JP7595810B1 - Change detection device and change detection method - Google Patents

Abstract

The change detection device is configured to include an image acquisition unit (1) that acquires time-series polarized SAR images obtained by observing an object with polarized SAR at a plurality of different times, and a polarization state determination unit (2) that determines, based on the power of the polarized SAR images acquired by the image acquisition unit (1), a combination of the polarization state of a transmitted radio wave radiated toward the object and the polarization state of a received radio wave, which is a radio wave reflected by the object, as a polarization state to be used for detecting a change in the object,

Description

The present disclosure relates to a change detection device and a change detection method.

2. Description of the Related Art There is a change detection device that detects changes in an observation target by comparing a plurality of Synthetic Aperture Radar (SAR) images, which are images of the observation target.
As such a change detection device, for example, Patent Document 1 discloses a change detection device that compares a reference image, which is an image of a displacement extraction target region, with a target image, which is an image of the displacement extraction target region captured at a time different from that of the reference image, and detects a change in the displacement extraction target region based on the comparison result between the reference image and the target image. Each of the reference image and the target image is a single-polarized SAR image.

JP 2023-65066 A

Depending on the combination of the polarization state of the transmitted radio wave related to the SAR image and the polarization state of the received radio wave related to the SAR image, the change in the displacement extraction target area may be clear or unclear. Specifically, when the polarization state of the transmitted radio wave related to the SAR image is, for example, vertical polarization, and the polarization state of the received radio wave related to the SAR image is, for example, horizontal polarization, the change in the displacement extraction target area may be clear. On the other hand, when the polarization state of the transmitted radio wave is, for example, horizontal polarization, and the polarization state of the received radio wave is, for example, vertical polarization, the change in the displacement extraction target area may be unclear.
In the change detection device disclosed in Patent Document 1, the SAR image is a single-polarized SAR image, not a multi-polarized SAR image obtained by multi-polarized SAR observation, and therefore the change detection device has a problem in that it cannot select a combination of the polarization state of the transmitted radio wave and the polarization state of the received radio wave that clearly shows a change in the displacement extraction target area.

The present disclosure has been made to solve the problems described above, and aims to obtain a change detection device that can select a combination of polarization states that clearly indicates changes in the object being observed.

The change detection device according to the present disclosure includes an image acquisition unit that acquires time-series polarized SAR images obtained by observing an observation target with a polarized SAR (Synthetic Aperture Radar) at a plurality of different times, and a power difference ΔPli between the two polarized SAR images acquired by the image acquisition unit. and a polarization state determination unit that determines a combination of the transmission polarization state and the polarization state of the received radio wave, which is the radio wave after reflection by the observation target, as a polarization state to be used for detecting a change in the observation target, while switching the polarization state of the transmission radio wave radiated towards the observation target based on n, wherein when an arbitrary transmission polarization state is selected, the polarization state determination unit calculates a reception polarization state corresponding to a maximum value of the difference ΔPlin in the power values of the multi-polarized SAR images at two times, and while switching the transmission polarization state, repeatedly performs a process of calculating the power value difference ΔPlin and a process of calculating the reception polarization state, and compares the differences ΔPlin for all combinations to determine an optimal combination of the transmission polarization state and the reception polarization state that maximizes the power value difference ΔPlin, and the polarization state determination unit includes a polarization component extraction unit that extracts a polarization component corresponding to the combination determined by the polarization state determination unit, from each of the multi-polarized SAR images acquired by the image acquisition unit.

According to the present disclosure, it is possible to select a combination of polarization states that clearly reveals changes in the object being observed.

1 is a configuration diagram showing a change detection device according to a first embodiment; 1 is a hardware configuration diagram showing hardware of a change detection device according to a first embodiment. FIG. FIG. 2 is a hardware configuration diagram of a computer in the case where the change detection device is realized by software, firmware, or the like. 4 is a flowchart showing a change detection method which is a processing procedure of the change detection device. 1 is an explanatory diagram showing an orientation angle ψ and an ellipsity angle χ. FIG. 6A is an explanatory diagram showing an example of the PS of Co-Pol with respect to a horizontal dipole, and FIG. 6B is an explanatory diagram showing an example of the PS of Cr-Pol with respect to a horizontal dipole. FIG. 11 is a configuration diagram showing a change detection device according to a second embodiment. FIG. 11 is a hardware configuration diagram showing hardware of a change detection device according to a second embodiment.

In order to explain the present disclosure in more detail, the form for implementing the present disclosure will be described below with reference to the attached drawings.

Embodiment 1.
FIG. 1 is a configuration diagram showing a change detection device according to a first embodiment.
FIG. 2 is a hardware configuration diagram showing the hardware of the change detection device according to the first embodiment.
The change detection device shown in FIG. 1 includes an image acquisition unit 1, a polarization state determination unit 2, a polarization component extraction unit 3, and a change detection unit 4.

The image acquisition unit 1 is realized by, for example, an image acquisition circuit 11 shown in FIG.
The image acquisition unit 1 acquires time-series polarimetric synthetic aperture radar (SAR) images obtained by observing an observation target at a plurality of mutually different times using a polarimetric SAR.
A multi-polarized SAR image contains a polarization component (hereinafter referred to as "hh") when the polarization state of the transmitted radio waves is horizontally polarized and the polarization state of the received radio waves is horizontally polarized, and a polarization component (hereinafter referred to as "hv") when the polarization state of the transmitted radio waves is horizontally polarized and the polarization state of the received radio waves is vertically polarized.
In addition, the multi-polarized SAR image includes a polarization component (hereinafter referred to as "vh") when the polarization state of the transmitted radio waves is vertically polarized and the polarization state of the received radio waves is horizontally polarized, and a polarization component (hereinafter referred to as "vv") when the polarization state of the transmitted radio waves is vertically polarized and the polarization state of the received radio waves is vertically polarized.
The image acquisition unit 1 outputs time-series polarized SAR images to each of the polarization state determination unit 2 and the polarization component extraction unit 3.

The polarization state determining unit 2 is realized by, for example, a polarization state determining circuit 12 shown in FIG.
The polarization state determining unit 2 acquires time-series polarimetric SAR images from the image acquiring unit 1 .
Based on the power of the polarized SAR image, the polarization state determination unit 2 determines a combination of the polarization state of the transmitted radio waves emitted toward the observation target and the polarization state of the received radio waves, which are radio waves reflected by the observation target, as the polarization state to be used for detecting changes in the observation target.
Specifically, the polarization state determination unit 2 repeatedly calculates the power difference between any two of the time-series multi-polarized SAR images, assuming that the polarization state of the transmitted radio wave is one of a plurality of polarization states. The polarization state determination unit 2 then compares the calculated differences with each other, and determines the polarization state of the transmitted radio wave based on the comparison result of the differences, and determines a combination of the polarization state of the transmitted radio wave and the polarization state of the received radio wave.

The polarization component extractor 3 is realized by, for example, a polarization component extractor circuit 13 shown in FIG.
The polarization component extractor 3 acquires time-series polarimetric SAR images from the image acquirer 1 .
The polarization component extractor 3 extracts, from each of the polarized SAR images, the polarization components corresponding to the combination determined by the polarization state determiner 2 .
The polarization component extraction unit 3 outputs each of the extracted polarization components to the change detection unit 4 .

The change detection unit 4 is realized by, for example, a change detection circuit 14 shown in FIG.
The change detection unit 4 acquires a plurality of polarization components from the polarization component extraction unit 3 .
The change detector 4 compares the powers of the multiple polarization components with each other.
The change detection unit 4 detects a change in the observation target based on the result of the power comparison.
The change detection unit 4 outputs the detection result of the change in the observation target to, for example, a monitoring device (not shown). An example of the monitoring device is a device that monitors the occurrence of disasters in a region.

In Fig. 1, it is assumed that each of the components of the change detection device, that is, the image acquisition unit 1, the polarization state determination unit 2, the polarization component extraction unit 3, and the change detection unit 4, is realized by dedicated hardware as shown in Fig. 2. That is, it is assumed that the change detection device is realized by an image acquisition circuit 11, a polarization state determination circuit 12, a polarization component extraction circuit 13, and a change detection circuit 14.
Each of the image acquisition circuit 11, the polarization state determination circuit 12, the polarization component extraction circuit 13, and the change detection circuit 14 corresponds to, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination of these.

The components of the change detection device are not limited to those realized by dedicated hardware, and the change detection device may be realized by software, firmware, or a combination of software and firmware.
The software or firmware is stored as a program in the memory of a computer. The computer means hardware that executes the program, and includes, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), a central processing unit, a processing unit, an arithmetic unit, a microprocessor, a microcomputer, a processor, or a DSP (Digital Signal Processor).

FIG. 3 is a hardware configuration diagram of a computer in the case where the change detection device is realized by software, firmware, or the like.
When the change detection device is realized by software, firmware, or the like, a program for causing a computer to execute the respective processing procedures in the image acquisition unit 1, the polarization state determination unit 2, the polarization component extraction unit 3, and the change detection unit 4 is stored in the memory 21. Then, a processor 22 of the computer executes the program stored in the memory 21.

In addition, Fig. 2 shows an example in which each of the components of the change detection device is realized by dedicated hardware, and Fig. 3 shows an example in which the change detection device is realized by software or firmware, etc. However, this is merely one example, and some of the components in the change detection device may be realized by dedicated hardware, and the remaining components may be realized by software or firmware, etc.

First, polarimetric SAR observation of an object will be described.
For example, the power P of a received signal observed by a radar is expressed by the following formula (1) using a polarization state vector p ^→t indicating the state of the transmitted polarization, a polarization state vector p ^→r indicating the state of the received polarization, and a scattering matrix S expressed in the HV basis. The received signal observed by the radar is a signal indicating a radio wave reflected by an object to be observed. In the specification document, because of electronic filing, the symbol "→" cannot be placed above the letters, so it is written as p ^→t or p ^→r . The HV basis indicates a vector space of horizontal polarization h and vertical polarization v.
Here, the polarization state vectors p ^→t , p ^→r are assumed to be expressed in the HV basis, but the basis may be any vector space of two mutually orthogonal polarized waves, and is not limited to the vector space of horizontal polarization h and vertical polarization v.

式（１）～（３）において、全ての要素は、複素数である。

In equations (1) to (3), all elements are complex numbers.

When the antenna vector A ^→ of the antenna mounted on the radar is expressed as in the following equation (5) and the covariance matrix C is expressed as in the following equation (6), the power P of the received signal can be transformed into the following equation (4).

式（４）～（６）において、Ｔは、転置行列を示す数学記号であり、＊は、共役を示す数学記号である。

In equations (4) to (6), T is a mathematical symbol indicating a transposed matrix, and * is a mathematical symbol indicating a conjugate.

The state in which scattering from multiple scatterers is incoherently superimposed can be expressed as an ensemble average by using the covariance matrix C, as shown in the following equation (7). The state in which scattering from multiple scatterers is incoherently superimposed is the state of the signal that is actually received.

The polarization state vector p ^→i can be expressed using the orientation angle ψ and the ellipsity angle χ as shown in the following equation (8).

If the orientation angle ψ and the ellipticity angle χ are defined on a general elliptically polarized wave as shown in FIG. 5, the angular ranges of the orientation angle ψ and the ellipticity angle χ are expressed by the following expressions (9) and (10).
-90deg≦ψ≦90deg (9)
-45deg≦χ≦45deg (10)
FIG. 5 is an explanatory diagram showing the orientation angle ψ and the ellipsity angle χ.
5, h indicates the direction of polarization of a horizontally polarized wave, and v indicates the direction of polarization of a vertically polarized wave. The ellipse in FIG. 5 represents an elliptical polarization.

When multiple scattering matrices S are obtained by performing multi-polarized SAR observations, the covariance matrix C can be obtained by ensemble averaging the multiple scattering matrices S. For the covariance matrix C shown in equation (7), the scattering coefficient can be derived while changing the orientation angle ψ and the elliptically angle χ in the angle range shown in equations (9) to (10). In particular, the figure showing the received power calculated for Co-Pol (Co-polarization), in which the polarization state of transmission and the polarization state of reception are the same, and Cr-Pol (Cross-polarization), in which the polarization state of transmission and the polarization state of reception are orthogonal, is called PS (Polarmetric Signature).

For example, the PS of a Co-Pol for a horizontal dipole is as shown in FIG. 6A, and the PS of a Cr-Pol for a horizontal dipole is as shown in FIG. 6B.
FIG. 6A is an explanatory diagram showing an example of the PS of Co-Pol with respect to a horizontal dipole, and FIG. 6B is an explanatory diagram showing an example of the PS of Cr-Pol with respect to a horizontal dipole.
PS is unique information that represents the state of the observation target, and its shape does not change unless the state of the observation target and the state of the radar sensor change. Currently, the bases widely used in polarimetric SAR observation are H polarization and V polarization, and their combinations are Co-Pol (HH polarization and VV polarization) and Cr-Pol (HV polarization and VH polarization), and the angles correspond as follows:
Co-Pol
:HH@(ψ ^t , χ ^t )=(0,0), VV@(ψ ^t , χ ^t )=(±90,0)
Cr-Pol
:VH@(ψ ^t , χ ^t )=(0,0), HV@(ψ ^t , χ ^t )=(±90,0)
(11)

Therefore, by using PS to represent the scattering state of the observed target, the sensitivity of the observed target to the polarization with respect to the change in the observed target can be defined as follows:
Polarization sensitivity=Effect of change in observed object on shape of PS Furthermore, by using the orientation angle ψ and elliptically angle χ for transmission and reception, and the parameter p _i (t) of the observed object, the polarization sensitivity can be expressed as in the following equation (12).

式（１２）において、ｐ_ｉは、時間又は入射角等の観測条件、あるいは、連続的に変化する物理量を示すものである。連続的に変化する物理量としては、例えば、観測対象の高さ、又は、観測対象の生育状況がある。

In formula (12), p _i represents an observation condition such as time or an incident angle, or a continuously changing physical quantity. The continuously changing physical quantity may be, for example, the height of the observation target or the growth state of the observation target.

The power P of the received signal shown in equation (7) can also be expressed as the following equation (13) by separating the transmission polarization state and the reception polarization state.

Optimization to maximize the power P of the received signal corresponds to maximizing the power P shown in equation (7) or the power P shown in equation (13), and can be optimized by using the Lagrange undetermined multiplier method.
If the Lagrange multiplier method is applied to equation (7), an eigenvalue problem such as that shown in equation (15) below is obtained.

式（１５）において、λは、固有値である。

In equation (15), λ is an eigenvalue.

Since <[C]> is a 4 × 4 matrix, four eigenvalues and four eigenvectors are obtained. The eigenvector corresponding to the largest eigenvalue among the four eigenvalues is the antenna vector that maximizes the power P of the received signal.
If the Lagrange multiplier method is applied to equation (13), an eigenvalue problem such as that shown in equation (16) below is obtained.

Because <[C']> is a 2x2 matrix, two eigenvalues and two eigenvectors are obtained. Of the two eigenvalues, the eigenvector corresponding to the largest eigenvalue is the receiving polarization state that maximizes the power P of the received signal.

The difference is that equation (15) optimizes both transmission and reception together, whereas equation (16) optimizes the reception polarization state for any transmission polarization state.
The former is a purely mathematical optimization, and while it is expected to obtain higher received power than the latter, there is no guarantee that the orientation angle ψ and the elliptically angle χ can be restored from the obtained antenna vector. In other words, there is a possibility that optimization will be performed in a physically impossible antenna state. On the other hand, the latter can always restore those angles and know the physical polarization state, but the maximization options are limited within those angle ranges.
In equation (13), the transmit polarization state is arbitrarily determined by the specified ψ ^t and χ ^t . Meanwhile, as shown in the following equations (17) to (18), ψ ^r and χ ^r can be restored from the optimized receive polarization state as follows:
By the above optimization, a pair (φ ^r , χ ^r ) can be determined for a pair (φ ^t , χ ^t ).

Next, the operation of the change detection device shown in FIG. 1 will be described.
FIG. 4 is a flow chart showing a change detection method which is a processing procedure of the change detection device.
The image acquisition unit 1 acquires time-series polarimetric SAR images obtained by performing polarimetric SAR observation of an observation target at a plurality of different times (step ST1 in FIG. 4).
The image acquisition unit 1 outputs time-series polarized SAR images to each of the polarization state determination unit 2 and the polarization component extraction unit 3.
For ease of explanation, it is assumed here that the image acquisition unit 1 acquires polarimetric SAR images at two times as time-series polarimetric SAR images.
Furthermore, of the polarimetric SAR images at two times, the power value of the polarimetric SAR image observed before the occurrence of an event is denoted as P _before , and the power value of the polarimetric SAR image observed after the occurrence of the event is denoted as P _after .

The polarization state determining unit 2 acquires polarimetric SAR images at two different times from the image acquiring unit 1 .
Based on the power of the polarized SAR images at two times, the polarization state determination unit 2 determines a combination of the polarization state of the transmitted radio waves emitted toward the observation target and the polarization state of the received radio waves, which are radio waves reflected by the observation target, as the polarization state to be used for detecting changes in the observation target.
Specifically, the polarization state determination unit 2 calculates a difference ΔP lin between a power value P _after of a polarimetric SAR image observed after an event occurs and a power value P _before of a polarimetric SAR image observed before the event occurs, as shown in the following equation (20 ₎ (step ST2 in FIG. 4 ).
ΔP _lin =P _after -P _before (20)

Here, the polarization state determination unit 2 acquires polarized SAR images at two times from the image acquisition unit 1, and calculates the difference ΔP _lin between the power value P _after of the polarized SAR image and the power value P _before of the polarized SAR image. However, this is merely an example, and the polarization state determination unit 2 may acquire polarized SAR images at multiple times from the image acquisition unit 1, and calculate the power difference ΔP _lin between any two of the polarized SAR images at the multiple times.

The difference ΔP _lin shown in equation (20) can be expressed as the following equation (21) by using equation (13).

The polarization state determination unit 2 arbitrarily selects the orientation angle ψ and the elliptically angle χ within the ranges shown in equation (9) and (10), thereby selecting an arbitrary transmission polarization state (ψ ^t , χ ^t ).
The polarization state determination unit 2 selects an arbitrary transmission polarization state (ψ ^t , χ ^t ) and then applies the Lagrange undetermined multiplier method to equation (16) to calculate the maximum value of the power value difference ΔP _lin as shown in the following equation (22).

Since <[C']> is a 2x2 matrix, two eigenvalues and two eigenvectors are obtained.
In order to maximize the polarization sensitivity shown in equation (12), the polarization state determination unit 2 determines the eigenvector v ^→ corresponding to the eigenvalue with the largest absolute value as the optimal reception polarization state. This makes it possible to handle cases where the power increases or decreases with the observation before and after, for example.

Specifically, the polarization state determination unit 2 selects an arbitrary transmission polarization state (ψ ^t , χ ^t ) and then calculates the reception polarization state (ψ ^r , χ ^r ) corresponding to the maximum value of the power value difference ΔP _lin as shown in the following equation (23) (step ST3 in FIG. 4).

The maximized power difference ΔP _lin corresponding to an arbitrary transmitting polarization state (ψ ^t , χ ^t ), ΔP _r (ψ ^t , χ ^t ), can be expressed using the receiving polarization state (ψ ^r , χ ^r ), as shown in the following equation (24).

The polarization state determination unit 2 repeatedly performs the process of calculating the power value difference ΔP _lin (step ST2 in FIG. 4 ) and the process of calculating the reception polarization state (ψ ^r , χ ^r ) (step ST3 in FIG. 4 ) while switching between the orientation angle ψ and the elliptically angle χ in the ranges shown in equation (9) and (10).
That is, the polarization state determination unit 2 repeatedly performs the calculation process of the power value difference ΔP lin (step ST2 in FIG. 4 ) and the calculation process of the reception polarization state (ψ ^r , χ ^r ) (step ST3 in FIG. 4 ) until the calculation of the reception polarization state (ψ ^r , χ ^r ) is completed for all combinations of the orientation _angle ψ and the elliptically angle χ (until step ST4 in FIG. 4 is YES) (if step ST4 in FIG. 4 is NO).
The polarization state determination unit 2 compares the absolute values of the differences ΔP _r (ψ ^t , χ ^t ) for each combination to identify the difference ΔP _r (ψ ^t , χ ^t ) with the maximum absolute value.
The polarization state determination unit 2 determines, as the optimal transmission polarization state (ψ ^t _opt , χ ^t _opt ), the transmission polarization state (ψ ^t _opt , χ t ^{opt )} associated with the orientation angle ψ ^t and the elliptically angle χ ^t included in the combination corresponding to the identified difference _ΔPr (ψ ^t , χ ^t ₎ from among multiple combinations (step ST5 in Figure 4).
As a result, the optimal polarization state (φ ^t _opt , χ ^t _opt , φ ^r _opt , χ ^r _opt ) that maximizes the difference ΔP _lin in power values is determined as shown in the following equations (25) and (26). (ψ ^t _opt , χ ^t _opt ) included in the optimal polarization state (ψ ^t _opt , χ ^t _opt , ψ ^r _opt , χ ^r _opt ) is the optimal transmitting polarization state (ψ ^t _opt , χ ^t _opt ), and (ψ ^r _opt , χ ^r _opt ) included in the optimal polarization state (ψ ^t opt, χ ^t _{opt ,} ψ ^r _opt , χ ^r _opt ) is the receiving polarization state (ψ ^r , χ ^r ) corresponding to the maximum value of the power value difference ΔP _lin .
The polarization state determining unit 2 outputs information indicating _{the optimal polarization state (ψ top} ^{, χ} _top , ψ ^r _opt , χ ^r _opt ) to the polarization component extracting unit 3 .

The polarization component extractor 3 acquires time-series polarimetric SAR images from the image acquirer 1 .
The polarization component extractor 3 obtains information indicating the optimal polarization state (ψ ^top _{, χ top , ψ r opt , χ r opt} ⁾ _from ^the _{polarization state} ^determiner 2 .
The polarized component extractor 3 extracts the polarized components corresponding to the optimal polarization state (φ ^top _{, χ top , φ r opt , χ r opt} ⁾ _from ^each _of ^the _polarized SAR images (step ST6 in FIG. 4).
For example, if ψ ^t _opt = χ ^t _opt = 0, ψ ^r _opt = 90, and χ ^r _opt = 0, the polarization component extraction unit 3 extracts the polarization component when the polarization state of the transmitted polarization is horizontal and the polarization state of the received radio wave is vertical.
For example, if ψ ^t _opt ==90, χ ^t _opt =0, and ψ ^r _opt =χ ^r _opt =0, the polarization component extraction unit 3 extracts the polarization component when the polarization state of the transmitted polarization is vertical polarization and the polarization state of the received radio wave is horizontal polarization.
The polarized component extractor 3 outputs the polarized components extracted from each of the polarized SAR images to the change detector 4 .

The change detection unit 4 acquires a plurality of polarization components from the polarization component extraction unit 3 .
The change detector 4 compares the powers of the multiple polarization components with each other.
The change detection unit 4 detects a change in the observation target based on the result of the power comparison (step ST7 in FIG. 4).
Specifically, if the difference between the power of a polarization component at time _t1 and the power of a polarization component at time _t2 among the multiple polarization components is equal to or greater than a threshold value Th, the change detection unit 4 determines that a change has occurred in the observed object between time _t1 and time _t2 .
If the difference between the power of the polarized component at time _t1 and the power of the polarized component at time _t2 is less than the threshold value Th, the change detection unit 4 determines that no change has occurred in the object being observed between time _t1 and time _t2 .
The threshold value Th may be stored in an internal memory of the change detection unit 4, or may be provided from outside the change detection device shown in FIG.
The change detection unit 4 outputs the detection result of the change in the observation target to, for example, a monitoring device (not shown).

In the above-described first embodiment, the change detection device is configured to include an image acquisition unit 1 that acquires time-series polarized SAR images obtained by polarized SAR observation of the observation target at multiple different times, and a polarization state determination unit 2 that determines, based on the power of the polarized SAR images acquired by the image acquisition unit 1, a combination of the polarization state of the transmitted radio wave radiated toward the observation target and the polarization state of the received radio wave, which is the radio wave reflected by the observation target, as the polarization state to be used for detecting changes in the observation target. Therefore, the change detection device can select a combination of polarization states that clearly indicates a change in the observation target.

In the change detection device shown in FIG. 1 , the polarization state determination unit 2 calculates a difference ΔP lin between a power value P _after of the polarized SAR image and a power value P _before of the polarized SAR image, as shown in equation (20), assuming that the power values of the polarized SAR images at two times are represented in a _{linear space} .
However, because the radar signal has a wide dynamic range, the power values of the polarized SAR images at two times may be expressed in logarithmic space (dB). In this case, the polarization state determination unit 2 assumes that the polarization state of the transmitted radio wave is one of a plurality of polarization states, and repeatedly calculates the ratio of the power of any two polarized SAR images among the time-series polarized SAR images. Specifically, the polarization state determination unit 2 calculates the ratio ΔP _log between the power value P _after of the polarized SAR image and the power value P _before of the polarized SAR image, as shown in the following formula (27).

In logarithmic space, as shown in the following equation (28), the ratio of power values ΔP _log is expressed as the ratio between the power value P _after and the power value P _before .

The polarization state determination unit 2 determines an arbitrary transmission polarization state (ψ ^t , χ ^t ) and then applies the Lagrange multiplier method to equation (16) to set an evaluation function f with λ as the Lagrange multiplier that optimizes the power value ratio ΔP _log , as shown in the following equation (29).

式（２９）において、ｄは、任意の定数である。

In equation (29), d is an arbitrary constant.

In this case, the polarization state determination unit 2 assumes that d is an arbitrary constant and maximizes the numerator of equation (28) under the premise that the following equation (30) holds. Differentiating equation (29) and setting it to 0, the following equation (31) is obtained.

The eigenvalue λ of <[C _before ]> ⁻¹ <[C _after ]> corresponds to the power ratio P _after /P _before , and according to equation (28), all the eigenvalues λ are positive real numbers.
Similar to the case where the power values of the polarimetric SAR images at two times are expressed in a linear space, the inverse of the power ratio P after /P _before must be taken into consideration not only when P _after > P _before but also when P _after < P _before in order to maximize the sensitivity of _{the power ratio P after /} P _before .
Therefore, equation (31) is transformed into the following equation (32).

From equation (29), the eigenvalue λ' is expressed as follows in equation (33).

As a result, the receiving polarization state that maximizes the sensitivity of the power ratio P _after /P _before becomes the eigenvector corresponding to the larger of the eigenvalue λ of <[C _before ]> ^-1 <[C _after ]> and the inverse λ' of the eigenvalue λ of <[C _before ]> ^-1 <[C _after ]>, as shown in the following equation (34). Therefore, the polarization state determination unit 2 selects the larger of the eigenvalue λ of <[C _before ]> ^-1 <[C _after ]> and the inverse λ' of the eigenvalue λ of <[C _before ]> ^-1 <[C _after ]>, and finds the eigenvector corresponding to the larger one.
Here, the polarization state determination unit 2 calculates the inverse λ' of the eigenvalue λ of <[C _before ]> ^-1 <[C _after ]> according to formula (31). However, this is merely an example, and the polarization state determination unit 2 may directly calculate the eigenvalue λ' of <[C _before ]> ^-1 <[C _after ]>. In this case, the polarization state determination unit 2 selects the larger eigenvalue between the eigenvalue λ of <[C _before ]> ^-1 <[C _after ]> and the eigenvalue λ' of <[C _before ]> ^-1 <[C _after ]>, as shown in the following formula (35), and obtains the eigenvector corresponding to the larger eigenvalue.

In the change detection device shown in Fig. 1, the polarization state determination unit 2 physically optimizes the power value difference _ΔPlin or the power ratio _Pafter / _Pbefore . However, this is merely an example, and the polarization state determination unit 2 may mathematically optimize the power value difference _ΔPlin or the power ratio _Pafter / _Pbefore .

When optimizing the power value difference ΔP _lin or the power ratio P _after /P _before mathematically instead of physically, the polarization state determination unit 2 uses a 4×4 covariance matrix <[C]> instead of a 2×2 covariance matrix <[C']> as shown in the following formula (36) or formula (37). Also, the polarization state determination unit 2 replaces the vector to be optimized from 2×1 R ^→* to 4×1 A → ^* as shown in formula (36) or formula (37). This increases the number of eigenvalues from 2 to 4.

In this case, an optimized reception polarization state ( ^ψr , ^χr ) is obtained from the optimal vector A ^→* . Therefore, the polarization state determination unit 2 does not need to determine an arbitrary transmission polarization state ( ^ψt , ^χt ). Therefore, a processing loop accompanying a change in the transmission polarization state ( ^ψt , ^χt ) is not required.

Embodiment 2.
In the second embodiment, a change detection device is described in which a change detection unit 6 compares the polarization states of a plurality of polarimetric SAR images determined by a polarization state determination unit 5 with each other, and detects a change in the observed object based on the comparison result of the polarization states.

Fig. 7 is a configuration diagram showing a change detection device according to embodiment 2. In Fig. 7, the same reference numerals as in Fig. 1 indicate the same or corresponding parts, and detailed description thereof will be omitted.
Fig. 8 is a hardware configuration diagram showing the hardware of the change detection device according to embodiment 1. In Fig. 8, the same reference numerals as in Fig. 2 indicate the same or corresponding parts, and therefore detailed description thereof will be omitted.
The change detection device shown in FIG. 7 includes an image acquisition unit 1, a polarization state determination unit 5, and a change detection unit 6.

The polarization state determining unit 5 is realized by, for example, a polarization state determining circuit 15 shown in FIG.
The polarization state determining unit 5 acquires time-series polarimetric SAR images from the image acquiring unit 1 .
The polarization state determining unit 5 determines a combination of the polarization state of the transmitted radio wave and the polarization state of the received radio wave as the polarization state for each polarized SAR image, based on the power of each polarized SAR image.

The change detection unit 6 is realized by, for example, a change detection circuit 16 shown in FIG.
The change detection unit 6 compares the polarization states for the multiple polarimetric SAR images determined by the polarization state determination unit 5 with each other.
The change detection unit 6 detects a change in the observation target based on the comparison result of the polarization state.

In Fig. 7, it is assumed that each of the components of the change detection device, that is, the image acquisition unit 1, the polarization state determination unit 5, and the change detection unit 6, is realized by dedicated hardware as shown in Fig. 8. That is, it is assumed that the change detection device is realized by an image acquisition circuit 11, a polarization state determination circuit 15, and a change detection circuit 16.
Each of the image acquisition circuit 11, the polarization state determination circuit 15 and the change detection circuit 16 may be, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC, an FPGA, or a combination thereof.

The components of the change detection device are not limited to those realized by dedicated hardware, and the change detection device may be realized by software, firmware, or a combination of software and firmware.
When the change detection device is realized by software, firmware, or the like, a program for causing a computer to execute the respective processing procedures in the image acquisition unit 1, the polarization state determination unit 5, and the change detection unit 6 is stored in a memory 21 shown in Fig. 3. Then, a processor 22 shown in Fig. 3 executes the program stored in the memory 21.

Also, Fig. 8 shows an example in which each of the components of the change detection device is realized by dedicated hardware, and Fig. 3 shows an example in which the change detection device is realized by software or firmware, etc. However, this is merely one example, and some of the components in the change detection device may be realized by dedicated hardware, and the remaining components may be realized by software or firmware, etc.

Next, the operation of the change detection device shown in FIG. 7 will be described.
The image acquisition unit 1 acquires time-series polarimetric SAR images obtained by performing polarimetric SAR observation of an observation target at a plurality of mutually different times.
The image acquisition unit 1 outputs time-series polarimetric SAR images to the polarization state determination unit 5 .

The polarization state determining unit 5 acquires time-series polarimetric SAR images from the image acquiring unit 1 .
The polarization state determining unit 5 determines a combination of the polarization state of the transmitted radio wave and the polarization state of the received radio wave as the polarization state for each polarized SAR image, based on the power of each polarized SAR image.
The process of determining the combination by the polarization state determining unit 5 will be specifically described below.

The power P of each multi-polarized SAR image can be expressed as follows using equation (13):

The polarization state determination unit 5 selects an arbitrary transmission polarization state (ψ ^t , χ ^t ) and then optimizes the power P of each polarimetric SAR image by applying the Lagrange undetermined multiplier method to equation (16), as shown in the following equation (39).

Since <[C']> is a 2x2 matrix, two eigenvalues and two eigenvectors are obtained.
In order to maximize the polarization sensitivity shown in equation (12), the polarization state determination unit 5 determines the eigenvector v ^→ corresponding to the eigenvalue with the maximum absolute value as the optimal receiving polarization state.

Specifically, the polarization state determination unit 5 selects an arbitrary transmission polarization state (ψ ^t , χ ^t ) and then calculates the reception polarization state (ψ ^r , χ ^r ) corresponding to the maximum value of power P, as shown in the following equation (40).

The maximized power P corresponding to an arbitrary transmit polarization state (ψ ^t , χ ^t ), P(ψ ^t , χ ^t ), can be expressed using the receive polarization state (ψ ^r , χ ^r ), as shown in the following equation (41):

The polarization state determining unit 5 calculates power P(ψ t , χ t ) as shown in the following equation (42) for an orientation angle ψ within the range shown in equation (9) and an elliptically angle χ within ^the range shown in equation ( ¹⁰ ).
Then, the polarization state determining unit 5 determines the transmission polarization state (φ ^t _opt , χ ^t _opt ) in which the absolute value of the power P(φ ^t , χ ^t ) is maximum.
From the above, the optimal polarization state (φ ^t _opt , χ ^t _opt , φ ^r _opt , χ ^r _opt ) that maximizes the power P is determined as shown in the following equation (43).
The polarization state determining unit 5 outputs information indicating _{the optimal polarization state (ψ top} ^{, χ} _top , ψ ^r _opt , χ ^r _opt ) to the change detecting unit 6 .

The change detection unit 6 obtains from the polarization state determination unit 5 information indicating optimal polarization states (φ ^top , χ _top , φ ^r _opt , χ ^r _opt ) as the polarization states due to multiple polarization for each polarized ^SAR _image .
The change detection unit 6 compares the polarization states of the time-series polarimetric SAR images with each other.
The change detection unit 6 detects a change in the observation target based on the comparison result of the polarization state.
Specifically, if the difference between the polarization states of multiple polarized SAR images, for example, between the polarization state at time _t1 and the polarization state at time _t2 , is equal to or greater than a threshold value Thp, the change detection unit 6 determines that a change has occurred in the observed object between time _t1 and time _t2 .
That is, if any of the differences between ψ ^t _opt at time _t1 and ψ ^t _opt at time _t2 , between χ ^t _opt at time _t1 and χ ^t _opt at time _t2 , between ψ ^r _opt at time _t1 and ψ ^r _opt at time _t2 , or between χ ^r _opt at time _t1 and χ ^r _opt at time _t2 , is greater than or equal to the threshold value Thp, the change detection unit 6 determines that a change has occurred in the object being observed between time _t1 and time _t2 .
If the difference between the polarization state at time _t1 and the polarization state at time _t2 is less than the threshold value Thp, the change detection unit 6 determines that no change has occurred in the observation target between time _t1 and time _t2 .
That is, if the difference between ψ ^t _opt at time _t1 and ψ ^t _opt at time _t2 , the difference between χ ^t _opt at time _t1 and χ ^t _opt at time _t2 , the difference between ψ ^r _opt at time _t1 and ψ ^r _opt at time _t2 , and the difference between χ ^r _opt at time _t1 and χ ^r _opt at time _t2 are all less than the threshold value Thp, the change detection unit 6 determines that no change has occurred in the object being observed between time _t1 and time _t2 .
The threshold value Thp may be stored in an internal memory of the change detection unit 6, or may be provided from outside the change detection device shown in FIG.
The change detection unit 6 outputs the detection result of the change in the observation target to, for example, a monitoring device (not shown).

In the above-described second embodiment, the polarization state determination unit 5 determines a combination of the polarization state of the transmitted radio wave and the polarization state of the received radio wave as the polarization state for each polarized SAR image based on the power of each polarized SAR image acquired by the image acquisition unit 1. The change detection device is configured so that the change detection unit 6 compares the polarization states for the multiple polarized SAR images determined by the polarization state determination unit 5 with each other, and detects a change in the observation target based on the comparison result of the polarization states. Therefore, the change detection device can detect a change in the observation target based on a combination of polarization states that clearly indicates a change in the observation target.

In addition, this disclosure allows for any combination of the embodiments, any modification of any component of each embodiment, or any omission of any component of each embodiment.

The present disclosure is suitable for a change detection device and a change detection method.

1 Image acquisition unit, 2 Polarization state determination unit, 3 Polarization component extraction unit, 4 Change detection unit, 5 Polarization state determination unit, 6 Change detection unit, 11 Image acquisition circuit, 12 Polarization state determination circuit, 13 Polarization component extraction circuit, 14 Change detection circuit, 15 Polarization state determination circuit, 16 Change detection circuit, 21 Memory, 22 Processor.

Claims (5)

an image acquisition unit that acquires time-series polarimetric synthetic aperture radar (SAR) images obtained by observing an observation target at a plurality of mutually different times;
a polarization state determination unit that determines a combination of a transmission polarization state and a polarization state of a received radio wave, which is a radio wave reflected by the observation target, while switching the polarization state of a transmission radio wave radiated toward the observation target, as a polarization state used for detecting a change in the observation target, based on a difference ΔPlin between power values of two polarized SAR images acquired by the image acquisition unit,
The polarization state determination unit includes:
when an arbitrary transmission polarization state is selected, a reception polarization state corresponding to a maximum value of the difference ΔPlin in power values between the polarized SAR images at two times is calculated, and while switching the transmission polarization state, the calculation process of the difference ΔPlin in power values and the calculation process of the reception polarization state are repeatedly performed, and the differences ΔPlin for all combinations are compared with each other to determine an optimal combination of the transmission polarization state and the reception polarization state that maximizes the difference ΔPlin in power values;
The polarization state determination unit includes a polarization component extraction unit that extracts polarization components corresponding to the combination determined by the polarization state determination unit from each of the polarized SAR images acquired by the image acquisition unit.
Change detection device. an image acquisition unit that acquires time-series polarimetric synthetic aperture radar (SAR) images obtained by observing an observation target at a plurality of mutually different times;
a polarization state determination unit that determines a combination of a transmission polarization state and a polarization state of a received radio wave, which is a radio wave reflected by the observation target, while switching the polarization state of a transmission radio wave radiated toward the observation target, as a polarization state used for detecting a change in the observation target, based on a difference ΔPlin between power values of two polarized SAR images acquired by the image acquisition unit,
The polarization state determination unit includes:
when an arbitrary transmission polarization state is selected, a reception polarization state corresponding to a maximum value of the difference ΔPlin in power values between the polarized SAR images at two times is calculated, and while switching the transmission polarization state, the calculation process of the difference ΔPlin in power values and the calculation process of the reception polarization state are repeatedly performed, and the differences ΔPlin for all combinations are compared with each other to determine an optimal combination of the transmission polarization state and the reception polarization state that maximizes the difference ΔPlin in power values;
a polarization component extracting unit that extracts, from each of the polarimetric SAR images acquired by the image acquiring unit, a polarization component corresponding to the combination determined by the polarization state determining unit;
a change detection unit that compares the powers of the multiple polarization components extracted by the polarization component extraction unit with each other and detects a change in the observation target based on a result of the comparison of the powers.
Change detection device. The polarization state determination unit, in the range shown in the following formula,
-90deg≦ψ≦90deg (9)
-45deg≦χ≦45deg (10)
The power difference ΔP is repeatedly calculated while switching the orientation angle ψ and the ellipsity angle χ.
3. A change detection device according to claim 1 or 2. The polarization state determination unit, in the range shown in the following formula,
-90deg≦ψ≦90deg (9)
-45deg≦χ≦45deg (10)
Solving the optimization problem for physically realizable antenna states,
3. A change detection device according to claim 1 or 2. A change detection method for a change detection device including an image acquisition unit and a polarization state determination unit including a polarization component extraction unit,
the image acquisition unit acquires time-series polarimetric SAR images obtained by observing an observation target with a polarimetric SAR (Synthetic Aperture Radar) at a plurality of mutually different times;
the polarization state determination unit determines, as a polarization state used for detecting a change in the observation target, a combination of a transmission polarization state and a polarization state of a received radio wave, which is a radio wave reflected by the observation target, while switching a polarization state of a transmission radio wave radiated toward the observation target, based on a difference ΔPlin between the power values of the two polarized SAR images acquired by the image acquisition unit;
the polarization component extraction unit extracts, from each of the polarized SAR images acquired by the image acquisition unit, a polarization component corresponding to the combination determined by the polarization state determination unit ;
The polarization state determination unit includes:
When an arbitrary transmission polarization state is selected, a reception polarization state corresponding to the maximum value of the power value difference ΔPlin between the polarized SAR images at two times is calculated, and while switching the transmission polarization state, the calculation process of the power value difference ΔPlin and the calculation process of the reception polarization state are repeated, and the differences ΔPlin for all combinations are compared with each other to determine an optimal combination of the transmission polarization state and the reception polarization state that maximizes the power value difference ΔPlin.
Change detection methods.

Images