization and imaging quality improvement after the satellite is launched into orbit.

[0084] The above embodiments only represent several implementation manners of the present application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. An on-orbit test method for a spaceborne SAR azimuth two-way antenna pattern, characterized in that: The on-orbit testing method of the spaceborne SAR azimuth two-way antenna pattern comprises: Step S1, obtaining the coordinates of each calibration device in the calibration field under the ground fixed system and the position of the satellite under the ground fixed system during the imaging period, so as to obtain the distance between the satellite and the center position of the calibration field; Step S2, using the coordinates of each calibration device in the ground fixed system, determining the rectangular window slice data corresponding to each calibration device in each single-view complex image; Step S3, using the rectangular window slice data to perform Doppler envelope extraction to determine the Doppler envelope curve corresponding to each calibration device; Step S4, using the distance between the satellite and the center of the calibration field to determine the satellite speed during the imaging period; Step S5, using the satellite velocity in the imaging period and the Doppler envelope curve corresponding to each calibration device, performing SAR azimuth two-way antenna pattern inversion to determine the spaceborne SAR azimuth two-way antenna pattern corresponding to each calibration device; Step S6: average the azimuth two-way antenna patterns of the spaceborne SAR corresponding to all calibration devices.

2. The on-orbit test method for the azimuth two-way antenna pattern of a spaceborne SAR according to claim 1, characterized in that: In step S1, the specific process of obtaining the coordinates of each calibration device in the calibration field under the ground fixed system includes: Step S11, selecting multiple calibration points from the calibration field, and deploying a calibration device at each calibration point; Step S12: adjusting the strongest scattering direction of each calibration device to the SAR beam center direction according to the satellite access time, SAR incident angle and side viewing angle; Step S13: measuring the scattering center position of each adjusted calibration device to determine the coordinates of each calibration device in the ground fixed system.

3. The on-orbit test method for the azimuth two-way antenna pattern of a spaceborne SAR according to claim 2, characterized in that: In step S2, the specific process of determining the rectangular window slice data corresponding to each calibration device in each single-view complex image includes: Step S21, performing imaging processing using calibration field echo data and satellite parameters transmitted by the satellite to obtain a single-view complex image; Step S22, using the coordinates of each calibration device in the ground fixed system, determining the position of each calibration device in each single-view complex image; Step S23: Taking each calibration device as the center, extracting the rectangular window slice data corresponding to each calibration device in the single-view complex image.

4. The on-orbit test method for the azimuth two-way antenna pattern of a spaceborne SAR according to any one of claims 1 to 3, characterized in that: In step S3, the specific process of Doppler envelope extraction includes: Step S31, performing range frequency domain transformation, frequency domain zero padding and range time domain transformation on the rectangular window slice data corresponding to each calibration device in sequence; Step S32, performing azimuth time domain zero padding and Doppler domain transformation on the rectangular window slice data after the range time domain transformation in sequence; Step S33: Calculate the average power of the rectangular window slice data after the Doppler domain transformation in the range direction to form a Doppler envelope curve.

5. The on-orbit test method for the azimuth two-way antenna pattern of a spaceborne SAR according to claim 4, characterized in that: The specific implementation process of step S4 includes: Step S41, using the distance between the satellite and the center of the calibration field to solve the Doppler history; Step S42, deriving the Doppler process in the azimuth time domain; Step S43: Perform linear fitting on the derivative results and then average them to obtain the satellite velocity in the ground-fixed system during the satellite imaging period.

6. The on-orbit test method for the azimuth two-way antenna pattern of a spaceborne SAR according to claim 5, characterized in that: In step S5, the specific process of inverting the azimuth two-way antenna pattern includes: Step S51, mapping is performed based on the correspondence between the azimuth angle and the Doppler frequency using the satellite velocity in the imaging period and the Doppler envelope curve of each calibration device to obtain the azimuth two-way antenna pattern corresponding to each calibration device; Step S52: Perform equal-angle interval interpolation fitting on the azimuth two-way antenna pattern corresponding to each calibration device to obtain the azimuth two-way antenna pattern of the spaceborne SAR corresponding to each calibration device.

7. The on-orbit test method for the azimuth two-way antenna pattern of a spaceborne SAR according to claim 6, characterized in that: In step S51, the corresponding relationship between the azimuth angle and the Doppler frequency is: Among them, f η is the Doppler frequency; λ is the central wavelength of the radar payload; V sat is the satellite speed during the imaging period; θ d is related to the Doppler frequency f η The corresponding azimuth.

8. The on-orbit test method for the azimuth two-way antenna pattern of a spaceborne SAR according to claim 7, characterized in that: In step S6, averaging is performed according to the following formula: in, It represents the two-way antenna pattern of the spaceborne SAR in azimuth after average processing; It represents the azimuth two-way antenna pattern corresponding to the nth calibration device after equi-angle interval interpolation fitting; n = 1, 2, ..., N, where N is the number of calibration devices deployed in the calibration field.

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