JP7779398B2 - Signal processing device and signal processing method - Google Patents

Description

The present invention relates to a signal processing device and a signal processing method.

Synthetic aperture radar (SAR) technology is a technology in which an antenna in a radar mounted on a flying object (such as an artificial satellite or an airplane) transmits and receives electromagnetic waves while the flying object is moving, and an aperture is artificially synthesized to obtain an image (SAR image) equivalent to that obtained by an antenna with a large aperture. Hereinafter, an artificial satellite will be taken as an example of the flying object. An artificial satellite will sometimes be referred to as a SAR satellite.

There is an increasing demand for SAR images capturing a wide area. There is also an increasing demand for high-resolution SAR images. Research into video SAR is also underway. To improve the resolution of SAR images, it is conceivable to increase the synthetic aperture length by pointing an antenna at the capture area for a long period of time. It is also conceivable to widen the capture area by increasing the squint angle of the antenna when performing squint photography (squint observation) (see, for example, Patent Document 1). In squint photography, the capture area is captured by tilting the antenna in the azimuth direction or the opposite direction. In squint photography, the antenna tilt may also fluctuate.

Patent Document 2 describes an example of a terrestrial projection method (terrestrial projection transformation).

JP 2012-093257 A International Publication No. 2010/149132

When squint photography is performed, when an image is generated based on a received signal (observation signal) received by a radar mounted on a satellite, a spectrum (intensity distribution) A is tilted, as illustrated in FIG. 17 . The degree of tilt depends on the squint angle. FIG. 17 is an explanatory diagram that schematically shows spectrum A after imaging based on the observation signal. Specifically, FIG. 17 shows spectrum A after the observation signal has been Fourier transformed. Spectrum A can also be considered to be a region where a signal exists. In FIG. 17 , rectangular region B, whose sides are parallel to the azimuth frequency axis and horizontal to the range frequency axis, shows a spectrum without tilt.

Spectrum A has a wide bandwidth in both the azimuth frequency direction and the range frequency direction. To sample all signals in spectrum A, it is necessary to sample region C, which includes spectrum A. Sampling is performed along both the azimuth frequency axis and the range frequency axis. For example, sampling region B of spectrum A, which is the same size as spectrum A, may result in aliasing. Therefore, sampling of region C, which is wider than spectrum A, is required. Sampling of region C increases the amount of data. That is, when squint imaging is performed, there is a problem that the amount of data in the radar image increases.

An object of the present invention is to suppress an increase in the amount of data of a radar image.

The signal processing device according to the present invention comprises an upsampling means for adding pixels around the spectrum, a zero Doppler imaging processing means for performing imaging processing based on a zero Doppler algorithm as an imaging algorithm, a rotation processing means for rotating the spectrum resulting from the imaging processing, and a downsampling means for performing processing to reduce the number of pixels around the spectrum, and creates information about the reflected signal from the signal in a data format represented along a first direction representing the direction in which a satellite radiates a signal in a direction different from an orthogonal direction orthogonal to the satellite's direction of travel, and a second direction orthogonal to the first direction in a plane formed by the direction of travel and the orthogonal direction.

In the signal processing method according to the present invention, a computer adds pixels around the spectrum, performs imaging processing based on a zero Doppler algorithm as an imaging algorithm, rotates the spectrum resulting from the imaging processing, and performs processing to reduce the number of pixels around the spectrum, and creates information about the reflected signal from the signal in a data format represented along a first direction representing the direction in which a satellite radiates a signal in a direction different from an orthogonal direction orthogonal to the satellite's direction of travel, and a second direction orthogonal to the first direction in a plane formed by the direction of travel and the orthogonal direction.

The signal processing program according to the present invention causes a computer to add pixels around the spectrum, perform imaging processing based on a zero Doppler algorithm as an imaging algorithm, rotate the spectrum resulting from the imaging processing, reduce the number of pixels around the spectrum, and create information about the reflected signal from the signal in a data format represented along a first direction representing the direction in which a satellite radiates a signal in a direction different from an orthogonal direction orthogonal to the satellite's direction of travel, and a second direction orthogonal to the first direction in a plane formed by the direction of travel and the orthogonal direction.

According to the present invention, an increase in the amount of data required to generate an image can be suppressed.

1 is an explanatory diagram for explaining a projection surface of a radar image, a flying object such as an artificial satellite, the earth's surface, and various directions. FIG. 2 is an explanatory diagram for explaining an example of a coordinate system of a radar image. FIG. 2 is an explanatory diagram for explaining an example of a coordinate system of a radar image. FIG. 2 is an explanatory diagram for explaining an example of a coordinate system of a radar image. 1 is a block diagram showing an example of the configuration of a signal processing device according to a first embodiment; FIG. 10 is a block diagram illustrating an example of the configuration of a signal processing device according to a second embodiment. 10 is a flowchart illustrating an operation of a signal processing device according to a second embodiment. FIG. 10 is a block diagram showing an example of the configuration of a signal processing device according to a third embodiment. 10 is a flowchart illustrating an operation of a signal processing device according to a third embodiment. FIG. 11 is an explanatory diagram for explaining a shift process in the third embodiment. FIG. 10 is a block diagram showing an example of the configuration of a signal processing device according to a fourth embodiment. FIG. 10 is an explanatory diagram for explaining zero Doppler. FIG. 13 is an explanatory diagram for explaining zero Doppler processing in the fourth embodiment. FIG. 10 is a block diagram showing an example of the configuration of a signal processing device according to a fifth embodiment. FIG. 13 is a block diagram showing an example of the configuration of a signal processing device according to a sixth embodiment. FIG. 10 is an explanatory diagram for explaining oblique coordinate information. FIG. 13 is a block diagram showing an example of the configuration of a signal processing device according to a seventh embodiment. FIG. 1 is a block diagram illustrating an example of a computer having a CPU. FIG. 2 is an explanatory diagram schematically showing a spectrum A obtained after a Fourier transform of an observed signal.

FIG. 1 is an explanatory diagram illustrating the projection plane of a radar image, a flying object, e.g., a satellite, the Earth's surface, and various directions. FIG. 1 shows the satellite's direction of flight and the antenna direction, which is the direction in which the satellite's antenna is pointed. The plane formed by these two directions is shown as the projection plane. Furthermore, a direction that is perpendicular to the flight direction and included in the projection plane is shown as the direction perpendicular to the flight direction. Hereinafter, the antenna direction is referred to as the range direction. Furthermore, the direction perpendicular to the satellite's flight direction is the direction in which Doppler becomes zero, as will be described later. Therefore, this direction is also referred to as the zero Doppler direction. The angle between the zero Doppler direction and the antenna direction is called the squint angle, and an angle greater than approximately 5 degrees is considered high squint.

In general, in radar images, the zero-Doppler direction and the range direction are often treated as the same. However, in the case of high squint, the difference cannot be ignored, so we will clearly distinguish them. Satellites are equipped with antennas. The antenna emits electromagnetic waves in the range direction. When the electromagnetic waves hit the area indicated by the ellipse and bounce back, the phase delay and reflection strength are recorded. Regarding radar image projection, for a given position on the satellite orbit, reflections from all positions at the same distance from that position within a plane perpendicular to the satellite's direction of motion are added together, and the result is recorded at the position that intersects with the radar image projection plane. Furthermore, as the satellite moves, the electromagnetic wave irradiation position shifts, and electromagnetic wave reflections from different ground positions are recorded for different satellite positions.

In synthetic aperture radar, the reflection of electromagnetic waves emitted with a small spread from a certain satellite position is calculated by combining the reflections of electromagnetic waves emitted with a spread from multiple satellite positions. The satellite position after this combination is referred to as azimuth, etc. Because there is no actual difference between azimuth and satellite position, we will not distinguish between them here. That is, the azimuth direction refers to the satellite's direction of travel. The coordinate axis extending in that direction is referred to as the azimuth axis. Note that while the explanation has been based on a flat Earth surface and a straight orbit parallel to it as shown in Figure 1, the same applies to a spherical Earth surface and a curved satellite orbit in an actual satellite. Furthermore, it is well known that for radar images acquired using a curved Earth surface and a satellite orbit, it is possible to approximately derive a flat Earth surface and a straight satellite orbit that can obtain a radar image similar to that acquired using a curved Earth surface and a satellite orbit. The squint angle in this approximate geometry is referred to as the effective squint angle, etc., but the two will not be distinguished in the following explanation.

2A to 2C are explanatory diagrams illustrating examples of coordinate systems for radar images. Specifically, FIGS. 2A to 2C show coordinate systems within the projection plane shown in FIG. 1. FIG. 2A shows a coordinate system with two axes: the direction of travel of the satellite 1 and a direction perpendicular to that. The axial direction of the coordinate system corresponds to the sampling direction. FIG. 2B shows a coordinate system with two axes: the direction of travel of the satellite 1 and the range direction of the satellite. FIG. 2C shows a coordinate system with two axes: the range direction of the satellite 1 and a direction perpendicular to that. Note that the term "grid" refers to each of the multiple vertical and horizontal lines that divide space. The rectangular meshes enclosed by the grid correspond to pixels. In other words, a radar image is composed of a large number of meshes. In FIGS. 2A and 2C, each mesh, i.e., each pixel, is clearly marked. Hereinafter, the range direction will be referred to as the "antenna direction."

When the coordinate system shown in FIG. 2A is used, the spectrum is tilted as shown in FIG. 17. As a result, the amount of data required to avoid aliasing increases. The observation signals are stored in, for example, a storage device. When the coordinate system shown in FIG. 2A is used, the time in the zero-Doppler direction corresponding to the observation signals is stored in the storage device, for example. Zero Doppler refers to the direction in which the Doppler caused by the satellite's movement is zero. Generally, the direction in which the Doppler is zero is a direction perpendicular to the satellite's movement velocity vector. In many cases, zero Doppler refers to the cross-orbit direction or a direction that takes into account the movement velocity of the observation target, which depends on the time on Earth. Furthermore, the time in the zero-Doppler direction is the time when the observation target passes directly in front of the satellite in the zero-Doppler direction (cross-orbit direction). In other words, satellite image processing systems are generally capable of calculating the satellite's position at each pixel by linking the satellite's orbit with the zero-Doppler time of each pixel.

When the coordinate system shown in FIG. 2B is used, the spectrum has a parallelogram shape. As a result, the amount of data required to avoid aliasing is reduced compared to when the coordinate system shown in FIG. 2A is used. However, when processing radar images, for example, when moving them, the processing load increases. When the coordinate system shown in FIG. 2B is used, as an example, the delay time with respect to the antenna direction and the time at which a scatterer would come in front of the antenna if the antenna had continued to point in the range direction during the imaging time are stored in a storage device in association with the observation signal. Note that the delay time is the time from when an electromagnetic wave is emitted until a reflected wave is received.

The coordinate system shown in Fig. 2C is used in the following embodiments. When sampling is performed along each of the two axes using the coordinate system shown in Fig. 2C, unnecessary signals do not need to be sampled. Therefore, an increase in the amount of data in the radar image is suppressed.

The coordinate system shown in Figure 2C can be used, for example, when a satellite radiates a signal in a direction different from a direction perpendicular to the satellite's direction of travel. The signal is radiated from a radar mounted on the satellite toward an observation target, such as the Earth's surface. The presence or absence of an observation target and the shape of the observation target can be observed depending on the signal strength of the reflected signal representing the reflection of the signal.

The intensity of the observed signal is expressed in a data format measured in the coordinate system shown in FIG. 2C , for example. One axis in the coordinate system is an axis along the direction in which the satellite radiates the signal (for convenience, this may be referred to as the "first direction"). When the squint angle is greater than 0, the first direction is a direction different from the orthogonal direction that is perpendicular to the satellite's direction of travel. The other axis is an axis along a direction perpendicular to the first direction (for convenience, this may be referred to as the "second direction") in the plane formed by the direction of travel and the orthogonal direction. Therefore, information about the reflected signal is created in a data format represented along the first direction and the second direction.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

Embodiment 1.
3 is a block diagram showing an example of the configuration of the signal processing device of the first embodiment. The signal processing device shown in FIG. 3 includes a tilt image generating unit 100 and a satellite observation data storage unit 130.

The satellite observation data storage unit 130 stores observation signals 131 and satellite information 132. The observation signals are signals received by radar on the artificial satellites. The satellite information includes the satellite position, antenna direction, bandwidth, etc.

The tilt image generating unit 100 generates, for example, the two axes illustrated in Fig. 2C based on satellite information, and then calculates the scattering intensity of each pixel using a predetermined method.

In other words, the tilt image generating unit (or information generating unit) 100 generates information about scattering intensity in a data format representing the information along a first direction and a second direction, as described above with reference to Fig. 2C. The first direction represents the direction in which a satellite radiates a signal in a direction different from the orthogonal direction perpendicular to the satellite's direction of travel. The second direction represents the direction perpendicular to the first direction in the plane formed by the direction of travel and the orthogonal direction.

Embodiment 2.
Fig. 4 is a block diagram showing an example of the configuration of a signal processing device according to the second embodiment. The signal processing device shown in Fig. 4 includes a tilt image generating unit 100 and a satellite observation data storage unit 130. In the second embodiment, the tilt image generating unit 100 includes a grid generating unit 111 and a back projection unit 112. The signal processing device according to the second embodiment corresponds to a specific example of the signal processing device according to the first embodiment shown in Fig. 3.

The grid generation unit 111 generates a grid in advance, as exemplified in Fig. 2C, based on satellite information. The back projection unit 112 obtains the scattering intensity of each pixel using back projection.

Next, the operation of the signal processing device of the first embodiment will be described with reference to the flowchart of FIG.

The grid generation unit 111 generates a grid based on satellite information (step S101). In step S101, the grid generation unit 111 generates a grid based on parameters that can be used to calculate resolution, such as bandwidth, and the antenna direction. The grid generation unit 111 generates the grid on two axes: the antenna direction and a direction that exists on a plane spanned by the vectors of the antenna direction and the satellite movement direction and is perpendicular to the antenna direction.

Note that the two axes do not have to be strictly orthogonal. Furthermore, the two axes do not have to be strictly on a plane subtended by the vectors of the antenna direction and the satellite movement direction. Since real artificial satellites fly along curves in space depending on the curvature of the Earth's surface, the antenna direction and the satellite movement direction are not fixed to a single direction. Therefore, the antenna direction may be a representative direction among multiple directions. In other words, the antenna direction may be a part of the multiple directions. Furthermore, the satellite movement direction may be a representative direction among multiple directions. In other words, the satellite movement direction may be a part of the multiple directions.

The back projection unit 112 performs back projection on each mesh formed by the grid (step S102). Back projection is a method of generating each pixel by calculating a correlation integral value for each pixel to be generated. That is, the back projection unit 112 can generate a radar image in an area including multiple meshes formed by multiple grids parallel to the antenna direction and a direction perpendicular to the antenna direction as two axes.

Embodiment 3.
FIG. 6 is a block diagram showing an example of the configuration of a signal processing device according to a third embodiment. The signal processing device shown in FIG. 6 includes a tilt image generating unit 100 and a satellite observation data storage unit 130. In the third embodiment, the tilt image generating unit 100 uses the Omega-K algorithm. In the third embodiment, the tilt image generating unit 100 includes a two-dimensional (2D) Fourier transform unit 113, a range spectrum shifting unit 114, an azimuth spectrum shifting unit 115, and a 2D inverse Fourier transforming unit 116. The signal processing device according to the third embodiment corresponds to another specific example of the signal processing device according to the first embodiment shown in FIG. 3. Note that a general Omega-K algorithm includes range spectrum shifting and azimuth spectrum shifting performed after the 2D Fourier transform.

In the third embodiment, for example, the delay time relative to the antenna direction and the time at which the scatterer would come in front of the antenna if the antenna were continuously pointed in the range direction during the shooting time are stored as satellite information 132 in the satellite observation data storage unit 130 in association with the observation signal.

The 2D Fourier transform unit 113 performs a two-dimensional Fourier transform on the observation signal. The range spectrum shift unit 114 performs spectrum shift processing in the range direction. The azimuth spectrum shift unit 115 performs spectrum shift processing in the azimuth direction. The 2D inverse Fourier transform unit 116 performs a two-dimensional inverse Fourier transform.

Next, the operation of the signal processing device of the third embodiment will be described with reference to the flowchart of Fig. 7 and the explanatory diagram of Fig. 8. Fig. 8 is an explanatory diagram for explaining shift processing.

The 2D Fourier transform unit 113 performs a two-dimensional Fourier transform on the observation signal (step S103). The range spectrum shift unit 114 shifts the spectrum in the range direction (step S104). In step S104, the range spectrum shift unit 114 shifts the spectrum by an amount corresponding to the azimuth frequency. Next, the azimuth spectrum shift unit 115 shifts the spectrum in the azimuth direction (step S105).

In the third embodiment, when the satellite information 132 in the satellite observation data storage unit 130 is used, the spectrum illustrated in the upper left part of Fig. 8 is obtained. Note that such a spectrum corresponds to the spectrum in the coordinate system illustrated in Fig. 2B. When the process of step S104 is performed, a spectrum curved in the azimuth direction, as illustrated in the upper right part of Fig. 8, is obtained.

A specific example of the shift process will be described below.

Let the range time be τ and the azimuth time be η. Let the observation signal be s(τ,η). Let the antenna direction be θ _sq . Let the range wave number corresponding to the wave number of the frequency of the electromagnetic wave be k _carrier .

Furthermore, the spatial frequency in the range direction is defined as _krg , and the spatial frequency in the azimuth direction is defined as _kaz . A two-dimensional spectrum S( _krg , _kaz ) is obtained by two-dimensional Fourier transform of the observation signal. The 2D Fourier transform unit 113 also performs scaling processing of (speed of light/2) in the range direction and scaling processing using the satellite velocity in the azimuth direction.

When the range spectrum shift unit 114 moves, i.e., shifts, the value of the position indicated by ( _krg , _kaz ) to ( _k'rg , _kaz ), the spectrum illustrated on the right side of the upper row in Fig. 8 is obtained. _k'rg is expressed by equation (1). In equation (1), _C1 is an arbitrary constant.

When _C1 is expressed by equation (2), the frequency characteristics of _k'rg are closest to the frequency characteristics of the original observed signal. When _C1 is expressed by equation (3), the center of the range band becomes 0, making it easiest to perform interpolation processing required for image movement processing, etc. Also, it becomes easier to avoid aliasing in the range direction.

In the third embodiment, the azimuth spectrum shifter 115 corrects the slope of the spectrum in the azimuth direction (see the right side of the top row in FIG. 8 ). Specifically, the azimuth spectrum shifter 115 shifts ( _k'rg , _kaz ) to ( _k'rg , _k'az ). Here, _k'az is expressed by equation (4). In equation (4), _C2 is an arbitrary constant. Note that _sinθsq corresponds to the slope of the spectrum illustrated in the right side of the top row in FIG. 8 .

The azimuth spectrum shifter 115 shifts ( _k'rg , _k'az ) to ( _k'rg , _k'az ), thereby obtaining a spectrum such as that shown in the lower part of Fig. 8. When _C2 = 0, the center of the azimuth band becomes 0, making it easier to avoid aliasing in the azimuth direction.

Note that tan θ _sq in equation (1) and sin θ _sq in equation (4) may be values close to tan θ _sq or sin θ _sq . Ideally, a precise squint angle is used based on control information of the satellite's onboard positioning system or antenna. However, if accurate positioning system information is unavailable, the squint angle may be calculated backward from the Doppler shift of the electromagnetic waves reflected from the Earth's surface. In imaging processing, a curved trajectory may be approximated as a straight line, or a curved Earth surface may be approximated as a flat surface. For example, the effective squint angle, θ _sq , may be used as the squint angle in the approximated geometry. The angle of incidence of the reflected wave as seen from the Earth's surface may also be used as θ _sq .

Thereafter, the 2D inverse Fourier transform unit 116 performs a two-dimensional inverse Fourier transform (step S106).

In the third embodiment, a spectrum such as that shown in the lower part of Fig. 8 can be obtained. That is, unnecessary signals are reduced, and an increase in the amount of data of a radar image is suppressed.

In the third embodiment, a radar image is also generated in an area including a plurality of meshes formed by a grid parallel to one of two axes, the antenna direction and a direction perpendicular thereto.

Embodiment 4.
Fig. 9 is a block diagram showing an example of the configuration of a signal processing device according to a fourth embodiment. The signal processing device shown in Fig. 9 includes a tilt image generating unit 100 and a satellite observation data storage unit 130. In the fourth embodiment, the tilt image generating unit 100 includes an upsampling unit 117, a zero-Doppler imaging processing unit 118, a rotation processing unit 119, and a downsampling unit 120. In the fourth embodiment, the tilt image generating unit 100 uses a general zero-Doppler algorithm as an imaging algorithm. The zero-Doppler algorithm refers to a general imaging method that performs imaging along two axes, the zero-Doppler direction and the satellite orbit direction.

The upsampling unit 117 performs processing to increase the number of pixels. The zero-Doppler imaging processing unit 118 performs imaging processing based on a zero-Doppler algorithm, i.e., zero-Doppler processing as imaging processing. The rotation processing unit 119 performs processing to rotate the spectrum resulting from the imaging processing. The downsampling unit 120 performs processing to reduce the number of pixels around the spectrum.

FIG. 10 is an explanatory diagram for explaining imaging processing using zero Doppler. When zero Doppler processing is used, time information called zero Doppler time is saved. Zero Doppler time differs from the time when the radar actually receives the signal. When the squint angle θ _sq is small, there is little discrepancy between the reception time and the zero Doppler time. Therefore, even if the radar image is projected onto the ground using a satellite position interpolated to the zero Doppler time, no discrepancy occurs. However, when the squint is high, there is a large difference between the reception time and the zero Doppler time, resulting in a discrepancy.

When zero-Doppler processing is used, the spectrum becomes tilted when an image is generated based on the observed signal, as shown in Fig. 17. As a result, aliasing is likely to occur.

In the fourth embodiment, aliasing is avoided even when zero Doppler processing is used.

11 is an explanatory diagram illustrating zero-Doppler processing. As illustrated in the upper left part of FIG. 11, the spectrum tilts when an image is generated based on the observed signal. Therefore, the upsampling unit 117 adds pixels with a pixel value of 0 around the actual spectrum so that the entire spectrum can be sampled (see the upper right part of FIG. 11).

The zero Doppler imaging processor 118 performs zero Doppler processing. A radar image is obtained by the zero Doppler processing. However, as illustrated in the lower left side of FIG. 11 , the spectrum is tilted. The rotation processor 119 performs rotation processing on the image to eliminate the spectral tilt. The spectral tilt is corrected. After the rotation processing by the rotation processor 119 is performed, the spectral tilt is eliminated, as illustrated in the lower center of FIG. 11 . The downsampling unit 120 deletes pixels in areas where no spectrum exists (see the lower right side of FIG. 11 ).

The signal processing device of the fourth embodiment can avoid aliasing by upsampling, and can suppress an increase in the data amount of a radar image by downsampling performed after rotation processing.

The above-described embodiments can also be applied to a bistatic configuration in which the transmitting antenna and the receiving antenna are located at different positions. In the case of a bistatic configuration, the bisector of the angle between the direction of the transmitting antenna relative to the target object and the direction of the receiving antenna relative to the target object can be set as one axis, and the perpendicular direction can be set as the other axis.

The above-described embodiments can also be applied to tomography, which performs three-dimensional synthetic aperture processing by capturing images from multiple orbits. In the case of tomography, for example, as with two-dimensional squint images, one axis is set to the antenna direction, and the other axis is set to a direction perpendicular to the antenna direction and within the plane formed by the satellite orbit and the antenna direction. Creating a three-dimensional grid with the normal direction to the plane formed by these two axes as the elevation direction can effectively prevent aliasing.

In bistatic tomography (where there is one transmitter satellite but multiple receiver satellites are flying), the above embodiments can be applied by using the bisector direction of the angle between the transmitter antenna direction and the receiver antenna direction instead of the antenna direction.

In the fourth embodiment, a radar image is also generated in an area including a plurality of meshes formed by a grid parallel to one of two axes, the antenna direction and a direction perpendicular thereto.

Embodiment 5.
The signal processing device of the fifth embodiment is effectively applied to a high-resolution mode or a wide-area mode. For example, in order to increase the resolution of a SAR image, it is conceivable to increase the synthetic aperture length by directing the antenna of a radar mounted on a satellite toward the imaging area for a long period of time. The mode in which processing to increase the resolution of a SAR image is executed is referred to as the high-resolution mode.

Furthermore, for example, the imaging area can be expanded by imaging while changing the squint angle of the antenna. A mode in which a wide area is imaged is called wide-area mode. Hereinafter, changing the squint angle of the antenna may be referred to as "swinging the antenna."

The signal processing device shown in FIG. 12 includes a tilt image generating unit 101, a satellite observation data storage unit 130, and a phase modulation estimating unit 200.

In the fifth embodiment, the tilt image generating unit 101 has the same function as the tilt image generating unit 100 in the first embodiment. However, the tilt image generating unit 101 performs imaging processing while taking into account the influence of antenna swing. For example, the imaging result includes phase modulation information that indicates phase modulation that depends on the direction of the antenna at each time. Therefore, it is possible to calculate a change in bandwidth that depends on the image position in the image from the phase modulation information, and to remove the influence.

In the fifth embodiment, the tilt image generating unit 101 outputs a radar image, and the phase modulation estimating unit 200 outputs phase modulation information. The phase modulation estimating unit 200 calculates the amount of phase modulation at each coordinate in a direction perpendicular to the antenna direction. Note that the amount of phase modulation is almost constant in the antenna direction.

When the satellite approaches the scatterer during photography, the Doppler frequency is high. When photography is performed with the distance between the satellite and the scatterer remaining roughly constant, the satellite neither approaches nor moves away from the scatterer, so the Doppler frequency is 0. When the antenna is swung, both the state in which the satellite approaches the scatterer and the state in which the distance between the satellite and the scatterer remains roughly constant appear repeatedly.

In other words, when the antenna is moved, the time at which the electromagnetic wave strikes the scatterer corresponding to each pixel in the image differs, resulting in a change in the bandwidth depending on the antenna direction at the time the electromagnetic wave strikes. As a result, phase modulation appears across the entire image. Since the bandwidth differs depending on the pixel position, for example, even if sampling is performed to prevent aliasing at the left edge of the image, aliasing may occur at the right edge of the image.

In the fifth embodiment, the signal processing device provides phase modulation information to a device such as an image processing device that uses the output of the signal processing device. Hereinafter, the device such as the image processing device will be referred to as another device.

When imaging a sufficiently distant object, the change in the band depending on the pixel position occurs as a shift in the band center, and the change in the bandwidth is quite small. Taking advantage of this, for example, by calculating the phase modulation that shifts the band center obtained from each pixel and its neighboring pixels from 0, and then removing the calculated phase modulation from the radar image, it is possible to perform image processing such as translation while avoiding aliasing.

Furthermore, when another device performs interference processing or the like, it can perform a process of removing phase modulation from radar images in the interference processing, then align the images by moving them relative to each other, and then add the phase modulation again before performing interference processing. Furthermore, the other device can perform interference processing between aligned images and interference processing between phase modulations added to each image, and add the result of the interference processing between phase modulations to the result of the interference processing between images, thereby preventing aliasing and performing interference processing that accurately maintains phase.

Furthermore, other devices can use phase modulation information to deform images without losing phase information while preventing aliasing. In particular, when imaging results are obtained in an oblique coordinate system as in the radar processing devices of the above embodiments, the following effects can also be obtained.

In other words, when another device calculates phase modulation information for the imaging results in an oblique coordinate system, there is no pixel position dependency in the antenna direction, and the phase modulation information can be approximated by a quadratic function in the direction perpendicular to the antenna direction, so processing such as expansion and contraction due to deformation can be easily performed as polynomial deformation processing.

Furthermore, when another device uses the signal processing device of the fifth embodiment to generate an image using interferometric SAR (Synthetic Aperture Radar Interferometry), the two radar images can be aligned with each other with the phase modulation removed, causing them to interfere with each other. The other device then adds the interference results between the phase modulations to the interference. By performing such processing, the other device can perform interference processing while avoiding aliasing and other problems during processing.

The phase modulation estimation unit 200 can calculate the phase modulation using a combination of coefficients used in an imaging algorithm. The phase modulation estimation unit 200 can also calculate the phase modulation from only the satellite orbit, the squint angle, and the gaze point position. The phase modulation estimation unit 200 can also calculate the phase modulation from only the satellite orbit, the position of the subject, and the gaze point position.

Embodiment 6.
Fig. 13 is a block diagram showing an example of the configuration of a signal processing device according to the sixth embodiment. The signal processing device shown in Fig. 13 includes a tilt image generating unit 100, a satellite observation data storage unit 130, an image processing unit 300, and a ground projection unit 301. The tilt image generating unit 102 has the same functions as the tilt image generating unit 100 in the first embodiment.

The configuration of the signal processing device of the sixth embodiment is a configuration in which an image processing unit 300 and a terrestrial projection unit 301 are added to the signal processing device of the first embodiment. Note that the image processing unit 300 and the terrestrial projection unit 301 may be added to a signal processing device of an embodiment other than the first embodiment.

However, in addition to the functions of the tilt image generating unit 100, the tilt image generating unit 102 also has a function of generating oblique coordinate information.

The image processing unit 300 performs image processing on the data, i.e., the radar image, generated by the tilt image generating unit 102. The ground projection unit 301 performs ground projection processing for projecting the image processing results onto the ground.

Image processing that narrows the spatial frequency may be performed, such as conversion to an absolute value image or change detection results. When the image processing unit 300 performs such image processing, it performs image processing using the data generated by the tilt image generation unit 102 as is, and then the ground projection unit 301 performs ground projection processing, thereby reducing the amount of calculation required for processing.

The oblique coordinate information includes the shift amount from the satellite orbit, the coordinate axis direction, etc. However, if the oblique coordinate information includes, for example, information such as that shown in Fig. 14, i.e., the range direction, the direction perpendicular to the range direction, and the image center distance, the terrestrial projection unit 301 can perform projection processing based on the satellite orbit. Note that the range direction and the direction perpendicular to the range direction are configured to be output from the tilt image generation unit 102 because they do not necessarily need to be precise information.

The terrestrial projection unit 301 executes the terrestrial projection process, for example, as follows.

The ground projection unit 301 calculates the satellite time at which each pixel in the radar image was received and the distance from the satellite orbit based on the pixel number and the oblique coordinate information.The ground projection unit 301 then performs ground projection.For example, the ground projection unit 301 calculates the intersection of a circle equidistant from the satellite orbit and a solid on the ground based on the three-dimensional shape of the ground.More simply, the ground projection unit 301 may calculate the intersection of the center of the imaging target point with the tangent plane of the Earth ellipsoid.

For example, when the tangent plane is a plane and projection is performed on a small area, the terrestrial projection unit 301 can realize projection processing and alignment correction processing by aligning the Ground Control Point, i.e., a point that reliably aligns the ground position with the SAR pixel position, and by simple deformation, by slightly modifying the method described in Patent Document 2. Note that while the method described in Patent Document 2 uses a coordinate system with two axes, the antenna direction and the satellite flight direction, the sixth embodiment is modified to use two axes, the antenna direction and a direction perpendicular to the antenna.

In the method described in Patent Document 2, a vector representing an axis in an image is first projected, and the projection position of each pixel is then calculated as a relative position obtained by adding the previously projected vector to the center position of the image.

Embodiment 7.
Fig. 15 is a block diagram showing an example of the configuration of a signal processing device according to the seventh embodiment. The signal processing device shown in Fig. 15 includes a tilt image generating unit 100, a satellite observation data storage unit 130, a vector data extracting unit 302, and a ground projection unit 301. The tilt image generating unit 102 has the same functions as the tilt image generating unit 100 in the first embodiment.

The signal processing device of the seventh embodiment has a configuration in which a vector data extraction unit 302 and a terrestrial projection unit 303 are added to the signal processing device of the first embodiment. Note that the vector data extraction unit 302 and the terrestrial projection unit 303 may be added to the signal processing device of any embodiment other than the first embodiment. The signal processing device of the seventh embodiment has a configuration in which the vector data extraction unit 302 is provided instead of the image processing unit 300 in the signal processing device of the sixth embodiment. Note that the terrestrial projection unit 303 performs terrestrial imaging processing in the same way as the terrestrial projection unit 301, but the input to the terrestrial projection unit 303 is vector data.

As in the sixth embodiment, the tilt image generating unit 102 has the function of generating oblique coordinate information in addition to the function of the tilt image generating unit 100 .

The vector data extraction unit 302 acquires vector data from, for example, a radar image that has been subjected to image processing. For example, if semantic segmentation is performed as the image processing, polygons that connect pixels or pixel groups can be acquired. The ground projection unit 301 performs ground projection processing based on the vector data.

The processing load of the ground projection process based on vector data is relatively small, that is, the signal processing device of the seventh embodiment can perform the ground projection process in a short period of time while preventing aliasing.

The vector data extraction unit 302 may include an interference data analysis function.

Each component in the above embodiment can be configured as a single piece of hardware, or as a single piece of software. Each component can also be configured as multiple pieces of hardware, or as multiple pieces of software. Furthermore, some of the components can be configured as hardware, and the other parts can be configured as software.

Each function in the above-described embodiments can be realized by a computer having a processor such as a CPU (Central Processing Unit), a memory, etc. For example, a program for implementing the method in the above-described embodiments may be stored in a storage device, and each function may be realized by executing the program stored in the storage device by the CPU.

16 is a block diagram showing an example of a computer having a CPU. The computer is implemented in a signal processing device. The CPU 1000 executes processing in accordance with a signal processing program stored in a storage device 1001, thereby realizing the functions of the signal processing device in the above-described embodiment.

Specifically, in the first embodiment, the computer realizes the functions of the gradient image generating unit 100. In the second embodiment, the computer realizes the functions of the grid generating unit 111 and the back projection unit 112. In the third embodiment, the computer realizes the functions of the 2D Fourier transform unit 113, the range spectrum shifting unit 114, the azimuth spectrum shifting unit 115, and the 2D inverse Fourier transforming unit 116. In the fourth embodiment, the computer realizes the functions of the upsampling unit 117, the zero Doppler imaging processing unit 118, the rotation processing unit 119, and the downsampling unit 120. In the fifth embodiment, the computer realizes the functions of the gradient image generating unit 100 and the phase modulation estimating unit 200. In the sixth embodiment, the computer realizes the functions of the gradient image generating unit 102, the image processing unit 300, and the ground projection unit 301. In the seventh embodiment, the computer realizes the functions of the gradient image generating unit 102, the vector data extracting unit 302, and the ground projection unit 303.

The storage device 1001 is, for example, a non-transitory computer readable medium. The non-transitory computer readable medium includes various types of tangible storage media. Specific examples of the non-transitory computer readable medium include magnetic recording media (for example, hard disks), magneto-optical recording media (for example, magneto-optical disks), CD-ROMs (Compact Disc-Read Only Memory), CD-Rs (Compact Disc-Recordable), CD-R/Ws (Compact Disc-ReWritable), semiconductor memories (for example, mask ROMs, PROMs (Programmable ROMs), and EPROMs (Erasable PROMs)).
), flash ROM).

The satellite observation data storage unit 130 can be realized by the storage device 1001 .

The program may also be stored in various types of transitory computer-readable media, to which the program is supplied, for example, via a wired or wireless communication path, i.e., via an electrical signal, an optical signal, or an electromagnetic wave.

The memory 1002 is realized by, for example, a RAM (Random Access Memory), and is a storage means for temporarily storing data when the CPU 1000 executes processing. A configuration is also conceivable in which a program held in the storage device 1001 or a temporary computer-readable medium is transferred to the memory 1002, and the CPU 1000 executes processing based on the program in the memory 1002.

The signal processing device may include a phase modulation estimation means (implemented by the phase modulation estimation unit 200 in the embodiment) that calculates the amount of phase modulation at each coordinate in a direction orthogonal to the antenna direction.

The signal processing device may include an image processing means (realized by the image processing unit 300 in the embodiment) that performs image processing on the radar image, and a ground projection means (realized by the ground projection unit 301 in the embodiment) that performs processing to project the results of the image processing onto the ground.

The signal processing device may include a vector data extraction means (in the embodiment, realized by the vector data extraction unit 302) that acquires vector data from an image obtained by image processing of a radar image, and a ground projection means (in the embodiment, realized by the ground projection unit 303) that performs ground projection processing based on the vector data.

Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

100, 101, 102 Tilt image generation unit 111 Grid generation unit 112 Back projection unit 113 2D Fourier transform unit 114 Range spectrum shift unit 115 Azimuth spectrum shift unit 116 2D inverse Fourier transform unit 117 Upsampling unit 118 Zero Doppler imaging processing unit 119 Rotation processing unit 120 Downsampling unit 130 Satellite observation data storage unit 131 Observation signal 132 Satellite information 200 Phase modulation estimation unit 300 Image processing unit 301, 303 Ground projection unit 302 Vector data extraction unit 1000 CPU
1001 storage device 1002 memory

Claims (5)

an upsampling means for adding pixels around the spectrum;
a zero Doppler imaging processing means for performing imaging processing based on a zero Doppler algorithm as an imaging algorithm;
a rotation processing means for rotating the spectrum resulting from the imaging processing;
downsampling means for performing processing to reduce the number of pixels around the spectrum;
A signal processing device that creates information about a reflected signal in response to a signal in a data format that represents a first direction that represents a direction in which a satellite that emits a signal in a direction different from an orthogonal direction that is orthogonal to the satellite's direction of travel radiates the signal, and a second direction that is orthogonal to the first direction in a plane formed by the direction of travel and the orthogonal direction. The signal processing device according to claim 1 , further comprising a phase modulation estimation unit that calculates a phase modulation amount at each coordinate in the second direction. a grid generating means for generating a plurality of grids having axes along the first direction and the second direction;
3. The signal processing device according to claim 1, further comprising back projection means for generating pixels between the plurality of grids by a back projection method. The computer
Add pixels around the spectrum,
performing imaging processing based on a zero Doppler algorithm as an imaging algorithm;
Rotating the spectrum resulting from the imaging process;
The number of pixels around the spectrum is reduced,
A signal processing method in which the computer creates information about a reflected signal in response to a signal in a data format that represents a first direction that represents a direction in which a satellite radiates a signal in a direction different from an orthogonal direction that is orthogonal to the satellite's direction of travel, and a second direction that is orthogonal to the first direction in a plane formed by the direction of travel and the orthogonal direction. On the computer,
Add pixels around the spectrum,
performing imaging processing based on a zero Doppler algorithm as an imaging algorithm;
Rotating the spectrum resulting from the imaging process;
The number of pixels around the spectrum is reduced.
A signal processing program for executing a process of creating information about a reflected signal in response to a signal in a data format that represents a first direction representing a direction in which a satellite radiates a signal in a direction different from an orthogonal direction that is orthogonal to the satellite's direction of travel, and a second direction that is orthogonal to the first direction in a plane formed by the direction of travel and the orthogonal direction.