CN110865346B - Satellite-borne SAR time parameter calibration method based on direct positioning algorithm - Google Patents
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Abstract
The invention provides a direct positioning algorithm-based satellite-borne SAR time parameter calibration method, which comprises the following steps: selecting a calibration area and arranging corner reflectors; adjusting the orientation of a corner reflector and measuring the position of a scattering center; imaging and image up-sampling processing; determining the nominal azimuth time and the slope distance of the corner reflector; fitting the phase center position and the speed of the SAR antenna; comprehensively processing the fitting coefficient; establishing a calibration equation set; solving a calibration equation set; atmospheric delay compensation; solving the azimuth slow time error and the slope error; averaging the processing results of the polygon reflector; averaging the multiple observation processing results; traversing and calibrating the form of the signals transmitted by the radar system, and finally obtaining the azimuth slow time correction parameters and the slant range correction parameters of the satellite-borne SAR. The satellite-borne SAR time parameter calibration method based on the direct positioning algorithm calibrates the errors of the azimuth slow time and the slant range system, thereby improving the geometric positioning precision and the imaging performance of the system.
Description
Technical Field
The invention relates to the technical field of synthetic aperture radars, in particular to a satellite-borne SAR time parameter calibration method based on a direct positioning algorithm.
Background
Synthetic Aperture Radar (SAR) is an active remote sensing device that can work all day long and all weather to obtain high resolution ground scene SAR images. Due to the device performance and other reasons, certain errors exist in the radar recorded distance fast time and azimuth slow time, the errors change along with the time width and the bandwidth of a transmitted signal, and the distance fast time errors are directly expressed as slant range errors. The existence of the error not only affects the positioning accuracy of the target, but also causes a frequency modulation error in the imaging process and affects the imaging performance, and for a high-resolution SAR, the influence is particularly serious, and the high-resolution SAR must be calibrated, so that the geometric positioning accuracy and the imaging performance of the radar are improved.
In view of this, a new calibration method for the satellite-borne SAR time parameter is urgently needed to be designed.
Disclosure of Invention
The invention aims to solve the problem of system errors of a satellite-borne SAR in the fast distance time and the slow azimuth time, and provides a satellite-borne SAR time parameter calibration method based on a direct positioning algorithm.
In order to achieve the above object, the technical solution of the present invention is to provide a direct positioning algorithm-based calibration method for satellite-borne SAR time parameters, comprising the following steps: the method comprises the following steps: selecting a calibration area and arranging corner reflectors; step two: adjusting the orientation of a corner reflector and measuring the position of a scattering center; step three: imaging and image up-sampling processing; step four: determining the nominal azimuth time and the slope distance of the corner reflector; step five: fitting the phase center position and the speed of the SAR antenna; step six: comprehensively processing the fitting coefficient; step seven: establishing a calibration equation set; step eight: solving a calibration equation set; step nine: atmospheric delay compensation; step ten: solving the azimuth slow time error and the slope error; step eleven: averaging the processing results of the polygon reflector; step twelve: averaging the multiple observation processing results; step thirteen: traversing and calibrating the form of the signals transmitted by the radar system, and finally obtaining the azimuth slow time correction parameters and the slant range correction parameters of the satellite-borne SAR.
Further, the step of selecting the calibration area and arranging the corner reflector specifically comprises: n scaling regions are selected, a corner reflector is arranged at the center of each scaling region, and the signal-to-noise ratio of the corner reflector to the background is not lower than 30 dB.
Further, the step of adjusting the orientation of the corner reflector and measuring the position of the scattering center specifically includes: a satellite observation working plan is made, and corresponding adjustment is carried out on the direction of the corner reflector according to the incidence angle of the satellite-borne SAR and the lower view angle, so that the direction of the strongest scattering of the corner reflector is coincident with the direction of the center of the radar beam; after the pointing direction is adjusted, the scattering center position of the corner reflector is accurately measured, and the coordinates of the corner reflector under the earth-center fixed connection coordinate system are obtained.
Further, the imaging and image up-sampling processing specifically includes: after the satellite works, transmitting wave data and data such as the position, the speed, the attitude and the like of the satellite downwards, and carrying out imaging processing on an echo by a ground processing system to obtain single-view complex image data; in the single-view complex image, each corner reflector is taken as a center, a rectangular window area with the size of L multiplied by L pixels is selected, and M multiplied by M two-dimensional up-sampling processing is carried out; the size L of the rectangular window is 32-128 pixels; and taking 256-1024 upsampling multiples M.
Further, the step of determining the nominal azimuth time and the slope distance of the corner reflector specifically comprises: in the up-sampled image, the scattering center position of the angle reflector is determined according to the position of the maximum energy value, and the nominal slow time t of the angle reflector is determined according to the parameters of radar imaging starting time, echo sampling starting time, pulse repetition frequency, pulse sampling interval, up-sampling multiple and the likeeNominal slope distance reAnd Doppler center f during corner reflector imagingdc。
Further, the step of fitting the phase center position and the speed of the SAR antenna specifically includes: and (3) taking position and speed data containing an imaging time range to perform fitting operation, wherein the position uses P-order fitting, the speed uses P-1-order fitting, and the fitting order P is more than or equal to 3. After fitting, position (x)s,ys,zs) Is represented in the following form by the following formula,
velocity (v)x,vy,vz) Is represented in the following form by the following formula,
the data time used in the fitting process needs to be longer than the radar imaging time, the time sampling interval in the fitting process corresponds to the up-sampling multiple of the image, and 1/(M & PRF) is taken, wherein M is the up-sampling multiple, and PRF is the radar pulse repetition frequency.
Furthermore, in the step of the fitting coefficient comprehensive processing, a comprehensive fitting coefficient calculation formula is as follows,
a0＝a′0；b0＝b′0；c0＝c′0
p＝1,2,...,P
after the fitting coefficients are comprehensively processed, the position and the speed of the phase center of the antenna are expressed in the following form
Further, the step of establishing the calibration equation set specifically includes: according to the SAR distance equation and the Doppler equation, a geometric calibration equation set is established as follows,
in the above formula, (x, y, z) is the corner reflector position, fdcThe Doppler center of a corner reflector in imaging is defined, lambda is the radar wavelength, and the parameters are known quantities; t is tcIs the real value of imaging center time r of corner reflectorcThe two parameters are unknown quantities and are parameters to be solved in the equation set. Modeling the calibration process as an unknown parameter t by establishing the calibration equation setc、rcTo solve the problem.
Further, the method for solving the scaling equation system is as follows: the iterative calculation formula of the (i + 1) th step in the iterative calculation is as follows,
wherein,
the velocity derivative with respect to slow time in the above equation is shown below,
in an iteration, t may be usede、reAs an iteration initial value, | F (t)c,rc)|≤10-3And | G (t)c,rc)|≤10-3As an iteration termination condition.
Further, the step of compensating for the atmospheric delay specifically includes: calculating to obtain an inclined distance error r caused by atmospheric delay according to the imaging time, the satellite position, the radar incident angle and the arrangement position of the corner reflector and through real-time atmospheric mode dataa；
The formula for solving the azimuth slow time error and the slope error is as follows:
Δt＝te-tc；Δr＝re-ra-rc
wherein, Δ t is the azimuth slow time error, and Δ r is the slant distance error.
The method uses a corner reflector with accurately measured arrangement positions as a calibration point to comprehensively fit position and speed data of a radar antenna phase center to obtain a fitting model of the position and the speed, establishes a calibration equation set consisting of a slant-range equation and a Doppler equation, solves the calibration process of the azimuth time and the range slant-range into a direct solving problem of the equation set, and solves the equation set by using an iterative algorithm to directly obtain the azimuth time error and the range slant-range error. After the atmospheric delay error is removed, the solving results of the corner reflectors are averaged, so that the single calibration precision is improved, and the multiple calibration results under the condition of the same time width and bandwidth are averaged, so that the precision can be further improved. And traversing different time-width and bandwidth combinations of the system to finally obtain the geometric calibration parameters of the satellite-borne SAR system.
The invention has the following beneficial effects: the invention provides a direct positioning algorithm-based satellite-borne SAR time parameter calibration method, which is used for calibrating azimuth slow time and slant distance system errors due to the fact that the direct expression of the distance direction fast time errors is slant distance errors, so that the geometric positioning accuracy and the imaging performance of a system are improved.
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In order to more clearly illustrate the embodiments or technical solutions of the present invention, the drawings used in the description of the embodiments will be briefly introduced below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without inventive efforts, wherein:
fig. 1 is a schematic flow chart of a direct positioning algorithm-based calibration method for a satellite-borne SAR time parameter provided by the invention.
Fig. 2 is a schematic diagram of the layout of cross-shaped corner reflectors in the satellite-borne SAR time parameter calibration method based on the direct positioning algorithm provided by the invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1, fig. 1 is a schematic flow chart of a direct positioning algorithm-based calibration method for a satellite-borne SAR time parameter, and the working flow of the method is described in detail below.
The method comprises the following steps: and selecting a calibration area and arranging a corner reflector. And selecting N calibration areas, wherein the calibration areas have weaker radar backscattering coefficients, and the peripheral undulating terrain, artificial buildings and the like can not cover and shield the calibration areas in the SAR image. And a corner reflector is arranged at the central position of each scaling region, and the signal-to-noise ratio of the corner reflector to the background is not lower than 30 dB.
Specifically, the corner reflector may be a three-sided corner reflector, or may be another type of corner reflector such as a two-sided corner reflector. The number of the corner reflectors cannot be less than 2, and the corner reflectors should be distributed dispersedly in the SAR observation range as much as possible, and a schematic diagram of the arrangement of 5 corner reflectors in a cross shape is given in fig. 2.
Step two: corner reflector pointing adjustment and scattering center position measurement. Specifically, a satellite observation work plan is firstly formulated, and corresponding adjustment is carried out on the direction of the corner reflector according to the radar incident angle and the lower view angle, so that the direction of the strongest scattering of the corner reflector is coincident with the direction of the center of a radar beam; after the pointing direction is adjusted, the scattering center position of the corner reflector is accurately measured, and coordinates (x, y, z) of the corner reflector under the earth-center fixed connection coordinate system are obtained.
Specifically, the accuracy of the measurement of the scattering center position of the corner reflector is related to the calibration accuracy, and the measurement accuracy is usually required to be higher than 0.5m (3 σ).
Step three: imaging and image up-sampling processing. Specifically, after the satellite works, wave data and data such as the position, the speed, the attitude and the like of the satellite are sent back downwards, and the ground processing system carries out imaging processing on an echo to obtain single-view complex image data; in a single-view complex image, each corner reflector is taken as a center, a rectangular window with the size of L multiplied by L pixels is selected, and M multiplied by M times of two-dimensional up-sampling processing is carried out through a Fourier transform method; the size L of the rectangular window is 32-128 pixels; and taking 256-1024 upsampling multiples M.
Specifically, the window size and the upsampling multiple are related to the scaling precision and the calculation amount, generally, the window size L is 64, and the upsampling multiple M is 512, so that excessive calculation amount cannot be increased on the premise of ensuring precision.
Step four: the corner reflector nominal azimuth time and slope distance are determined. Specifically, in the image after the up-sampling, the scattering center position of the corner reflector is determined according to the position of the maximum energy value, and the nominal slow time t of the corner reflector is determined according to the parameters such as the radar imaging starting time, the echo sampling starting time, the pulse repetition frequency, the pulse sampling interval, the up-sampling multiple and the likeeNominal slope distance reAnd Doppler center f during corner reflector imagingdc。
Step five: and (5) fitting the phase center position and the speed of the SAR antenna. Specifically, fitting operation is carried out on position and speed data including an imaging time range, P-order fitting is used for the position, P-1-order fitting is used for the satellite speed, and the fitting order P is more than or equal to 3.
In specific implementation, the fitting order P is usually 3, so that better fitting accuracy can be obtained. Position (x) when the fitting order is 3s,ys,zs) Is represented in the following form by the following formula,
velocity (v)x,vy,vz) Is represented in the following form by the following formula,
the data time used in the fitting is longer than the radar imaging time, the time sampling interval in the fitting process is corresponding to the up-sampling multiple of the image, and 1/(M & PRF) is taken, wherein M is the up-sampling multiple, and PRF is the radar pulse repetition frequency.
The position and speed data directly recorded by the satellite are usually the position and speed of the navigation device, so in actual operation, the position and speed of the phase center of the SAR antenna are firstly obtained according to the installation position parameters of the satellite navigation device and the SAR antenna in the satellite body coordinate system.
Step six: and comprehensively processing the fitting coefficients. In order to improve the position and speed fitting accuracy of the antenna phase center, the coefficients after position and speed fitting are averaged to obtain a final fitting result. The overall fitting coefficient calculation formula is as follows,
a0＝a′0；b0＝b′0；c0＝c′0
p＝1,2,...,P
after the fitting coefficients are comprehensively processed, the position and the speed of the phase center of the antenna are expressed in the following form
Taking the fitting order P as 3 as an example, the final fitting coefficient calculation formula is as follows,
in this case, after the fitting coefficient integration processing, the antenna phase center position and velocity are expressed in the following form.
Step seven: and establishing a calibration equation set. According to the SAR distance equation and the Doppler equation, the following geometric calibration equation system is established,
in the above formula, (x, y, z) is the corner reflector position, fdcThe Doppler center of a corner reflector in imaging is defined, lambda is the radar wavelength, and the parameters are known quantities; t is tcIs the real value of imaging center time r of corner reflectorcThe two parameters are unknown quantities and are parameters to be solved in the equation set.
Step eight: and solving a calibration equation system. Solving a positioning equation by using an iterative algorithm to obtain the imaging center time t of the corner reflectorcAnd an oblique distance rc。
Specifically, the iterative calculation formula of the (i + 1) th step in the iterative calculation is as follows,
wherein,
the velocity derivative with respect to slow time in the above equation is shown below,
when the fitting order P is 3, the equation for the velocity derivative with respect to slow time in the above equation is shown below,
in an iteration, t is typically usede、reAs an iteration initial value, | F (t)c,rc)|≤10-3And | G (t)c,rc)|≤10-3As an iteration termination condition.
Step nine: and (4) compensating atmospheric delay. Calculating to obtain an inclined distance error r caused by atmospheric delay through real-time atmospheric mode data according to parameters such as imaging time, satellite position, radar incident angle, arrangement position of a corner reflector and the likea。
Step ten: and solving the azimuth slow time error and the slope error. The slow time error delta t and the slant distance error delta r of the system azimuth direction are solved through the calculation results, and the calculation formula is shown as follows.
Δt＝te-tc；Δr＝re-ra-rc
Step eleven: the results of the polygon reflector processing are averaged. Specifically, the operations are performed on each corner reflector, so that the solving results of N azimuth slow time errors and slope errors can be obtained, the results are subjected to average processing, and the estimation accuracy is improved. The formula for the averaging process is:
step twelve: the results of multiple observation treatments were averaged. Specifically, wave positions with the same time width and bandwidth of radar transmission signals are used, a corner reflector is imaged for multiple times, the operations are repeated, and the processing result is averaged to improve the error estimation precision.
Step thirteen: traversing the combination of the time and the bandwidth of the signal transmitted by the radar system, finally obtaining the azimuth slow time correction parameter and the slant range correction parameter of the satellite-borne SAR, and compensating the azimuth slow time correction parameter and the slant range correction parameter in a ground processing system so as to improve the positioning accuracy and the imaging effect of the system.
The invention also provides a device with a storage function, wherein program data are stored on the device, the program data are executed by a processor, and the satellite-borne SAR time parameter calibration method based on the direct positioning algorithm is disclosed by the invention.
The device with storage function may be at least one of a server, a floppy disk drive, a hard disk drive, a CD-ROM reader, a magneto-optical disk reader, and the like.
The invention has the following beneficial effects: the invention provides a direct positioning algorithm-based satellite-borne SAR time parameter calibration method, which is used for calibrating azimuth slow time and slant distance system errors due to the fact that the direct expression of the distance direction fast time errors is slant distance errors, so that the geometric positioning accuracy and the imaging performance of a system are improved.
The above description is only an embodiment of the present invention, and not intended to limit the scope of the present invention, and all modifications of equivalent structures and equivalent processes, which are made by the present specification, or directly or indirectly applied to other related technical fields, are included in the scope of the present invention.
Claims (6)
1. A direct positioning algorithm-based satellite-borne SAR time parameter calibration method is characterized by comprising the following steps:
the method comprises the following steps: selecting a calibration area and arranging corner reflectors, wherein the steps of selecting the calibration area and arranging the corner reflectors specifically comprise: selecting N scaling regions, and arranging a corner reflector at the center of each scaling region, wherein the signal-to-noise ratio of the corner reflector to the background is not lower than 30 dB;
step two: the method specifically comprises the following steps of corner reflector pointing adjustment and scattering center position measurement: a satellite observation working plan is made, and corresponding adjustment is carried out on the direction of the corner reflector according to the incidence angle of the satellite-borne SAR and the lower view angle, so that the direction of the strongest scattering of the corner reflector is coincident with the direction of the center of the radar beam; after the pointing direction is adjusted, accurately measuring the scattering center position of the corner reflector to obtain coordinates (x, y, z) of the corner reflector under a geocentric fixed connection coordinate system;
step three: imaging and image up-sampling processing, wherein the imaging and image up-sampling processing specifically comprises the following steps: after the satellite works, transmitting wave data and satellite position, speed and attitude data downwards, and performing imaging processing on echoes by a ground processing system to obtain single-view complex image data; in the single-view complex image, each corner reflector is taken as a center, a rectangular window area with the size of L multiplied by L pixels is selected, and M multiplied by M two-dimensional up-sampling processing is carried out; the size L of the rectangular window is 32-128 pixels; taking 256-1024 upsampling times M;
step four: determining the nominal azimuth time and the slope distance of the corner reflector specifically comprises the following steps: in the up-sampled image, the scattering center position of the corner reflector is determined according to the position of the maximum energy value, and the nominal slow time t of the corner reflector is determined according to the radar imaging starting time, the echo sampling starting time, the pulse repetition frequency, the pulse sampling interval and the up-sampling multiple parameterseNominal slope distance reAnd Doppler center f during corner reflector imagingdc；
Step five: fitting the phase center position and the speed of the SAR antenna;
step six: comprehensively processing the fitting coefficient;
step seven: establishing a calibration equation set;
step eight: solving a calibration equation set;
step nine: atmospheric delay compensation;
step ten: solving the azimuth slow time error and the slope error;
step eleven: averaging the per corner reflector processing results;
step twelve: averaging the multiple observation processing results;
step thirteen: traversing the combination of the time and the bandwidth of the signal transmitted by the radar system, and finally obtaining the azimuth slow time correction parameter and the slant range correction parameter of the satellite-borne SAR.
2. The direct positioning algorithm-based time parameter calibration method for the satellite-borne SAR, as recited in claim 1, wherein the step of fitting the phase center position and the speed of the SAR antenna specifically comprises: taking position and speed data containing an imaging time range to perform fitting operation, wherein the position uses P-order fitting, the speed uses P-1-order fitting, the fitting order P is more than or equal to 3, and when the fitting order is 3, the position (x)s,ys,zs) Is represented in the following form by the following formula,
velocity (v)x,vy,vz) Is represented in the following form by the following formula,
the data time used in the fitting is longer than the radar imaging time, the time sampling interval in the fitting process is corresponding to the up-sampling multiple of the image, and 1/(M & PRF) is taken, wherein M is the up-sampling multiple, and PRF is the radar pulse repetition frequency.
3. The method for calibrating the time parameter of the satellite-borne SAR based on the direct positioning algorithm as claimed in claim 2, wherein in the step of the fitting coefficient synthesis processing, the synthetic fitting coefficient calculation formula is as follows,
a0＝a′0；b0＝b′0；c0＝c′0
p＝1,2,...,P
after the fitting coefficients are comprehensively processed, the position and the speed of the phase center of the antenna are expressed in the following form
4. The method for calibrating the satellite-borne SAR time parameter based on the direct positioning algorithm as claimed in claim 3, wherein the step of establishing the calibration equation set specifically comprises: according to the SAR distance equation and the Doppler equation, a geometric calibration equation set is established as follows,
in the above formula, (x, y, z) is the corner reflector position, fdcThe Doppler center of a corner reflector in imaging is defined, lambda is the radar wavelength, and the parameters are known quantities; t is tcIs the real value of imaging center time r of corner reflectorcThe two parameters are unknown quantities and are parameters to be solved in an equation set, and a calibration process is modeled into an unknown parameter t by establishing the calibration equation setc、rcTo solve the problem.
5. The direct positioning algorithm-based time parameter calibration method for the satellite-borne SAR, as recited in claim 4, wherein the step of solving the calibration equation set specifically comprises: solving a positioning equation by using an iterative algorithm to obtain the imaging center time t of the corner reflectorcAnd an oblique distance rcThe iterative calculation formula is as follows,
wherein,
the velocity derivative with respect to slow time in the above equation is shown below,
in the iteration, use te、reAs an iteration initial value, | F (t)c,rc)|≤10-3And | G (t)c,rc)|≤10-3As an iteration termination condition.
6. The direct positioning algorithm-based calibration method for the satellite-borne SAR time parameters according to claim 5, characterized in that the atmospheric delay compensation step specifically comprises: calculating to obtain an inclined distance error r caused by atmospheric delay according to the imaging time, the satellite position, the radar incident angle and the arrangement position of the corner reflector and through real-time atmospheric mode dataa；
The formula for solving the azimuth slow time error and the slope error is as follows:
Δt＝te-tc；Δr＝re-ra-rc
wherein, Δ t is the azimuth slow time error, and Δ r is the slant distance error.
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