CN112946592B - Doppler correction method and system for SAR along with distance space variation - Google Patents
Doppler correction method and system for SAR along with distance space variation Download PDFInfo
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Abstract
The invention discloses a Doppler correction method and a Doppler correction system for SAR along with distance space-variant, and relates to the technical field of SAR imaging. The method comprises the following steps: establishing a two-dimensional error Doppler calculation model; processing the two-dimensional error Doppler calculation model, and removing the error value of the roll angle; carrying out distance dimension block processing on any scene SAR data, respectively estimating the Doppler center frequency of each block, and calculating the estimated value of an azimuth angle; and obtaining a corrected Doppler value according to the estimated value of the azimuth angle and the one-dimensional error Doppler calculation model. The Doppler correction method provided by the invention is suitable for SAR imaging, reduces the complexity of a two-dimensional error Doppler calculation model, improves the calculation efficiency, is suitable for Doppler calculation of batch SAR data along with space variation of distance, and meets the requirements of batch, high-efficiency processing and high-precision imaging.
Description
Technical Field
The invention relates to the technical field of SAR imaging, in particular to a Doppler correction method and a Doppler correction system for SAR along with space variation of distance.
Background
Due to the non-uniformity of the flight speed and the variation of the beam pointing angle, the doppler center frequency of the airborne SAR is generally space-variant along the azimuth. However, in an actual system, there is a possibility that the doppler center frequency becomes space-variant along the distance dimension for various reasons. Generally, irradiation of radar beams in a three-dimensional space needs to be controlled from two dimensions, namely an azimuth dimension and a distance dimension, so that the beams can be well prevented from being distorted in the space, and the pointing direction can be adjusted at any time according to the attitude of an airplane. If the control of the wave beam in the three-dimensional space is only one-dimensionally controllable, for example, no servo is provided, the linear array antenna is only electronically scanned along the phased array of the azimuth dimension, and the azimuth wave beam width is narrow, for example, within 1 degree, when the system is installed on a medium and small platform with unstable airplane attitude, the phenomenon of space variation of Doppler center frequency along the distance dimension is particularly easy to occur.
However, at present, the doppler of each azimuth position and distance position is usually obtained by an estimation method during the imaging process, and the calculation efficiency of the method is too low to meet the processing requirement of batch data.
Disclosure of Invention
The invention aims to solve the technical problem of the prior art and provides a Doppler correction method and a Doppler correction system for SAR along with space variation of distance.
The technical scheme for solving the technical problems is as follows:
a range-space-variant doppler correction method for SAR, comprising:
establishing a two-dimensional error Doppler calculation model, wherein the two-dimensional error Doppler calculation model comprises an azimuth error value and a roll angle error value;
processing the two-dimensional error Doppler calculation model according to the distance relation between the SAR and the target, and removing the error value of the roll angle to obtain a one-dimensional error Doppler calculation model;
acquiring SAR data of any scene, carrying out distance dimension block processing on the SAR data, respectively estimating the Doppler center frequency of each block, and calculating the estimation value of an azimuth angle according to the Doppler center frequency of each block;
and obtaining a corrected Doppler value according to the estimated value of the azimuth angle and the one-dimensional error Doppler calculation model.
Another technical solution of the present invention for solving the above technical problems is as follows:
a range-space-variant doppler correction system for SAR, comprising:
the device comprises a first modeling unit, a second modeling unit and a third modeling unit, wherein the first modeling unit is used for establishing a two-dimensional error Doppler calculation model, and the two-dimensional error Doppler calculation model comprises an azimuth error value and a roll angle error value;
the second modeling unit is used for processing the two-dimensional error Doppler calculation model according to the distance relation between the SAR and a target, removing the error value of the roll angle and obtaining a one-dimensional error Doppler calculation model;
the processing unit is used for acquiring SAR data of any scene, performing distance dimension blocking processing on the SAR data, respectively estimating the Doppler center frequency of each block, and calculating the estimation value of an azimuth angle according to the Doppler center frequency of each block;
and the correction unit is used for obtaining a corrected Doppler value according to the estimated value of the azimuth angle and the one-dimensional error Doppler calculation model.
The invention has the beneficial effects that: according to the Doppler correction method provided by the invention, the roll angle error in the two-dimensional error Doppler calculation model is optimized and simplified by combining with the SAR imaging geometric relation to obtain the one-dimensional error Doppler calculation model, only the one-dimensional antenna installation attitude error is obtained by estimation according to scene data, and the Doppler for imaging can be realized by combining with the one-dimensional error Doppler calculation model, so that the complexity of the two-dimensional error Doppler calculation model is reduced, the calculation efficiency is improved, the method is suitable for Doppler calculation of batch SAR data with space variation along with distance, and the requirements of batch, high-efficiency processing and high-precision imaging are met.
Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
Drawings
FIG. 1 is a schematic flow chart diagram of a Doppler correction method with distance space-variant according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of measured data Doppler two-dimensional space-variant provided by an embodiment of the Doppler correction method with distance space-variant of the present invention;
FIG. 3 is a schematic diagram of a Doppler distance dimensional space variation provided by an embodiment of the Doppler correction method with distance space variation according to the present invention;
FIG. 4 is a diagram illustrating the difference between the Doppler estimated value and the calculated value according to the embodiment of the Doppler correction method with distance space-variant of the present invention;
FIG. 5 is a schematic diagram of an image location spectrum corrected according to Doppler calculated values according to an embodiment of the Doppler correction method with distance space-variant of the present invention;
fig. 6 is a schematic structural framework diagram of the doppler correction system with space-variant distance according to the embodiment of the present invention.
Detailed Description
The principles and features of the present invention will be described with reference to the following drawings, which are illustrative only and are not intended to limit the scope of the invention.
As shown in fig. 1, a schematic flowchart is provided for an embodiment of the distance-dependent space-variant doppler correction method according to the present invention, where the distance-dependent space-variant doppler correction method is used for SAR, and includes:
s1, establishing a two-dimensional error Doppler calculation model, wherein the two-dimensional error Doppler calculation model comprises an azimuth angle error value and a roll angle error value.
In the SAR described above, a radar beam is emitted from an antenna, reaches a ground target, and returns to the antenna, and a unit vector between the radar and the target and a velocity vector of the radar in a three-dimensional space are related by definition. The airborne equipment has larger vibration during operation, so that the installation attitude angle of the antenna is time-varying, and measurement errors exist in the unit vector and the velocity vector due to various reasons, so that the calculated Doppler is not accurate, especially for a high-frequency range radar system, the wavelength is smaller, and the calculation error caused by the inaccurate measurement of the unit vector and the velocity vector is even larger than the azimuth bandwidth, which causes serious influence on imaging.
Because the current SAR generally adopts high-precision inertial measurement equipment, the influence of the measurement error of the radar speed on Doppler can be ignored. However, the influence of the vibration of the medium and small flying platforms on the antenna attitude is not negligible in the high-frequency band and narrow-beam radar system. The measurement error of an instrument of the antenna attitude and the change error of the antenna attitude caused by the aircraft vibration can be collectively called as attitude error, so that a two-dimensional error Doppler calculation model containing the error value of the azimuth angle and the error value of the roll angle is firstly established, and if the attitude error value is known, the accurate Doppler frequency can be calculated.
The Doppler of the SAR is space-variant along with the direction, and for a Doppler SAR system which is space-variant along with the distance, the Doppler frequency is two-dimensional space-variant along with the distance and the direction and exactly corresponds to two-dimensional errors of an antenna attitude azimuth angle and a roll angle of a two-dimensional error Doppler calculation model.
As shown in fig. 2, an exemplary actually measured doppler schematic diagram varying with distance and azimuth in two dimensions is provided, in fig. 2, the abscissa is the azimuth doppler frequency, the ordinate is the number of distance points, a true doppler value can be obtained through a scene of SAR data, and then substituted into a two-dimensional error doppler calculation model, so that errors of an azimuth angle and a roll angle can be reversely deduced, and further, the doppler of all other strip data of the frame can be calculated and imaged according to the obtained angle error value and the two-dimensional error doppler calculation model.
Although the two-dimensional errors of the attitude azimuth angle and the roll angle can be obtained by resolving through the constructed two-dimensional error Doppler calculation model, the calculation amount is large, the efficiency is not high, and the modeling is complex, so that the obtained two-dimensional error Doppler calculation model is continuously processed as follows.
And S2, processing the two-dimensional error Doppler calculation model according to the distance relation between the SAR and the target, and removing the error value of the roll angle to obtain the one-dimensional error Doppler calculation model.
It should be noted that SAR imaging is an approximation process, and both accuracy and efficiency are considered. Likewise, the doppler center calculation also allows for some error. Based on this, the two-dimensional error Doppler calculation model can be simplified by combining the SAR imaging geometric relation.
And calculating a true roll angle through the height between the radar and the ground and an included angle between a connecting line of the radar and the target and a vertical line, and replacing an initial roll angle and a roll angle error in the two-dimensional error Doppler calculation model, so that only one attitude error, namely an azimuth error, exists in the two-dimensional error Doppler calculation model to obtain the one-dimensional error Doppler calculation model.
And S3, acquiring SAR data of any scene, carrying out distance dimension blocking processing on the SAR data, respectively estimating the Doppler center frequency of each block, and calculating the estimation value of the azimuth angle according to the Doppler center frequency of each block.
For example, the size may be 512 range gates, the SAR data may be partitioned into blocks, and there may be no overlap between blocks.
Optionally, after obtaining the doppler center frequency of each block, the doppler center frequency can be substituted into a one-dimensional error doppler calculation model to reversely deduce the estimated value α of the azimuth angle 0 +Δα。
It should be noted that the block processing of the SAR data belongs to the prior art, the SAR data may be divided into a preset number of range block data according to a distance dimension by a preset program, the doppler center frequency of each block is estimated by the existing doppler center frequency estimation method, and then the estimation value of the attitude error, that is, the estimation value of the azimuth angle, is obtained by the bisection method or the least square method.
And S4, obtaining a corrected Doppler value according to the estimated value of the azimuth angle and the one-dimensional error Doppler calculation model.
As shown in fig. 3, a doppler distance dimension space-variant diagram is given, in which distance-divided small blocks are used as abscissa and doppler is used as ordinate, in the diagram, a solid line represents a doppler estimation value, a dotted line represents a corrected doppler calculation value calculated by the method provided by the present invention, fig. 4 is a difference value between the doppler calculation value and the estimation value, and an ordinate of fig. 4 is a doppler difference value between the calculation value and the estimation value of fig. 3, and it can be seen from fig. 4 that a doppler error value fluctuates around 0Hz and a fluctuation range is substantially within an allowable range.
It should be understood that the doppler estimation value indicated by the solid line in fig. 3 can be obtained by a doppler estimation algorithm in the SAR data processing, and the doppler estimation algorithm alone is mature and will not be described herein again.
Fig. 5 shows an exemplary corrected image orientation spectrum according to the calculated doppler value, and comparing fig. 1 and 5 shows that the distance space variation of doppler is corrected.
According to the Doppler correction method provided by the embodiment, through combining with the SAR imaging geometric relation, the roll angle error in the two-dimensional error Doppler calculation model is optimized and simplified to obtain the one-dimensional error Doppler calculation model, according to scene data, only the one-dimensional antenna installation attitude error needs to be obtained through estimation, the Doppler used for imaging can be realized by combining with the one-dimensional error Doppler calculation model, the complexity of the two-dimensional error Doppler calculation model is reduced, the calculation efficiency is improved, the method is suitable for Doppler calculation of SAR data in batches along with space variation of distance, and the requirements of batch, efficient processing and high-precision imaging are met.
Optionally, in some possible embodiments, the establishing a two-dimensional error doppler calculation model specifically includes:
in an airborne SAR, radar beams are emitted from an antenna, reach a ground target and then return to the antenna, and a Doppler calculation model of the SAR is established according to definition: .
Initial measurement alpha of azimuth angle of antenna attitude 0 Initial measurement value r of roll angle 0 And introducing the attitude error value delta alpha of the azimuth angle and the attitude error value delta r of the roll angle into a Doppler calculation model to obtain a two-dimensional error Doppler calculation model:
wherein f is dc Is the doppler value, λ is the wavelength, R is the unit vector between the radar and the target, and V is the velocity vector of the radar in three-dimensional space.
It will be appreciated that the unit vector R between the radar and the target and the velocity vector V of the radar in three-dimensional space should be within the same coordinate system.
Optionally, in some possible embodiments, the processing the two-dimensional error doppler computation model according to the distance relationship between the SAR and the target, and removing an error value of a roll angle to obtain a one-dimensional error doppler computation model specifically includes:
acquiring the height H between the radar and the ground and the included angle R between the connecting line of the radar and the target and the vertical line r Then, the true roll angle corresponding to the unit vector R is:
substituting the real roll angle corresponding to the unit vector R into the two-dimensional error Doppler calculation model to obtain a one-dimensional error Doppler calculation model:
wherein r is 0 + Δ R is the true roll angle corresponding to the unit vector R, and the right side of the true roll angle equation is the range gate corresponding to the image.
It should be noted that the height H between the radar and the ground and the included angle R between the connecting line between the radar and the target and the vertical line r This can be measured by a ranging tool, for example, by a laser ranging device on the SAR.
Optionally, in some possible embodiments, calculating an estimate of the azimuth angle according to the doppler center frequency of each block specifically includes:
and fitting the Doppler center frequency of each block by a bisection method search or a least square method to obtain an estimated value of the azimuth angle.
For example, according to a one-dimensional error Doppler calculation model, the azimuth error amount Δ α and the Doppler center frequency f of each block dc There is a one-to-one correspondence of functional relationships that vary monotonically over a small range. Obtaining different values of delta alpha according to a one-dimensional error Doppler calculation model to obtain different f dc Observation f dc The change rule of (2) can search the best estimated value of delta alpha by dichotomy, and the Doppler f calculated according to the one-dimensional error Doppler calculation model when the delta alpha is used as the estimated value dc The estimated value alpha of the azimuth angle is obtained by being closest to the Doppler estimated along the distance block, and the error value delta alpha of the azimuth angle can be regarded as a real error value at the moment 0 +Δα。
Optionally, in some possible embodiments, the method further includes:
and imaging according to the corrected Doppler value.
In the steps, the Doppler changing along with the distance can be quickly corrected only through the one-dimensional attitude error, so that the method is suitable for SAR Doppler calculation imaging with high efficiency in batches.
It is to be understood that some or all of the various embodiments described above may be included in some embodiments.
As shown in fig. 6, a schematic structural framework diagram is provided for an embodiment of the distance-dependent space-variant doppler correction system of the present invention, where the distance-dependent space-variant doppler correction system is used for SAR, and the system includes:
the first modeling unit 1 is used for establishing a two-dimensional error Doppler calculation model, and the two-dimensional error Doppler calculation model comprises an azimuth angle error value and a roll angle error value;
the second modeling unit 2 is used for processing the two-dimensional error Doppler calculation model according to the distance relation between the SAR and the target, removing the error value of the roll angle and obtaining a one-dimensional error Doppler calculation model;
the processing unit 3 is used for acquiring SAR data of any scene, performing distance dimension block processing on the SAR data, respectively estimating the Doppler center frequency of each block, and calculating the estimation value of the azimuth angle according to the Doppler center frequency of each block;
and the correction unit 4 is used for obtaining a corrected Doppler value according to the estimated value of the azimuth angle and the one-dimensional error Doppler calculation model.
It should be noted that, the above units may be integrated in a terminal device, for example, the units may be a computer in an SAR signal receiving and processing system, the processing unit 3 may be wirelessly connected to the SAR echo receiving device through a communication unit, such as a 4G module, a 5G module, or a WIFI module, or may be in wired connection with the SAR echo receiving device through a data line and a communication interface, the processing unit 3 acquires SAR data from the SAR echo receiving device, and after correcting doppler, imaging may be performed through a display console.
The Doppler correction system provided by the embodiment optimizes and simplifies the roll angle error in the two-dimensional error Doppler calculation model by combining the SAR imaging geometric relation to obtain the one-dimensional error Doppler calculation model, only one-dimensional antenna installation attitude error needs to be obtained by estimation according to scene data, doppler for imaging can be realized by combining the one-dimensional error Doppler calculation model, the complexity of the two-dimensional error Doppler calculation model is reduced, the calculation efficiency is improved, the system is suitable for Doppler calculation of batch SAR data along with space variation of distance, and the requirements of batch, high-efficiency processing and high-precision imaging are met.
Optionally, in some possible embodiments, the first modeling unit 1 is specifically configured to establish a doppler calculation model of the SAR:
initial measurement alpha of azimuth angle of antenna attitude 0 Initial measurement value r of roll angle 0 And introducing the attitude error value delta alpha of the azimuth angle and the attitude error value delta r of the roll angle into a Doppler calculation model to obtain a two-dimensional error Doppler calculation model:
wherein f is dc Is the doppler value, λ is the wavelength, R is the unit vector between the radar and the target, and V is the velocity vector of the radar in three-dimensional space.
Optionally, in some possible embodiments, the second modeling unit 2 is specifically configured to obtain a height H between the radar and the ground and an angle R between a line connecting the radar and the target and a vertical line r Then, the true roll angle corresponding to the unit vector R is:
substituting the real roll angle corresponding to the unit vector R into the two-dimensional error Doppler calculation model to obtain a one-dimensional error Doppler calculation model:
wherein r is 0 And + deltar is the true roll angle corresponding to the unit vector R.
Optionally, in some possible embodiments, the processing unit 3 is specifically configured to find an estimate of the azimuth angle by fitting the doppler center frequency of each block by a binary search or a least-squares method.
Optionally, in some possible embodiments, the method further includes:
and the imaging unit is used for imaging according to the corrected Doppler value.
It is to be understood that some or all of the various embodiments described above may be included in some embodiments.
It should be noted that the above embodiments are product embodiments corresponding to previous method embodiments, and for the description of the product embodiments, reference may be made to corresponding descriptions in the above method embodiments, and details are not repeated here.
The reader should understand that in the description of this specification, reference to the description of the terms "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.
In the several embodiments provided in the present application, it should be understood that the disclosed apparatus and method may be implemented in other ways. For example, the above-described method embodiments are merely illustrative, and for example, the division of steps into only one logical functional division may be implemented in practice in another way, for example, multiple steps may be combined or integrated into another step, or some features may be omitted, or not implemented.
While the invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (6)
1. A range-space-variant doppler correction method for SAR, comprising:
establishing a two-dimensional error Doppler calculation model, wherein the two-dimensional error Doppler calculation model comprises an azimuth error value and a roll angle error value;
processing the two-dimensional error Doppler calculation model according to the distance relation between the SAR and the target, and removing the error value of the roll angle to obtain a one-dimensional error Doppler calculation model;
acquiring SAR data of any scene, carrying out distance dimension block processing on the SAR data, respectively estimating the Doppler center frequency of each block, and calculating the estimation value of an azimuth angle according to the Doppler center frequency of each block;
obtaining a corrected Doppler value according to the estimated value of the azimuth angle and the one-dimensional error Doppler calculation model;
establishing a two-dimensional error Doppler calculation model, which specifically comprises the following steps:
establishing a Doppler calculation model of SAR:
initial measurement alpha of azimuth angle of antenna attitude 0 Initial measurement value r of roll angle 0 And introducing the attitude error value delta alpha of the azimuth angle and the attitude error value delta r of the roll angle into the Doppler calculation model to obtain a two-dimensional error Doppler calculation model:
wherein f is dc Is a Doppler value, λ is a wavelength, R is a unit vector between the radar and the target, and V is a velocity vector of the radar in three-dimensional space;
according to the distance relation between the SAR and the target, the two-dimensional error Doppler calculation model is processed, the error value of the roll angle is removed, and the one-dimensional error Doppler calculation model is obtained, and the method specifically comprises the following steps:
acquiring the height H between a radar and the ground and the included angle R between a connecting line of the radar and a target and a vertical line r Then, the true roll angle corresponding to the unit vector R is:
substituting the real roll angle corresponding to the unit vector R into the two-dimensional error Doppler calculation model to obtain a one-dimensional error Doppler calculation model:
wherein r is 0 And + deltar is the true roll angle corresponding to the unit vector R.
2. The method for spatially varying range-dependent doppler correction for a SAR of claim 1, wherein calculating an estimate of an azimuth angle from the doppler center frequency of each block comprises:
and fitting the Doppler center frequency of each block by a bisection method search or a least square method to obtain an estimated value of the azimuth angle.
3. The method of range-space-variant doppler correction for SAR of claim 1 or 2, further comprising:
and imaging according to the corrected Doppler value.
4. A range-space-variant doppler correction system for SAR, comprising:
the device comprises a first modeling unit, a second modeling unit and a third modeling unit, wherein the first modeling unit is used for establishing a two-dimensional error Doppler calculation model, and the two-dimensional error Doppler calculation model comprises an azimuth error value and a roll angle error value;
the second modeling unit is used for processing the two-dimensional error Doppler calculation model according to the distance relation between the SAR and a target, removing the error value of the roll angle and obtaining a one-dimensional error Doppler calculation model;
the processing unit is used for acquiring SAR data of any scene, performing distance dimension blocking processing on the SAR data, respectively estimating the Doppler center frequency of each block, and calculating the estimation value of an azimuth angle according to the Doppler center frequency of each block;
the correction unit is used for obtaining a corrected Doppler value according to the estimated value of the azimuth angle and the one-dimensional error Doppler calculation model;
the first modeling unit is specifically configured to establish a doppler calculation model of the SAR:
initial measurement alpha of azimuth angle of antenna attitude 0 Initial measurement value r of roll angle 0 The attitude error value delta alpha of the azimuth angle and the attitude error value delta r of the roll angle are introduced into the Doppler calculation model to obtain a two-dimensional error DopplerLe calculation model:
wherein f is dc Is a Doppler value, λ is a wavelength, R is a unit vector between the radar and the target, and V is a velocity vector of the radar in three-dimensional space;
the second modeling unit is specifically used for acquiring the height H between the radar and the ground and the included angle R between a connecting line of the radar and the target and a vertical line r If the true roll angle corresponding to the unit vector R is:
substituting the real roll angle corresponding to the unit vector R into the two-dimensional error Doppler calculation model to obtain a one-dimensional error Doppler calculation model:
wherein r is 0 And + deltar is the true roll angle corresponding to the unit vector R.
5. The system of claim 4, wherein the processing unit is specifically configured to obtain the estimated value of the azimuth by fitting the doppler center frequency of each block by a binary search or a least-squares method.
6. The range-space-variant Doppler correction system for SAR according to claim 4 or 5, further comprising:
and the imaging unit is used for imaging according to the corrected Doppler value.
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