Dual-baseline spaceborne sar faraday rotation angle estimation method

Abstract

The application provides a dual-baseline spaceborne SAR Faraday rotation angle estimation method and belongs to the field of SAR ionospheric effect compensation.The method is based on the dual-baseline spaceborne SAR in a ping-pong working mode, Faraday rotation angle estimation is realized by using full polarization data, a measurement scattering matrix calculation model affected by the Faraday rotation effect is established under the condition that two satellites are respectively taken as a transmitting side and a receiving side, then intermediate parameters are obtained by adding and subtracting the measurement scattering matrix elements, complex parameters are constructed, the complex parameters are conjugated and multiplied, two Faraday rotation angles are estimated after mean filtering and noise reduction of the calculation results, and the Faraday rotation effect compensation is used.The method realizes the Faraday rotation angle estimation of the dual-baseline SAR, the estimation result is relatively accurate, and the estimation error does not change much under different signal-to-noise ratios.

Description

Technical Field

[0001] This invention relates to the field of ionospheric effect compensation in spaceborne synthetic aperture radar (SAR), and specifically to a dual-base spaceborne SAR Faraday rotation angle estimation method. Background Technology

[0002] Fully polarimetric SAR transmits and receives electromagnetic waves using different polarization modes. The four sets of HH, HV, VH, and VV data obtained can comprehensively acquire the target's polarization scattering characteristics. When electromagnetic waves pass through the ionosphere, the polarization surface rotates relative to the incident wave under the influence of the electromagnetic field; this is known as the Faraday rotation effect, and the rotation angle is called the Faraday rotation angle (FRA). HH represents horizontally polarized transmission and horizontally polarized reception data, HV represents horizontally polarized transmission and vertically polarized reception data, VH represents vertically polarized transmission and horizontally polarized reception data, and VV represents vertically polarized transmission and vertically polarized reception data.

[0003] Bistatic SAR systems, which acquire rich target scattering information, are a relatively new type of SAR system developed in recent years. For example, Tandem-X, consisting of two TerraSAR-X satellites, employs a dual-transmit / dual-receive mode to detect elevation information. For spaceborne SAR, at lower operating frequencies, the ionosphere can affect the SAR signal. The Faraday rotation effect causes the polarization surface to deflect during electromagnetic wave propagation, with the deflection angle being the Faraday rotation angle, resulting in measurement errors of the target's polarization information. Currently, spaceborne SAR ionospheric Faraday rotation angle estimation is designed for monostatic SAR, using fully polarimetric data to estimate the Faraday rotation angle. Because the signal propagation path of bistatic SAR differs from that of monostatic SAR, the Faraday rotation angles on the transmit and receive paths are different. Therefore, monostatic SAR methods cannot be applied to bistatic SAR; hence, research is needed on Faraday rotation angle estimation methods specifically for bistatic SAR. Summary of the Invention

[0004] This invention addresses the problem of ionospheric Faraday rotation angle estimation in dual-base spaceborne SAR systems by providing a method for estimating the Faraday rotation angle of dual-base spaceborne SAR. The method utilizes a ping-pong working mode based on dual-base spaceborne SAR and fully polarimetric data to achieve Faraday rotation angle estimation.

[0005] This invention provides a method for estimating the Faraday rotation angle of a dual-satellite SAR system. The dual-satellite SAR system employs a ping-pong operating mode to achieve Faraday rotation angle estimation. Both dual-satellite SAR systems have transmit and receive capabilities, operating in a ping-pong mode. In this mode, the two SAR satellites alternately act as the transmitter and receiver. The transmitter alternately transmits horizontally and vertically polarized electromagnetic waves, while the receiver simultaneously receives both horizontally and vertically polarized electromagnetic waves. Because the transmission and reception cross different ionospheric positions, the method is affected by the two Faraday rotation angles from the two satellites to the target. The method for estimating the Faraday rotation angle of a dual-satellite SAR system of this invention includes the following steps:

[0006] Step 1: Acquire the fully polarimetric image of the dual-base satellite-borne SAR in ping-pong mode, and measure the scattering matrix from the fully polarimetric image; label the two satellites as A and B, and assume that when satellite A transmits and satellite B receives, the measured scattering matrix affected by the Faraday rotation effect is as follows. Suppose that when satellite B transmits and satellite A receives, the measurement scattering matrix affected by the Faraday rotation effect is obtained as follows: Subscript HH represents horizontal polarization transmission and horizontal polarization reception, HV represents horizontal polarization transmission and vertical polarization reception, VH represents vertical polarization transmission and horizontal polarization reception, and VV represents vertical polarization transmission and vertical polarization reception.

[0007] Step 2, estimate the Faraday rotation angles from the two stars to the target, including steps 21-24 below;

[0008] Step 21: Perform addition and subtraction calculations on the scattering matrix elements of the two stars to obtain intermediate parameters. ,as follows:

[0009] ;in, It is the Faraday rotation angle from satellite B to the target. It is the Faraday rotation angle from satellite A to the target. This represents the target scattering matrix when satellite A transmits and satellite B receives the data.

[0010] Step 22, based on intermediate parameters Constructing complex parameters as follows:

[0011] j is the imaginary unit;

[0012] Step 23, and Multiplying by their conjugates, we get: ;

[0013] Step 24: After denoising the results from Step 23 by mean filtering, calculate the estimated values ​​of the two Faraday rotation angles. as follows:

[0014] ;in This indicates that mean filtering is being performed, and arg represents the argument calculation;

[0015] Using the estimated Faraday rotation angle Compensation for the Faraday rotation effect is performed.

[0016] Compared to existing technologies, the advantages and positive effects of this invention are as follows: Existing methods for estimating the ionospheric Faraday rotation angle of single-static SAR cannot be applied to bistatic SAR. Therefore, it is necessary to study a Faraday rotation angle estimation method for bistatic SAR. Based on this, the method of this invention achieves the estimation of the Faraday rotation angle of bistatic spaceborne SAR in ping-pong mode. By acquiring fully polarized data using the ping-pong working mode of the bistatic spaceborne SAR, the measurement scattering matrix affected by the Faraday rotation effect is obtained. Then, the Faraday rotation angle from the bistatic SAR to the target is estimated using the elements of the measurement scattering matrix to compensate for the Faraday rotation effect. This invention designs a method for estimating the Faraday rotation angle from the bistatic SAR to the target. Experimental verification shows that the estimation results are relatively accurate, and the estimation error does not change significantly under different signal-to-noise ratios. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the implementation of the dual-base spaceborne SAR Faraday rotation angle estimation method according to an embodiment of the present invention.

[0018] Figure 2 This is a schematic diagram of a rainforest scene where dual-base satellite-borne dual-station SAR is applied according to an embodiment of the present invention;

[0019] Figure 3 This is the Faraday rotation angle from satellite B to the target under a signal-to-noise ratio of 10 dB according to an embodiment of the present invention. The estimation error diagram;

[0020] Figure 4 This embodiment of the invention demonstrates the effect of a signal-to-noise ratio of 20dB on... The estimation error diagram;

[0021] Figure 5 This is the Faraday rotation angle from satellite A to the target under a signal-to-noise ratio of 10 dB according to an embodiment of the present invention. Estimated error plot;

[0022] Figure 6 This embodiment of the invention demonstrates the effect of a signal-to-noise ratio of 20dB on... The estimated error plot. Detailed Implementation

[0023] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention will be further described in detail below with reference to the accompanying drawings.

[0024] This invention provides a method for estimating the Faraday rotation angle of dual-base satellite-borne SAR. The dual-base satellite-borne SAR adopts a ping-pong working mode. In the TanDEM-X satellite, the ping-pong working mode was used. In this mode, the two satellites alternately transmit signals and use the SAR image itself to estimate the Faraday rotation angle. The estimated Faraday rotation angle is then used to compensate for the Faraday rotation effect.

[0025] The dual-satellite SAR in this invention employs a ping-pong operating mode, with two satellites designated A and B. Both satellites' SAR systems possess both transmit and receive capabilities. Satellite A's SAR alternately transmits horizontally and vertically polarized electromagnetic waves, while Satellite B's SAR simultaneously receives both. Satellites A and B alternate in transmission. Due to the different ionospheric penetration points for transmission and reception, the method is affected by two Faraday rotation angles. One implementation flow of this invention is as follows: Figure 1 As shown, a scattering matrix calculation model for dual-base spaceborne SAR in ping-pong mode is first established.

[0026] With satellite A transmitting and satellite B receiving, the scattering matrix affected by the Faraday rotation effect is expressed as follows:

[0027] (1)

[0028] in, This represents the measured scattering matrix under the condition that satellite A transmits and satellite B receives. This represents the target scattering matrix under the condition that satellite A transmits and satellite B receives. and These represent the Faraday rotation angles from satellite B to the target and from satellite A to the target, respectively. It is a noise matrix.

[0029] With satellite B transmitting and satellite A receiving, the scattering matrix affected by the Faraday rotation effect is as follows:

[0030] (2)

[0031] in, This represents the measured scattering matrix under the condition that satellite B transmits and satellite A receives. This represents the target scattering matrix under the condition that satellite B transmits and satellite A receives the data; It is a noise matrix.

[0032] According to the table tennis mode, Then equation (2) can be written as:

[0033] (3)

[0034] Under ideal conditions, noise can be ignored. Expanding equations (1) and (3) above, we can obtain:

[0035] (4)

[0036] (5)

[0037] The invention implements a Faraday rotation effect compensation method for dual-base spaceborne SAR by acquiring a fully polarized image of a dual-base spaceborne SAR in ping-pong mode, measuring the scattering matrix from the fully polarized image, and then deriving the scattering matrix. The specific implementation steps include the following four steps, which are described below.

[0038] Step 1: Perform addition and subtraction calculations using equations (4) and (5), that is, perform addition and subtraction calculations using the measured scattering matrix to obtain the intermediate parameters. as follows;

[0039] (6)

[0040] Step Two, according to Constructing complex parameters ,as follows;

[0041] (7)

[0042] Where j is the imaginary unit.

[0043] Step 3, Conjugate multiplication, , Conjugate multiplication is as follows:

[0044] (8)

[0045] Step 4: After averaging and filtering the results from the previous step to reduce noise, the estimated Faraday rotation angle can be calculated. .

[0046] (9)

[0047] in arg indicates averaging, i.e., performing mean filtering; arg indicates calculating the argument.

[0048] Example:

[0049] This invention uses a uniform rainforest scene from ALOS PALSAR2 fully polarimetric data for simulation verification, such as... Figure 2-6 As shown, the sizes are all 800 in the azimuth direction and 800 in the distance direction, in pixels, and the Faraday rotation angle is -40 to 40 degrees.

[0050] Based on equation (1), the elements of the measured scattering matrix under the condition of satellite A transmitting and satellite B receiving are obtained from real SAR images. Based on equation (2), the measured scattering matrix element image under the condition of satellite A transmitting and satellite B receiving is obtained from real SAR images. With noise added, experiments were conducted at signal-to-noise ratios (SNR) of 20 dB and 10 dB, respectively, to implement the method of this invention:

[0051] Step 1: Calculate intermediate parameters by adding or subtracting elements from the measured scattering matrix. ;

[0052] Step 2: Based on intermediate parameters Constructing complex parameters ;

[0053] Step 3, Conjugate multiplication, , Conjugate multiplication;

[0054] Step 4: Estimate the Faraday rotation angle using formula (9) In this embodiment of the invention, the window sizes selected for mean filtering are 50×50, 100×100 and 200×200, respectively.

[0055] Depend on Figure 3-6 The experimental results are as follows:

[0056] Figure 3 and Figure 4 The Faraday rotation angle is given for signal-to-noise ratios of 10 dB and 20 dB, respectively. The estimation error, Figure 5 and Figure 6 The Faraday rotation angle is given for signal-to-noise ratios of 10 dB and 20 dB, respectively. The estimation error, with the horizontal and vertical axes in the figure representing respectively. and The values ​​are shown in the right-hand bar chart, where different colors represent different error values, and the unit is degrees. The estimation error varies with... and Not much has changed.

[0057] Except for the technical features described in the specification, all other technologies are known to those skilled in the art. Descriptions of well-known components and technologies are omitted in this invention to avoid redundancy and unnecessary limitation. The embodiments described above do not represent all embodiments consistent with this application. Various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of this invention are still within the protection scope of this invention.

Claims

1. A method for estimating the Faraday rotation angle in a dual-baseline space-borne SAR, characterized in that, For dual-baseline spaceborne SAR system, dual-baseline spaceborne SAR has both transmitting and receiving functions, adopts ping-pong working mode, two-star SAR alternately acts as transmitting side and receiving side, the transmitting side alternately transmits horizontal polarization and vertical polarization electromagnetic waves, and the receiving side simultaneously receives horizontal polarization and vertical polarization electromagnetic waves; Because the transmitting and receiving pass through different positions of ionosphere, they are affected by two Faraday rotation angles from two stars to the target; The compensation method includes the following steps: Step one, obtain the full polarimetric image of the dual-baseline spaceborne SAR in ping-pong mode, measure the scattering matrix from the full polarimetric image; mark the two stars as A and B, and assume that when A star transmits and B star receives, the measured scattering matrix affected by the Faraday rotation effect is , and assume that when B star transmits and A star receives, the measured scattering matrix affected by the Faraday rotation effect is ; the subscript HH represents horizontal polarization transmission and horizontal polarization reception, HV represents horizontal polarization transmission and vertical polarization reception, VH represents vertical polarization transmission and horizontal polarization reception, and VV represents vertical polarization transmission and vertical polarization reception; Step two, estimating the Faraday rotation angle from two stars to the target, including the following steps 21-24; Step 21, add and subtract the two measured scattering matrix elements to obtain an intermediate parameter As follows: ; where is the Faraday rotation angle from B-star to target, is the Faraday rotation angle from A-star to target, denotes the target scattering matrix when A-star transmits and B-star receives; Step 22, constructing intermediate parameters Constructing complex parameters As follows: ; j is the imaginary unit; Step 23, to and are respectively conjugated to obtain: ; Step 24, after the mean filtering and noise reduction of the result of step 23, the estimated value of the two Faraday rotation angles is calculated As follows: ; wherein denotes performing mean filtering, and arg denotes finding the argument. Utilizing the estimated faraday rotation angle Faraday rotation effect compensation is performed.

2. The method of claim 1, wherein, The step one, mark two stars as A and B, in the ping-pong working mode, when A star transmits and B star receives, the measurement scattering matrix influenced by the Faraday rotation effect is calculated as follows: ; When Astar transmits and Bstar receives, the measurement scattering matrix affected by the Faraday rotation effect is calculated as follows: ; wherein represents the target scattering matrix for B-star transmit, A-star receive; , are noise matrices; According to the ping-pong mode, The measured scattering matrix is then rewritten as ; In the ideal case, the noise is ignored, and the elements in the measured scattering matrix are as follows: ； 。

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