CN114236622B - Method for disambiguating Faraday rotation angle estimated value in spaceborne P-band PolSAR image - Google Patents
Method for disambiguating Faraday rotation angle estimated value in spaceborne P-band PolSAR image Download PDFInfo
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Abstract
The invention discloses a method for disambiguating an estimated value of a Faraday rotation angle in a satellite-borne P-band PolSAR image, which comprises the following steps: s1, performing fast Fourier transform on a satellite-borne P-band PolSAR image along a distance direction to obtain a PolSAR image of a distance frequency domain; s2, carrying out frequency domain Faraday rotation angle calculation on the PolSAR image of the distance frequency domain by using a B & B estimator; s3, judging whether inconsistent ambiguity exists in the Faraday rotation angle, if so, carrying out ambiguity consistency processing, otherwise, executing the step S4; s4, calculating a change value of a Faraday rotation angle in the bandwidth, and calculating ambiguity; s5, obtaining a change curve of the Faraday rotation angle relative to the distance frequency according to the ambiguity calculated in the step S4. The invention can realize the self-adaptive deblurring of the FRA estimated value of the satellite-borne P-band PolSAR image, thereby obtaining an accurate curve of the FRA along with the change of the distance frequency.
Description
Technical Field
The invention relates to the technical field of space-based microwave remote sensing and geophysical intersection, in particular to a method for disambiguating Faraday rotation angle estimated values in a satellite-borne P-band PolSAR image.
Background
The satellite-borne P-band PolSAR system has become one of the important directions of the development of the satellite-borne SAR in recent years due to the good penetration capability of the satellite-borne P-band PolSAR system to leaf clusters and shallow earth surface, and the band system can play an important role in the fields of global biomass inversion, carbon water circulation monitoring, military target reconnaissance and the like. However, the ionosphere FR (Faraday Rotation) effect is quite severe due to operation in the P-band, and the single pass FRA (FR Angle, faraday Rotation) for the peak year of solar activity can reach 321 degrees. FR effects can lead to polarization measurement errors, affecting the inversion performance of the feature characteristic parameters. In addition, the FR effect causes a reduction in interference phase coherence, and this decorrelation effect is actually due to aliasing of the data of each polarized channel caused by the FR effect.
Based on the combination of elements of the polarization cross-correlation matrix, domestic and foreign scholars propose a plurality of FR estimators, such as estimators of B & B, freeman, so that the estimation of FRA in the PolSAR data is realized, and the method is successfully applied to the L-band PolSAR actual measurement data (ALOS PALSAR). However, these FR estimators all have the problem of blurring the FRA estimate, i.e. the FRA estimate differs from the true value by n pi/2, n being an integer, called ambiguity. Because the FRA of the L-band system is generally not more than 40 degrees, the FRA estimated value blurring does not need to be considered, and the existing FR estimator can be directly applied to the L-band PolSAR measured data; however, the FRA of the P-band system can reach 321 degrees, so that the ambiguity of the FRA estimation value must be considered, otherwise, the accurate FRA cannot be estimated. In order to solve the problem, a plurality of deblurring methods based on a scaling target, a specific feature and a constraint FRA are proposed by a learner, but the methods mainly depend on a specific scene, so that the application range is limited.
Disclosure of Invention
The invention aims to provide a method for disambiguating an estimated value of a Faraday rotation angle in a satellite-borne P-band PolSAR image, which aims to overcome the defects in the prior art.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
The method for disambiguating the Faraday rotation angle estimated value in the spaceborne P-band PolSAR image comprises the following steps:
S1, performing fast Fourier transform on a satellite-borne P-band PolSAR image M along a distance direction to obtain a PolSAR image M of a distance frequency domain;
S2, carrying out frequency domain Faraday rotation angle calculation on the PolSAR image M of the distance frequency domain by using a B & B estimator;
s3, judging whether inconsistent ambiguity exists in the Faraday rotation angle, if so, carrying out ambiguity consistency processing, otherwise, executing the step S4;
s4, calculating a change value of a Faraday rotation angle in the bandwidth, and calculating ambiguity;
s5, obtaining a change curve of the Faraday rotation angle relative to the distance frequency according to the ambiguity calculated in the step S4.
Further, the formula for calculating the faraday rotation angle in the frequency band in the step S2 is as follows:
Wherein arg () is an angular function, Representing windowed averages, M a X1 is the window size, Z hv、Zvh is the equivalent circularly polarized substrate Z corresponds to two cross channel substrates, Z is expressed as:
Further, the step S3 specifically includes:
Firstly, calculating the difference omega 'B&B[i]＝ΩB&B[i]-ΩB&B[i-1],i＝2,3,...,Nr of Faraday rotation angles, if omega' B&B [ i ] has different value points larger than pi/4, then inconsistent ambiguity exists, and continuing to execute;
Then, an ambiguity consistency process is performed, including: if there is a outlier point, its coordinate is i 0, adding pi/2 to omega B&B[1：i0 -1; if there are two outlier points whose coordinates are i 0、j0, then Ω B&B[1：i0 -1 is added to pi and Ω B&B[i0：j0 -1 is added to pi/2.
Further, the calculation formula of the change value of the faraday rotation angle in the bandwidth in the step S4 is as follows:
ΔΩB&B＝ΩB&B[istart]-ΩB&B[iend]
wherein i start represents the coordinate corresponding to the initial frequency and has Round () means rounding, i end means the coordinates corresponding to the cut-off frequency, and there are/>
Based on the characteristic that the faraday rotation angle is inversely proportional to the square of the frequency, the following two formulas are obtained:
Wherein, omega c is FRA true value corresponding to carrier frequency, and omega c＝ΩB&B[icenter ] +npi/2,
The ambiguity is calculated by subtracting the two equations to the following equation:
further, the formula of the change curve of the faraday rotation angle with respect to the distance frequency in the step S5 is as follows:
Compared with the prior art, the invention has the advantages that: the method is suitable for PolSAR data of different ground object scenes, can realize correction of P-band PolSAR dispersion FR effect on one hand, and can realize inversion of ionosphere electron total quantity on the other hand. The invention can realize the self-adaptive deblurring of the FRA estimated value of the satellite-borne P-band PolSAR image, thereby obtaining an accurate curve of the FRA along with the change of the distance frequency.
Drawings
In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a flowchart of a method for disambiguating an estimated value of a faraday rotation angle in a spaceborne P-band PolSAR image according to the present invention.
Fig. 2 is a P-band on-board PolSAR pseudo-color image (Pauli decomposition) used for simulation in the present invention.
Fig. 3 is a pseudo-color image of a PolSAR under the influence of ionospheric dispersion FR effect in the present invention, the center FRA corresponding to fig. 3 (a) is 30 degrees, and the center FRA corresponding to fig. 3 (b) is 120 degrees.
Fig. 4 is a graph showing the change of the FRA with distance frequency estimated for the frequency domain FR in the present invention. FIG. 4 (a) shows that the FRA estimated value is varied from 40.8857 to 24.3939 degrees; in FIG. 4 (b), the FRA estimated value is varied from-45 to 45 degrees.
Fig. 5 is a graph of FRA estimation for the ambiguity equalization process of fig. 4 (b) in the present invention.
Fig. 6 shows FRA exact inversion results, (a) 30 degrees for the corresponding central FRA, and (b) 120 degrees for the corresponding central FRA.
Fig. 7 shows the deviation of the FRA exact inversion result from the FRA true value, with (a) 30 degrees for the corresponding center FRA and (b) 120 degrees for the corresponding center FRA.
Detailed Description
The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making clear and defining the scope of the present invention.
Inputting satellite-borne P-band PolSAR single-view complex image data M influenced by ionosphere FR effect, wherein the data comprises four polarized channel imagesThe number of azimuth points of each polarized channel image is M a, the number of distance points is N r, the known carrier frequency is F c, the system bandwidth is B r, and the distance sampling rate is F r.
Referring to fig. 1, the embodiment discloses a method for disambiguating an estimated value of a faraday rotation angle in a satellite-borne P-band PolSAR image, which comprises the following steps:
S1, performing fast Fourier transform on a space-borne P-band PolSAR image M along a distance direction to obtain a PolSAR image M of a distance frequency domain:
M＝FFTr(M)
S2, carrying out frequency domain Faraday rotation angle calculation on the PolSAR image M with the distance frequency domain by using a B & B estimator, wherein the method specifically comprises the following steps:
Frequency domain FR (faraday rotation) estimation is performed with a B & B estimator:
Wherein arg () is an angular function, Representing windowed average, M a X1 is the window size, Z hv、Zvh is the equivalent circularly polarized substrate Z corresponds to two cross channel substrates, Z can be represented as
S3, judging whether inconsistent ambiguity exists in the Faraday rotation angle, if so, carrying out ambiguity consistency processing, and otherwise, executing step S4.
In step S2, a curve of the change of the estimated value of the FRA (i.e. the faraday rotation angle) with the distance frequency is obtained, but the estimated value is limited to a range of-45 to 45 degrees due to the effect of the angle taking function, compared with the true value, the fuzzy error of n pi/2 exists, and the n ambiguity is inconsistent in the whole frequency domain range. Aiming at the problem, the solution provided by the embodiment is as follows:
Firstly, judging whether inconsistent ambiguity exists, namely solving a difference omega 'B&B[i]＝ΩB&B[i]-ΩB&B[i-1],i＝2,3,...,Nr of FRA estimated values, if omega' B&B [ i ] exists a different value point larger than pi/4, proving that the inconsistent ambiguity exists, and carrying out ambiguity consistency processing, wherein the specific processing method comprises the following steps: if there is an outlier point whose coordinates are i 0, then Ω B&B[1：i0 -1 is added to pi/2; if two outlier points exist, the coordinates of which are i 0、j0 respectively, adding pi to omega B&B[1：i0 -1 and pi to omega B&B[i0：j0 -1; by analogy, an estimate of the FRA (i.e., faraday rotation angle) with consistent ambiguity can be obtained.
S4, calculating a change value of a Faraday rotation angle in the bandwidth, and calculating ambiguity, wherein the change value is specifically as follows:
Calculating the change value of FRA (Faraday rotation angle) in the bandwidth: ΔΩ B&B＝ΩB&B[istart]-ΩB&B[iend ], where i start denotes a coordinate corresponding to the start frequency, and has Round () means rounding, i end means the coordinates corresponding to the cut-off frequency, and there are/>
Based on the characteristic that FRA is inversely proportional to the square of frequency, the following two formulas are obtained:
Wherein, omega c is FRA true value corresponding to carrier frequency, and omega c＝ΩB&B[icenter ] +npi/2,
And then subtracting the two formulas to obtain the following equation, and finally estimating the ambiguity:
s5, according to the ambiguity calculated in the step S4, a change curve of the Faraday rotation angle relative to the distance frequency is obtained, and the change curve is pushed to the formula:
Fig. 2 is a P-band on-board PolSAR pseudo-color image (Pauli decomposition) for simulation. Since there is no P-band SAR satellite system running on-orbit at present, the correction research for the P-band PolSAR ionosphere FR effect mainly depends on simulation. The method is characterized in that airborne P-band PolSAR image data is taken as input, the center frequency is f c =500 MHz, the system bandwidth is B r =135 MHz, and the airborne P-band PolSAR image data corresponds to the situation that ionospheric dispersion FR effect is not affected.
Fig. 3 is a PolSAR pseudo-color image affected by ionospheric dispersion FR effects. Ionospheric dispersion FR effects are injected in the distance frequency domain, where the center FRA corresponding to fig. 3 (a) is 30 degrees and the center FRA corresponding to fig. 3 (b) is 120 degrees. As shown in the figure, under the influence of ionospheric dispersion FR effect, the polarization measurement error caused by the FR effect changes the original components of each RGB color channel, thereby influencing the subsequent application of PolSAR such as ground object classification, polarization interference and the like.
Fig. 4 is a graph showing the change of the FRA with distance frequency obtained by estimating the frequency domain FR. As shown in fig. 4 (a), the FRA estimated value variation range is 40.8857 to 24.3939 degrees; as shown in FIG. 4 (b), the FRA estimated value range was-45 to 45 degrees, and the mutant FRA outlier was visible to the naked eye.
Fig. 5 is a FRA estimation curve after the ambiguity equalization process of fig. 4 (b). Through the third step, whether inconsistent ambiguity exists can be automatically judged, and then ambiguity equalization processing is carried out on the graph (b) of the inconsistent ambiguity exists, namely FRA corresponding to the distance frequency smaller than 469.7MHz is added with 90 degrees. As shown in fig. 5, the FRA estimate varies from 69.9248 to 3.0788 degrees and has consistent ambiguity.
Fig. 6 shows the FRA exact inversion results with (a) 30 degrees for the corresponding central FRA and (b) 120 degrees for the corresponding central FRA. Further calculating FRA variation value in bandwidth, wherein DeltaOmega B&B is 16.4918 DEG and 66.8460 DEG respectively, and deducing to obtain ambiguityAnd respectively 0 and 1, and finally obtaining the FRA accurate inversion result. As shown in fig. 6, the FRA estimation curve matches the FRA true value well.
Fig. 7 shows the deviation of the FRA exact inversion result from the FRA true value, with (a) 30 degrees for the corresponding center FRA and (b) 120 degrees for the corresponding center FRA. The estimated deviation mainly originates from factors such as channel noise, amplitude-phase imbalance, crosstalk and the like of the onboard P-band PolSAR data, and it is generally considered that when the FRA is smaller than 5 degrees, polarization measurement errors caused by the FR effect can be ignored. As shown in fig. 7, the absolute deviation does not exceed 1.5 degrees, and the accuracy requirement of FR estimation can be completely satisfied.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, the patentees may make various modifications or alterations within the scope of the appended claims, and are intended to be within the scope of the invention as described in the claims.
Claims (4)
1. The method for disambiguating the Faraday rotation angle estimated value in the satellite-borne P-band PolSAR image is characterized by comprising the following steps of:
s1, performing fast Fourier transform on a space-borne P-band PolSAR image M along a distance direction to obtain a PolSAR image of a distance frequency domain
S2, obtaining a PolSAR image of the distance frequency domain by using a B & B estimatorCarrying out frequency domain Faraday rotation angle calculation;
s3, judging whether inconsistent ambiguity exists in the Faraday rotation angle, if so, carrying out ambiguity consistency processing, otherwise, executing the step S4;
s4, calculating a change value of a Faraday rotation angle in the bandwidth, and calculating ambiguity;
S5, obtaining a change curve of the Faraday rotation angle relative to the distance frequency according to the ambiguity calculated in the step S4;
the step S3 specifically includes:
first, the difference in Faraday rotation angle is calculated If it isIf there is a outlier point greater than pi/4, there is inconsistent ambiguity, and continuing to execute;
then, an ambiguity consistency process is performed, including: if there is a outlier point, its coordinate is i 0, it will Plus pi/2; if there are two outlier points whose coordinates are i 0、j0, then the/>Plus pi, willPlus pi/2.
2. The method for resolving ambiguity of faraday rotation angle estimation value in a spaceborne P-band PolSAR image according to claim 1, wherein the formula for calculating faraday rotation angle in the frequency band in step S2 is:
Wherein arg () is an angular function, Represents windowed average, M a X1 is the size of window,/>Is equivalent to circularly polarized substrate/>Corresponding two cross channel substrates,/>Expressed as:
3. the method for resolving ambiguity of the faraday rotation angle estimation value in the spaceborne P-band PolSAR image according to claim 1, wherein the calculation formula of the change value of the faraday rotation angle in the bandwidth in step S4 is:
wherein i start represents the coordinate corresponding to the initial frequency and has Round () means rounding, i end means the coordinates corresponding to the cut-off frequency, and there are/>
Based on the characteristic that the faraday rotation angle is inversely proportional to the square of the frequency, the following two formulas are obtained:
Wherein, Is FRA true value corresponding to carrier frequency, and/>
The ambiguity is calculated by subtracting the two equations to the following equation:
4. The method for resolving ambiguity of faraday rotation angle estimation value in a spaceborne P-band PolSAR image according to claim 1, wherein the formula of the change curve of faraday rotation angle with respect to distance frequency in step S5 is:
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| US6914553B1 (en) * | 2004-11-09 | 2005-07-05 | Harris Corporation | Synthetic aperture radar (SAR) compensating for ionospheric distortion based upon measurement of the Faraday rotation, and associated methods |
| CN101551450A (en) * | 2009-05-06 | 2009-10-07 | 北京航空航天大学 | A building approach for space-borne polarization SAR Faraday rotation effect correction platform |
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| US6914553B1 (en) * | 2004-11-09 | 2005-07-05 | Harris Corporation | Synthetic aperture radar (SAR) compensating for ionospheric distortion based upon measurement of the Faraday rotation, and associated methods |
| CN101551450A (en) * | 2009-05-06 | 2009-10-07 | 北京航空航天大学 | A building approach for space-borne polarization SAR Faraday rotation effect correction platform |
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