Bistatic spaceborne SAR Faraday rotation angle estimation method

Abstract

The invention provides a bistatic spaceborne SAR Faraday rotation angle estimation method, and belongs to the field of SAR ionosphere effect compensation. According to the method, a ping-pong working mode is adopted based on a double-base satellite-borne SAR, Faraday rotation angle estimation is achieved through full-polarization data, a measurement scattering matrix calculation model influenced by the Faraday rotation effect is established when two satellites serve as a transmitting party and a receiving party respectively, then addition and subtraction calculation is conducted on measurement scattering matrix elements, intermediate parameters are obtained, and the measurement scattering matrix is obtained. And constructing complex parameters, performing conjugate multiplication on the complex parameters, performing mean filtering and noise reduction on a calculation result, and estimating two Faraday rotation angles for Faraday rotation effect compensation. According to the method, the Faraday rotation angle estimation of the bistatic SAR is realized, the estimation result is relatively accurate, and the estimation error has little change under different signal-to-noise ratios.

Description

Technical Field

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

[0002] Fully polarized SAR transmits and receives electromagnetic waves in different polarization modes. The measured 4 groups of HH, HV, VH, and VV data can fully obtain the polarization scattering characteristics of the target. When electromagnetic waves pass through the ionosphere, the polarization plane rotates relative to the incident wave under the influence of the electromagnetic field, which is the Faraday rotation effect. The rotation angle is the Faraday rotation angle (FRA). HH represents horizontal polarization transmission and horizontal polarization reception data, HV represents horizontal polarization transmission and vertical polarization reception data, VH represents vertical polarization transmission and horizontal polarization reception data, and VV represents vertical polarization transmission and vertical polarization reception data.

[0003] The bistatic SAR system acquires rich target scattering information and is a new type of SAR system developed in recent years. For example, Tandem-X, which includes two TerraSAR-X satellites, adopts a one-transmit and two-receive working mode to realize elevation information detection. For spaceborne SAR, when the operating frequency is low, the ionosphere will affect the SAR signal. The Faraday rotation effect will cause the polarization plane to deflect during the propagation of electromagnetic waves. The deflection angle is the Faraday rotation angle, which causes measurement errors of the target's polarization information. At present, the estimation of the Faraday rotation angle of the ionosphere of spaceborne SAR is aimed at monostatic SAR, and the Faraday rotation angle is estimated using full polarization data. Since the signal propagation path of bistatic SAR is different from that of monostatic SAR, the Faraday rotation angle on the transmitting and receiving paths is different. The method of monostatic SAR cannot be applied to bistatic SAR, so it is necessary to study the Faraday rotation angle estimation method for bistatic SAR. Summary of the invention

[0004] Aiming at the problem of ionospheric Faraday rotation angle estimation of a dual-base satellite-borne SAR system, the present invention provides a dual-base satellite-borne SAR Faraday rotation angle estimation method. The dual-base satellite-borne SAR adopts a ping-pong working mode and uses full polarization data to realize Faraday rotation angle estimation.

[0005] The present invention provides a method for estimating the Faraday rotation angle of a dual-base satellite-borne SAR, and the dual-base satellite-borne SAR adopts a ping-pong working mode to realize the Faraday rotation angle estimation. The dual-base satellite-borne SAR system has both transmitting and receiving functions, and adopts a ping-pong working mode. In this mode, the two-satellite SAR alternately acts as a transmitter and a receiver, and the transmitter alternately transmits horizontally polarized and vertically polarized electromagnetic waves, and the receiver simultaneously receives horizontally polarized and vertically polarized electromagnetic waves; since the transmission and reception pass through different positions of the ionosphere, they will be 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-base satellite-borne SAR of the present invention comprises the following steps:

[0006] Step 1: Obtain the full polarization image of the dual-base satellite-borne SAR in ping-pong working mode, and measure the scattering matrix from the full polarization image; mark the two stars as A and B, and assume that when star A transmits and star B receives, the measured scattering matrix affected by the Faraday rotation effect is obtained as , when B star transmits and A star receives, the measurement scattering matrix affected by the Faraday rotation effect is obtained as ; 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;

[0007] Step 2, estimating and obtaining the Faraday rotation angle from the two satellites to the target, including the following steps 21 to 24;

[0008] Step 21: Add and subtract the scattering matrix elements of the two stars to obtain the intermediate parameters ,as follows:

[0009] ;in, is the Faraday rotation angle from star B to the target, is the Faraday rotation angle from star A to the target, Represents the target scattering matrix when A satellite transmits and B satellite receives;

[0010] Step 22: According to the intermediate parameters Constructing complex parameters as follows:

[0011] ; j is the imaginary unit;

[0012] Step 23: and By conjugating and multiplying them respectively, we get: ;

[0013] Step 24: Calculate the estimated values ​​of the two Faraday rotation angles after filtering and denoising the result of step 23. as follows:

[0014] ;in Indicates mean filtering, arg indicates the angle of argument;

[0015] Using the estimated Faraday rotation angle Compensate for Faraday rotation effects.

[0016] Compared with the prior art, the advantages and positive effects of the present invention are: the existing ionospheric Faraday rotation angle estimation method of the monostatic SAR cannot be applied to the bistatic SAR, so it is necessary to study the Faraday rotation angle estimation method for the bistatic SAR, based on which, the method of the present invention realizes the estimation of the Faraday rotation angle of the bistatic satellite-borne SAR in the ping-pong mode, obtains full polarization data by adopting the ping-pong working mode of the bistatic satellite-borne SAR, obtains the measurement scattering matrix affected by the Faraday rotation effect, and then uses the measurement scattering matrix elements to estimate the Faraday rotation angle from the bistatic SAR to the target, which is used to compensate for the Faraday rotation effect. The method of the present invention designs an estimation method for the Faraday rotation angle from the bistatic SAR to the target, and it is verified by experiments that the estimation result is relatively accurate, and the estimation error does not change much under different signal-to-noise ratios. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a flow chart of the implementation of the Faraday rotation angle estimation method of the dual-base satellite-borne SAR in an embodiment of the present invention;

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

[0019] Figure 3 is the Faraday rotation angle from satellite B to the target at a signal-to-noise ratio of 10 dB in the embodiment of the present invention. Estimation error plot of ;

[0020] Figure 4 The embodiment of the present invention is a signal-to-noise ratio of 20dB. Estimation error plot of ;

[0021] Figure 5 is the Faraday rotation angle from satellite A to the target at a signal-to-noise ratio of 10 dB in the embodiment of the present invention. Error plots of estimates;

[0022] Figure 6 The embodiment of the present invention is a signal-to-noise ratio of 20dB. Error plot of the estimate. DETAILED DESCRIPTION

[0023] In order to facilitate those skilled in the art to understand and implement the present invention, the present invention is further described in detail and in depth below with reference to the accompanying drawings.

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

[0025] The dual-base satellite-borne SAR in the method of the present invention adopts a ping-pong working mode, marking two satellites as A and B, and the SAR systems of the two satellites have transmitting and receiving functions. The A-star SAR transmits horizontally polarized and vertically polarized electromagnetic waves alternately, and the B-star SAR receives horizontally polarized and vertically polarized electromagnetic waves at the same time. The B-star SAR transmits horizontally polarized and vertically polarized electromagnetic waves alternately, and the A-star SAR receives horizontally polarized and vertically polarized electromagnetic waves at the same time. A and B transmit alternately. Since the transmission and reception pass through different positions of the ionosphere, they will be affected by two Faraday rotation angles. An implementation process of an embodiment of the present invention is as follows: Figure 1 As shown, firstly, a scattering matrix calculation model of dual-base spaceborne SAR in ping-pong working mode is established.

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

[0027] (1)

[0028] in, represents the measurement scattering matrix when A is transmitting and B is receiving, It represents the target scattering matrix when A is transmitting and B is receiving. and They represent the Faraday rotation angles from star B to the target and from star A to the target respectively; is the noise matrix.

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

[0030] (2)

[0031] in, represents the measurement scattering matrix when B satellite transmits and A satellite receives, Represents the target scattering matrix when B satellite transmits and A satellite receives; is the noise matrix.

[0032] According to the ping-pong mode, , then formula (2) can be written as:

[0033] (3)

[0034] In an ideal situation, the noise can be ignored. Expanding equations (1) and (3) above yields:

[0035] (4)

[0036] (5)

[0037] By acquiring a fully polarized image of a dual-base satellite-borne SAR in ping-pong working mode, measuring a scattering matrix from the fully polarized image, and then based on the derivation of the scattering matrix, the present invention implements a method for compensating the Faraday rotation effect of a dual-base satellite-borne SAR. The specific implementation steps include the following 4 steps, which are described below respectively.

[0038] Step 1: Add and subtract equations (4) and (5), that is, add and subtract the measured scattering matrix to obtain the intermediate parameters: as follows;

[0039] (6)

[0040] Step 2: According to Constructing complex parameters ,as follows;

[0041] (7)

[0042] Here, j is the imaginary unit.

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

[0044] (8)

[0045] Step 4: After filtering and denoising the result of the previous step, the estimated Faraday rotation angle can be calculated. .

[0046] (9)

[0047] in It means to find the average, that is, to perform mean filtering; arg means to find the angle.

[0048] Example:

[0049] The embodiment of the present invention uses a uniform rainforest scene of full polarization data of ALOS PALSAR2 for simulation verification. Figure 2-6As shown, the size is 800 in azimuth × 800 in distance, the unit is pixel, and the Faraday rotation angle is -40~40, the unit is degree.

[0050] According to formula (1), the measured scattering matrix elements are obtained from the real SAR image when A is transmitting and B is receiving: According to formula (2), the measured scattering matrix element image is obtained by using the real SAR image when A is transmitting and B is receiving , adding noise, and conducting experiments at signal-to-noise ratios (SNR) of 20 dB and 10 dB, respectively, to implement the method of the present invention:

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

[0052] Step 2: According to the intermediate parameters Constructing complex parameters ;

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

[0054] Step 4: Estimate the Faraday rotation angle using formula (9) In the embodiment of the present 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 when the signal-to-noise ratio is 10dB and 20dB respectively. The estimation error of Figure 5 and Figure 6 The Faraday rotation angle is given when the signal-to-noise ratio is 10dB and 20dB respectively. The estimated error of and The bar graph on the right uses different colors to represent different error values, and the unit is degree. and Not much change.

[0057] Except for the technical features described in the specification, all of them are known technologies to those skilled in the art. The present invention omits the description of known components and known technologies to avoid redundancy and unnecessary limitation of the present invention. The implementation methods described in the above embodiments do not represent all implementation methods consistent with the present application. On the basis of the technical solution of the present invention, various modifications or variations that can be made by those skilled in the art without creative labor are still within the protection scope of the present invention.

Claims

1. A method for estimating the Faraday rotation angle of a dual-base satellite-borne SAR, characterized in that: The dual-base satellite-borne SAR both has the functions of transmitting and receiving, and adopts a ping-pong working mode. The two satellite SARs alternately serve as the transmitter and the receiver. The transmitter alternately transmits horizontally polarized and vertically polarized electromagnetic waves, and the receiver simultaneously receives horizontally polarized and vertically polarized electromagnetic waves. The compensation method comprises the following steps: Step 1: Obtain the full polarization image of the dual-base satellite-borne SAR in ping-pong working mode, and measure the scattering matrix from the full polarization image; mark the two stars as A and B, and assume that when star A transmits and star B receives, the measured scattering matrix affected by the Faraday rotation effect is obtained as , when B star transmits and A star receives, the measurement scattering matrix affected by the Faraday rotation effect is obtained as ; 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 2, estimating and obtaining the Faraday rotation angle from the two satellites to the target, including the following steps 21 to 24; Step 21, perform addition and subtraction calculations on the two measured scattering matrix elements to obtain the intermediate parameters ,as follows: ;in, is the Faraday rotation angle from star B to the target, is the Faraday rotation angle from star A to the target, Represents the target scattering matrix when A satellite transmits and B satellite receives; Step 22: According to the intermediate parameters Constructing complex parameters as follows: ; j is the imaginary unit; Step 23: and By conjugating and multiplying them respectively, we get: ； Step 24: Calculate the estimated values ​​of the two Faraday rotation angles after filtering and denoising the result of step 23. as follows: ;in Indicates mean filtering, arg indicates the angle of argument; Using the estimated Faraday rotation angle Compensate for Faraday rotation effects.

2. The method according to claim 1, characterized in that In the step 1, two stars are marked as A and B. In the ping-pong working mode, when star A transmits and star B receives, the measurement scattering matrix affected by the Faraday rotation effect is calculated as follows: ； When star A transmits and star B receives, the measured scattering matrix affected by the Faraday rotation effect is calculated as follows: ； in, Represents the target scattering matrix when B satellite transmits and A satellite receives; , are all noise matrices; According to the ping-pong mode, , then the measured scattering matrix can be rewritten as ； In an ideal situation, the noise is negligible, and the elements in the measured scattering matrix are as follows: ； 。

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