Signal processing device, signal processing method, and program

Abstract

This signal processing apparatus is provided with a transformation unit, a reference point generating unit, a calculation unit, an interpolation function generating unit, and an interpolation processing unit. The transformation unit performs Fourier transformation of a reflection signal representing a reflection from a scatterer into first signal data. The reference point generating unit generates a plurality of reference points. The calculation unit calculates the phase of each of second signal data obtained by Fourier transformation of reflection signals for cases in which radar irradiation is performed virtually for the plurality of reference points. The interpolation function generating unit generates an interpolation function using the phase of each of the second signal data. The interpolation processing unit interpolates the first signal data using the interpolation function, and performs inverse Fourier transformation of the first signal data. The interpolation function generating unit generates an interpolation function using a term that is based on a phase relating to the center frequency of radar radiated from a flying object. The interpolation processing unit performs correction processing that corrects for an effect caused by the term based on the phase relating to the center frequency.

Description

Technical Field

[0001] The present invention relates to a signal processing apparatus, a signal processing method, and program to

Background Art

[0002] A technique related to a radar sensor having a plurality of transmission antennas, and a technique for estimating the relative speed of a target is disclosed in Patent Document 1.

[0003] A technique related to a synthetic aperture radar device, and a technique for reducing an error occurring in the relative positional relationship between a fixed target and a moving target and superimposing images of both is disclosed in Patent Document 2.

[0004] A technique related to a synthetic aperture radar device that clarifies a method for determining the optimum degree N of a polynomial and obtains an image with improved resolution is disclosed in Patent Document 3.

[0005] A technique related to the Omega-K Algorithm in a SAR image generation method is disclosed in Non-Patent Document 1.

[0006] A technique related to SVD-Stolt in a SAR image generation method is disclosed in Non-Patent Document 2.

[0007] A technique related to the Extended Wavenumber domain in a SAR image generation method is disclosed in Non-Patent Document 3.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Non-licensed literature

[0009]

Non-licensed literature 1

Non-licensed Document 2

[0010] Patent documents 2 and 3 mentioned above disclose technologies related to SAR (Synthetic Aperture Radar) images. However, the SAR image generation methods described in these documents may result in a deterioration of imaging performance.

[0011] One example of the object of the present invention is to provide a signal processing device, a signal processing method, and a recording medium that can improve imaging performance, in view of the above-mentioned problems. [Means for solving the problem]

[0012] According to one aspect of the present invention, From the flying object In the shooting area Reflected signal representing the reflection from a scatterer to the radar being irradiated. A signal processing device that generates SAR images based on the following, The reflected signal A transformation means for performing a Fourier transform on the first signal data in the frequency domain, In the plane defined by the direction of travel of the flying object and the direction of illumination from which the flying object illuminates the radar, A reference point generation means for generating multiple reference points, A calculation means for calculating the phase of each of the second signal data in the frequency domain obtained by Fourier transforming the reflected signals when the radar is virtually irradiated to the plurality of reference points, An interpolation formula generation means generates an interpolation formula using the phase of each of the second signal data calculated by the calculation means, Interpolation processing means that interpolates the first signal data converted by the conversion means using the interpolation formula and performs an inverse Fourier transform on it. 、 A SAR image generation means that generates a SAR image based on the signal data after the inverse Fourier transform, Equipped with, The interpolation formula generation means generates the interpolation formula using a term based on the phase with respect to the center frequency of the radar emitted from the flying object. The interpolation processing means is provided as a signal processing device that performs a correction process to correct the effects caused by the aforementioned term based on the phase with respect to the center frequency.

[0013] According to one aspect of the present invention, One or more computers, From the flying object In the shooting area Reflected signal representing the reflection from a scatterer to the radar being irradiated. A signal processing method for generating SAR images based on the following, The reflected signal The first signal data in the frequency domain is Fourier transformed, In the plane defined by the direction of travel of the flying object and the direction of illumination from which the flying object illuminates the radar, Generate multiple reference points, The phase of the second signal data in the frequency domain obtained by Fourier transforming the reflected signals when the radar is virtually irradiated to the plurality of reference points is calculated for each of the above. An interpolation equation is generated using the phase of each of the aforementioned second signal data. The first signal data is interpolated using the interpolation formula and then subjected to an inverse Fourier transform. The process includes generating a SAR image based on the signal data after the inverse Fourier transform, By generating the aforementioned interpolation formula,The interpolation equation is generated using a term based on the phase of the central frequency of the radar emitted from the flying object. By interpolating using the aforementioned interpolation formula, A signal processing method is provided that performs a correction process to correct the effects caused by the aforementioned term based on the phase with respect to the center frequency.

[0014] According to one aspect of the present invention, On the computer, From the flying object In the shooting area Reflected signal representing the reflection from a scatterer to the radar being irradiated. A program that generates SAR images based on the following: The reflected signal Fourier transform of the first signal data in the frequency domain death , In the plane defined by the direction of travel of the flying object and the direction of illumination from which the flying object illuminates the radar, Generate multiple reference points, The phase of the second signal data in the frequency domain obtained by Fourier transforming the reflected signals when the radar is virtually irradiated onto the plurality of reference points is calculated for each of them. death , An interpolation equation is generated using the phase of each of the aforementioned second signal data. death , The first signal data is interpolated using the interpolation formula and then subjected to an inverse Fourier transform. death , The computer is instructed to generate a SAR image based on the signal data after the inverse Fourier transform. By generating the aforementioned interpolation formula, The interpolation equation is generated using a term based on the phase of the central frequency of the radar emitted from the aforementioned flying object. death , By interpolating using the aforementioned interpolation formula, Correction processing is performed to correct the effects caused by the above item based on the phase with respect to the center frequency. 、 Professional Mu It will be provided. [Effects of the Invention]

[0015] According to one aspect of the present invention, a signal processing device, a signal processing method, and a program capable of improving imaging performance can be obtained. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic diagram illustrating how a satellite illuminates the Earth's surface with radar. [Figure 2] This is a schematic diagram illustrating how a satellite illuminates a scatterer on the Earth's surface with radar. [Figure 3] This is a schematic diagram illustrating signals related to artificial satellites. [Figure 4] This is a block diagram showing an overview of the signal processing device according to the first embodiment. [Figure 5] This is a schematic diagram illustrating the reference point F in the first embodiment. [Figure 6] This figure shows an example of the hardware configuration of a signal processing device. [Figure 7] This diagram shows a conventional method for generating multiple reference points. [Figure 8] This is a block diagram showing an overview of the signal processing device according to the second embodiment. [Figure 9] This is a flowchart showing the process from when the interpolation formula generation unit generates the interpolation formula. [Figure 10] This is a flowchart showing the details of how interpolation formulas are generated. [Figure 11] This is a flowchart illustrating the process of generating SAR images. [Figure 12] This is a flowchart showing how the reference point generation unit according to the third embodiment generates the reference point F. [Figure 13] This is a block diagram showing an overview of the signal processing device according to the fourth embodiment. [Figure 14] This is a flowchart showing the processing of the coordinate system transformation unit according to the fourth embodiment. [Figure 15] This is a block diagram showing an overview of the signal processing device according to the fifth embodiment. [Figure 16] This is a flowchart showing the processing of the position information conversion unit according to the fifth embodiment. [Figure 17] This is a block diagram showing an overview of the signal processing device according to the sixth embodiment. [Figure 18] This is a flowchart showing the process by which the interpolation formula generation unit according to the sixth embodiment generates an interpolation formula. [Figure 19]This is a flowchart showing the details of the interpolation formula generation method according to the sixth embodiment. [Figure 20] This is a flowchart illustrating the process of generating a SAR image in the sixth embodiment. [Figure 21] This is a block diagram showing an overview of the signal processing device according to the seventh embodiment. [Modes for carrying out the invention]

[0017] Embodiments of the present invention will be described below with reference to the drawings. In all drawings, similar components are denoted by the same reference numerals, and their descriptions are omitted as appropriate.

[0018] Synthetic Aperture Radar (SAR) technology is a technique that artificially synthesizes apertures so that images (SAR images) equivalent to those obtained with an antenna with a large aperture can be obtained by having an antenna mounted on a moving object (such as a satellite or airplane) transmit and receive electromagnetic waves (radar). In the following, we will use a satellite (SAR satellite) as an example of a flying object.

[0019] (Term definition) Figure 1 is a schematic diagram showing satellite 5 irradiating the Earth's surface E with radar. Using Figure 1, we define the terms used in this embodiment. Although the Earth's surface E is actually a spherical shape with irregularities, Figure 1 simply represents the surface as a flat plane.

[0020] DR1, shown in Figure 1, indicates the direction of travel of satellite 5. Each of the parallelograms drawn along the direction of travel DR1 represents satellite 5. These then show the trajectory of satellite 5.

[0021] In synthetic aperture radar, the reflection of electromagnetic waves emitted from multiple satellite positions with a certain spread (width) is combined to calculate the reflection of electromagnetic waves that would occur if emitted from a certain satellite position with a smaller spread (width). The resulting combined satellite position is called the azimuth. In practice, there is no difference between the azimuth and the satellite position, so we will not distinguish between azimuth and satellite position here. That is, the azimuth direction is the direction of travel DR1. The coordinate axes that unfold in the azimuth direction are called the azimuth axes.

[0022] DR2 is the vertical direction DR2 that is perpendicular to the azimuth direction DR1 (=direction of motion DR1) within plane P. The vertical direction DR2 is the direction in which the Doppler effect is zero. The direction in which the Doppler effect is zero is also called the zero Doppler direction. Hereafter, the vertical direction DR2 will also be referred to as the zero Doppler direction DR2.

[0023] Surface P is defined by the azimuth direction DR1 and the illumination direction DR3. Irradiation direction DR3 is the direction in which satellite 5 illuminates with its radar. Irradiation direction DR3 is the direction in which the center of satellite 5's radar is moving. Irradiation direction DR3 is also the direction in which satellite 5's antenna is pointing. In the case of squint, illumination direction DR3 is set at an oblique angle with respect to the zero-Doppler direction DR2. Hereafter, illumination direction DR3 will also be referred to as range direction DR3.

[0024] Satellite 5 emits radar (electromagnetic waves) towards the Earth's surface E and receives reflected signals representing the radar's reflection. The radar emitted from Satellite 5's antenna in the range direction DR3 hits the imaging area R, and the phase delay and intensity of the reflected signals are recorded. A SAR image is then formed using the data related to these reflected signals. Because the radar has a certain width, it is emitted in a cone shape towards the Earth's surface E. The imaging area R in Figure 1 shows the region of the Earth's surface E that is illuminated by the radar.

[0025] The angle between the zero-Doppler direction DR2 and the range direction DR3 is called the squint angle θ. sq Let's assume that the range direction DR3 is θ relative to the zero Doppler direction DR2. sq It is inclined. Squint angle θ sq A high squint angle is defined as a value exceeding approximately 5 degrees. A representative squint angle is, for example, the squint angle of artificial satellite 5 in a given satellite orbit that corresponds to the time the antenna was pointed towards it for the longest period of time.

[0026] Generally, in image formation of SAR images, the zero-Doppler direction and the range direction are often treated as approximately the same. However, in the case of high-squint images, the difference in orientation between the zero-Doppler direction and the range direction cannot be ignored, so it is preferable to treat them as clearly distinct.

[0027] (Explanation of coordinate systems) Figure 2 is a schematic diagram showing how satellite 5 illuminates a scatterer N on the Earth's surface E with its radar. Figure 2 shows what happens when satellite 5 illuminates a scatterer N in the imaging area R on the Earth's surface E with its radar. The scatterer N represents a hypothetical point on the Earth's surface E that, upon being illuminated by radar, reflects radar in various directions. Figure 2 will be used to explain the coordinate system used in this specification.

[0028] (ξ-r system) The beam crossing time (ξ) is the time when satellite 5a is directly in front of a certain scatterer N in the range direction DR3 (=irradiation direction DR3). p Let's assume the beam passes through at azimuth time ξ. p The squint angle θ is sq This is the time when the scatterer N is directly in front of the antenna (radar) if the angle is not changed (=typical squint angle). ξ is the beam passage azimuth time. p This is the time when the center of the radar beam emitted by satellite 5a passes through each scatterer N.

[0029] Beam passage azimuth time ξ pIn this case, the distance from the artificial satellite 5a to the scatterer N in the range direction DR3 is defined as the range r.

[0030] Hereinafter, a coordinate system in which one axis represents the range and the other axis represents the beam passing azimuth time ξ p is referred to as the ξ-r system.

[0031] (η-ρ system) The time when the artificial satellite 5b arrives in front of a certain scatterer N in the zero Doppler direction DR2 (= vertical direction DR2) is defined as the closest approach time η p . The closest approach time η p is also the time when the distance between a certain scatterer N and the artificial satellite 5b is closest.

[0032] At the closest approach time η p , the distance from the artificial satellite 5a to the scatterer N in the zero Doppler direction DR2 is defined as the zero Doppler range ρ.

[0033] Hereinafter, a coordinate system in which one axis represents the zero Doppler range ρ and the other axis represents the closest approach time η p is referred to as the η-ρ system.

[0034] Although the terms and coordinate systems have been described based on the flat ground surface E and the orbit which is a straight line parallel to it as shown in FIG. 1, the same applies to the spherical ground surface E and the curved satellite orbit. Also, it is known that for the radar image obtained by the curved ground surface and the curved satellite orbit, the radar image can be obtained by approximating the ground surface as a plane and the satellite orbit as a straight line. The squint angle in this approximate geometry is called the effective squint angle etc., but they are not particularly distinguished in the following description.

[0035] FIG. 3 is a schematic diagram for explaining the signal related to the artificial satellite 5. The radar mounted on the artificial satellite 5 irradiates (or emits) electromagnetic wave pulses (pulse signals) to the observation area (imaging area R) one after another.

[0036] The left diagram in Figure 3 shows the relationship between the azimuth time η and the range time τ (described later) when a radar is irradiated onto a certain scatterer N in a ξ-r system. In the left diagram of Figure 3, the horizontal axis represents the azimuth time η. The azimuth time η is also the emission time of the pulse. In the left diagram of Figure 3, the vertical axis represents the range time τ. The range time τ represents the time from when the pulse is emitted to a location at a range r distance until the reflected signal (reflected wave) is received. The range time can also be said to be the elapsed time from the timing when the radar signal is emitted until the reflected signal representing the reflection of the radar signal is received. The vertical axis may also be represented by the round-trip distance, which is the value obtained by multiplying the speed of light by the range time, or by the range r (distance), which is half of that round-trip distance.

[0037] The width of the elliptical shape in the left diagram of Figure 3 represents the intensity of the reflected signal. For example, the beam transit azimuth time ξ in the left diagram of Figure 3. p In this configuration, the radar of satellite 5 is pointed directly at scatterer N, resulting in the strongest signal intensity.

[0038] The right-hand figure in Figure 3 shows the data obtained by performing a two-dimensional Fourier transform on the data shown in the left-hand figure of Figure 3 in the frequency domain. The horizontal axis in the right-hand figure of Figure 3 represents the azimuth frequency f obtained by converting the azimuth time η to the frequency domain. η In the right-hand figure of Figure 3, the vertical axis represents the range frequency f obtained by converting the range time τ to the frequency domain. τ The center frequency is f. c This is the central frequency of the radar (electromagnetic wave) frequency band emitted by artificial satellite 5, and it is predetermined for each artificial satellite 5.

[0039] A SAR image is formed by aggregating the data related to the reflected signals from numerous scatterers N, as shown in the left and right diagrams of Figure 3. In other words, the data related to the radar's reflected signals, as shown in the left and right diagrams of Figure 3, can also be called pixel data. Pixel data is a value associated with each of the two-dimensional grids. In the region shown in the left diagram of Figure 3, the two axes of the grid are range time τ and azimuth time η, so that each two-dimensional grid records the phase and absolute value (also called amplitude or signal strength) of each scatterer N as pixel data, associated with the range time τ and azimuth time η. In the region shown in the right diagram of Figure 3, the two axes of the grid are range frequency and azimuth frequency, so that each two-dimensional grid records the phase and absolute value of each scatterer N as pixel data, associated with the range frequency and azimuth frequency.

[0040] (First Embodiment) Figure 4 is a block diagram illustrating the overview of the signal processing device 1 according to the first embodiment. The signal processing device 1 comprises a conversion unit 10, a reference point generation unit 20, a calculation unit 30, an interpolation formula generation unit 40, and an interpolation processing unit 50.

[0041] The conversion unit 10 performs a two-dimensional Fourier transform on the reflected signal to first signal data in the frequency domain. The reflected signal is a signal representing the reflection from the scatterer N to the radar emitted from the artificial satellite 5. The reflected signal may be stored in the memory unit 2, or it may be the signal immediately after being received from the artificial satellite 5. The data format of the first signal data is as shown in the right-hand figure of Figure 3.

[0042] Figure 5 is a schematic diagram illustrating the reference point F in the first embodiment. The reference point generation unit 20 generates a plurality of reference points F arranged in a direction that is inclined diagonally with respect to the zero-Doppler direction DR2 (= vertical direction DR2). In the first embodiment, the direction that is inclined diagonally is the range direction DR3 (= irradiation direction DR3). That is, the reference point generation unit 20 generates the reference points F in the ξ-r system, not the η-ρ system. The reference point generation unit 20 generates the reference points F in the ξ-r system. sq Using the data related to this, we identify the range direction DR3.

[0043] A reference point F corresponds to a virtual point selected from the imaging area R shown in Figure 1. A reference point F is generated, for example, at the center of the imaging area R. A reference point F is a point generated to improve imaging accuracy around the reference point F. A reference point F is selected, for example, by the user. For example, 30 reference points F are selected, but there may be more or fewer. Multiple reference points F have different ranges r at beam passage azimuth time ξ1 (see r1 to r3 in Figure 5). For example, multiple reference points F are generated starting from the center of the imaging area R, with ranges r ranging from -30 [km] to 30 [km].

[0044] Returning to Figure 4, the calculation unit 30 calculates the phase of the second signal data in the frequency domain using the reference point F. The second signal data in the frequency domain is the signal data obtained after a two-dimensional Fourier transform of the reflected signal when the radar is virtually irradiated onto the reference point F. The second signal data in the frequency domain is linked to the range frequency and azimuth frequency, similar to the right diagram in Figure 3. The calculation unit 30 calculates the phase of the second signal data in the frequency domain for each of the reflected signals obtained after a Fourier transform when the radar is virtually irradiated onto multiple reference points F.

[0045] The interpolation formula generation unit 40 generates an interpolation formula using the phase of each second signal data calculated by the calculation unit 30. The interpolation formula is used to improve the processing accuracy of the interpolation processing unit 50, which will be described later, and to improve the imaging accuracy. Specific examples of the interpolation formula will be described in the second embodiment and subsequent embodiments.

[0046] The interpolation processing unit 50 interpolates the first signal data converted by the conversion unit 10 using the interpolation formula generated by the interpolation formula generation unit 40 and also performs an inverse Fourier transform on it. The data format of the signal data after the inverse Fourier transform is the same as the data described in the left diagram of Figure 3. The interpolation processing unit 50 then outputs the signal data after the inverse Fourier transform. For example, the interpolation processing unit 50 may output to a SAR image generation unit (not shown) that generates a SAR image. A SAR image is formed by collecting a large number of signal data after the inverse Fourier transform.

[0047] (Example hardware configuration) Figure 6 shows an example of the hardware configuration of the signal processing device 1. The signal processing device 1 includes a bus 1010, a processor 1020, a memory 1030, a storage device 1040, an input / output interface 1050, and a network interface 1060.

[0048] Bus 1010 is a data transmission path for the processor 1020, memory 1030, storage device 1040, input / output interface 1050, and network interface 1060 to send and receive data to and from each other. However, the method of connecting the processor 1020 and the other components to each other is not limited to bus connection.

[0049] The 1020 processor is a processor implemented in components such as the CPU (Central Processing Unit) and GPU (Graphics Processing Unit).

[0050] Memory 1030 is a main memory device implemented using RAM (Random Access Memory), etc.

[0051] The storage device 1040 is an auxiliary storage device implemented as a removable media such as an HDD (Hard Disk Drive), SSD (Solid State Drive), or memory card, or as ROM (Read Only Memory), and has a recording medium. The recording medium of the storage device 1040 stores program modules that implement each function of the signal processing device 1 (for example, the conversion unit 10, the reference point generation unit 20, the calculation unit 30, the interpolation formula generation unit 40, and the interpolation processing unit 50). The processor 1020 reads each of these program modules into the memory 1030 and executes them, thereby realizing each function corresponding to that program module. Alternatively, the storage device 1040 may function as a storage unit 2 connected to the signal processing device 1.

[0052] The input / output interface 1050 is an interface for connecting the signal processing device 1 with various input / output devices.

[0053] The network interface 1060 is an interface for connecting the signal processing device 1 to a network. This network may be, for example, a LAN (Local Area Network) or a WAN (Wide Area Network). The network interface 1060 may connect to the network via a wireless connection or a wired connection. The signal processing device 1 may communicate with the satellite 5 via the network interface 1060.

[0054] Figure 7 shows a conventional method for generating multiple reference points. Traditionally, reference points were generated in the zero-Doppler direction, but this resulted in a problem where imaging performance deteriorated when the squint angle became large (e.g., 5 degrees or more).

[0055] In the signal processing device 1 according to the first embodiment, the imaging performance when generating SAR images can be improved by generating a reference point F based on a new coordinate system (ξ-r system) that takes the squint angle into consideration.

[0056] In other words, this signal processing device provides a signal processing device 1 that can improve imaging performance.

[0057] (Second Embodiment) Figure 8 is a block diagram illustrating the overview of the signal processing device 1 according to the second embodiment. In the signal processing device 1 according to the second embodiment, the processing of the reference point generation unit 20, the calculation unit 30, the interpolation formula generation unit 40, and the interpolation processing unit 50 differs from that of the first embodiment.

[0058] The calculation unit 30 according to the second embodiment further uses the orbital data of the artificial satellite 5, the data relating to the azimuth frequency interval, and the data relating to the range frequency interval to calculate the phase of the second signal data corresponding to the reference point F. The orbital data of the artificial satellite 5 is data relating to the orbit of the artificial satellite 5, and includes, for example, data relating to the position of the artificial satellite 5 from the Earth, and data relating to the velocity vector of the artificial satellite 5 (magnitude and direction of the velocity of the artificial satellite 5). The orbital data of the artificial satellite 5 may be stored in the storage unit 3 or obtained from the artificial satellite 5.

[0059] The data regarding the azimuth frequency interval refers to the interval at which data is acquired relative to the azimuth frequency axis (see the horizontal axis in the right-hand diagram of Figure 3). For example, if the interval is 100 Hz, the phase of the second signal data will be calculated at 100 Hz intervals relative to the azimuth frequency axis for a given reference point F. The data regarding the azimuth frequency interval may be predetermined by the user.

[0060] The data regarding the range frequency interval refers to the interval at which data is acquired relative to the range frequency axis (see the vertical axis in the right-hand diagram of Figure 3). For example, if the interval is 10,000 Hz, the phase of the second signal data will be calculated at 10,000 Hz intervals relative to the range frequency axis for a given reference point F. The data regarding the range frequency interval may be predetermined by the user.

[0061] The interpolation processing unit 50 according to the second embodiment includes a bulk compression processing unit 52 that performs bulk compression processing, a mapping processing unit 53 that performs mapping processing, and a resampling processing unit 56 that performs resampling processing. Details of each processing unit will be described later.

[0062] (Flow of interpolation formula generation) Figure 9 is a flowchart showing the process by which the interpolation formula generation unit 40 generates an interpolation formula. The process by which the interpolation formula generation unit 40 generates an interpolation formula will be explained using Figure 9.

[0063] In step S100, the reference point generation unit 20 obtains information about the representative squint angle from the storage unit 4. The reference point generation unit 20 may also obtain information about the representative squint angle from the satellite 5. As for how to determine the representative squint angle, for example, the squint angle of the satellite 5 at the time when the antenna was pointed most sharply may be used as the representative squint angle. As another example, in imaging modes where the squint angle does not change at all during imaging (strip map mode), it is desirable to use the squint angle at that time. As yet another example, in imaging modes where the antenna is moved slightly (about 1-2 degrees) (called spotlight mode, etc.), it is desirable to use the angle of the antenna at the center of the range in which the antenna moves as the representative squint angle.

[0064] In step S110, the reference point generation unit 20 generates multiple reference points in the range direction DR3 (=irradiation direction DR3) using the acquired representative squint angle. The reference point generation unit 20 generates multiple reference points in the ξ-r system.

[0065] In step S120, the calculation unit 30 calculates the phase of the second signal data (two-dimensional spectrum) in the frequency domain for each reference point F.

[0066] In step S130, the interpolation formula generation unit 40 generates an interpolation formula using the phases of each second signal data calculated by the calculation unit 30. The details of the interpolation formula generation method are explained in the following flowchart.

[0067] Figure 10 is a flowchart showing the details of the interpolation equation generation method. The process of generating the interpolation equation will be explained using Figure 10. As a prerequisite, we assume that the following relationship is obtained.

number

[0068] The left-hand side of equation (1) is a relational expression relating to the phase of the second signal data (two-dimensional spectrum). The phase data of the second signal data for each reference point F, calculated by the calculation unit 30, is calculated using the azimuth frequency intervals and range frequency intervals defined by the above-mentioned azimuth frequency interval data and range frequency interval data. Equation (2) represents the relational expression relating to the phase of the second signal data.

number

[0069] f τ The range frequency for a certain reference point F is f η The azimuth frequency with respect to a certain reference point F is r p The range for a given reference point F is η. p This is the azimuth time (= beam azimuth time) relative to a reference point F. The coordinate system is defined as ξ-r.

[0070] In step S131, the interpolation equation generation unit 40 subtracts the phase with respect to the center of the imaging region R from the phase of the second signal data. The phase with respect to the center of the imaging region R is the phase of the second signal data when a reference point F is set at the center of the imaging region R. The phase with respect to the center of the imaging region R is the first term on the right-hand side of equation (1), and is denoted as equation (3). Equation (3) is the phase itself calculated by the calculation unit 30 for the point corresponding to the center among the reference points F. Note that this phase is calculated using the azimuth frequency and range frequency defined by the data on the interval of the azimuth frequency and the data on the interval of the range frequency. Equation (3) is also a term based on the phase of the second signal data when the radar is virtually irradiated onto the reference point F placed at the center of the imaging region R.

number

[0071] r c η is the range relative to the reference point F located at the center of the imaging area R. cThis is the azimuth time (= beam passage azimuth time) with respect to a reference point F located at the center of the imaging region R. All reference points in the ξ-r system are η c For the azimuth time (=beam pass-through azimuth time), different ranges r p They will be lined up in this order (see Figure 5).

[0072] In step S131, the process of subtracting equation (3) from equation (2) is performed. That is, for each of the reference points F, the phase at the range frequency and azimuth frequency calculated by the calculation unit 30 is subtracted from each of the phases at the range frequency and azimuth frequency calculated by the calculation unit 30 for the reference point F at the center of the imaging region R. At this time, in the ξ-r system, for each of the reference points F, η c and η p Since they are equal, η in equation (1) c and η p The term relating to the difference (2πf η (η p -η c )) can be ignored in step S132 described later. Note that the third term on the right side of equation (1) (2πf η (η p -η c ) is also a phase-based term that represents a shift in the direction of travel DR1 for each of the multiple reference points F.

[0073] In step S132, the interpolation equation generation unit 40 uses the subtraction result obtained in step S131 to calculate an interpolation equation that can be expressed by the following equation (4). p This represents the range.

number

[0074] Note that in equation (4), u(f τ ,f η Let equation (5) be the function shown by ). Equation (5) is the function of each azimuth frequency f η For the range frequency f τ This function depends only on [specific factor].

number

[0075] Note that in equation (4), v(r p ,f η Let equation (6) be the function shown by ). Equation (6) is given by each azimuth frequency f η This is a function that depends only on the distance between satellite 5 and reference point F (in this case, range r).

number

[0076] In step S133, the interpolation equation generation unit 40 transmits information about the function shown in equation (5) (interpolation equation) to the second interpolation unit 54 and information about the function shown in equation (6) (interpolation equation) to the third interpolation unit 57.

[0077] The second interpolation unit 54 transmits information about the function shown in equation (5) to the mapping processing unit 53, which will be described later. The third interpolation unit 57 transmits information about the function shown in equation (6) to the resampling processing unit 56, which will be described later.

[0078] The first interpolation unit 51 obtains data relating to the phase of the second signal data at a reference point F located at the center of the imaging area R calculated by the calculation unit 30 from the calculation unit 30 and transmits it to the bulk compression processing unit 52. Note that the phase data relating to the second signal data is the same as that obtained by subtracting in step S131 (= equation (3)). Note that the phase data relating to the second signal data obtained above is discrete in the azimuth frequency axis and the range frequency axis, so the first interpolation unit 51 performs interpolation processing in the azimuth frequency axis and the range frequency axis before transmitting it to the bulk compression processing unit 52.

[0079] Regarding the interpolation formula generation flow described above, each interpolation formula may be calculated by referring to, for example, the SVD-STOLT method described in Non-Patent Document 2. In this embodiment, the novel part is that the interpolation formula generation process is performed in the ξ-r system instead of the conventional η-ρ system.

[0080] (SAR image generation flow) Figure 11 is a flowchart illustrating the process of generating a SAR image. The process of generating a SAR image will be explained using Figure 11.

[0081] In step S200, the conversion unit 10 performs a two-dimensional Fourier transform on the reflected signal from the scatterer N into a first signal data in the frequency domain.

[0082] In step S210, the bulk compression processing unit 52 performs bulk compression on the first signal data using the phase data of the second signal data at the reference point F interpolated by the first interpolation unit 51. The bulk compression processing is the same as the Reference function multiply (bulk compression) described in Non-Patent Literature 1.

[0083] The bulk compression process will now be explained in more detail. The bulk compression processing unit 52 multiplies the first signal data generated by the two-dimensional Fourier transform performed by the transformation unit 10 by a reference signal. The reference signal is the complex conjugate of the ideal response (range time) from the scatterer N, assuming that the scatterer N exists at the reference point F (a reference point located at the center of the imaging area R), after its Fourier transform. The reference signal is obtained as a complex number with an absolute value of 1, having a phase with a negative sign added to the phase obtained from the first interpolation unit 51.

[0084] In step S220, the mapping processing unit 53 performs a mapping process using the information about the function shown in equation (5) received from the second interpolation unit 54. The mapping process is similar to, for example, the "Stolt Interpolation" described in Non-Patent Document 2.

[0085] In step S230, the range inverse Fourier transform unit 55 performs a (one-dimensional) inverse Fourier transform on the signal processed by the mapping processing unit 53 in the range direction.

[0086] In step S240, the resampling processing unit 56 performs a resampling process. The resampling process is a process that changes the coordinate system. The resampling processing unit 56 resamples according to equation (6) received from the third interpolation unit 57. That is, in the mapping process performed in step S220, equation (5) is mapped to the new range frequency, so in the inverse range Fourier transform of the mapping result, v(r) is obtained by the definition of equation (4) and the properties of the Fourier transform shift. p ,f η ) at the position of r p A signal exists for this. This can be expressed by the inverse function of v in equation (6) as r p The resampling processing unit 56 performs a resampling process on the data generated by the range inverse Fourier transform unit 55.

[0087] In step S250, the azimuth inverse Fourier transform unit 58 performs a (one-dimensional) inverse Fourier transform on the resampled data in the azimuth direction DR1.

[0088] The data generated through steps S200 to S250 is in a data format where the vertical axis represents range time τ and the horizontal axis represents azimuth time η (see Figure 3, left). A large number of these data points are collected to generate a SAR image. Note that in the left figure of Figure 3, the signal data is depicted as an ellipse, but after processing through steps S200 to S250, the signal data will be depicted as points (or rectangles).

[0089] The flows described in Figures 9, 10, and 11 may be executed simultaneously or separately. The interpolation formula may be calculated in advance by executing the flows described in Figures 9 and 10 beforehand.

[0090] As described above, according to the signal processing device 1 of the second embodiment, the calculation unit 30 further uses the orbital data of the artificial satellite 5, the data relating to the azimuth frequency interval, and the data relating to the range frequency interval to calculate the phase of the second signal data corresponding to the reference point F. This makes it possible to calculate the phase of the second signal data for each reference point F with high accuracy.

[0091] Furthermore, the interpolation processing unit 50 includes a bulk compression processing unit 52, a mapping processing unit 53, and a resampling processing unit 56. This allows the interpolation processing unit 50 to perform interpolation processing with high accuracy.

[0092] (Third embodiment)

[0093] Figure 12 is a flowchart showing how the reference point generation unit 20 generates the reference point F according to the third embodiment. In the signal processing device 1 according to the third embodiment, the processing of the reference point generation unit 20 differs from the processing of the reference point generation unit 20 according to the first embodiment.

[0094] The reference point generation unit 20 according to the third embodiment generates a plurality of reference points F using position data relating to the position of the artificial satellite 5, irradiation data relating to the irradiation direction DR3, and terrain data. The position data relating to the position of the artificial satellite 5, irradiation data relating to the irradiation direction DR3, and terrain data will be described later.

[0095] The process by which the reference point generation unit 20 according to the third embodiment generates the reference point F will be described below with reference to Figure 12.

[0096] In step S300, the reference point generation unit 20 acquires positional data relating to the position of the satellite 5 from the satellite 5 (it may also be acquired from the storage unit). This positional data is the beam passage azimuth time ξ when the radar of the satellite 5 illuminates a representative point in the imaging area R (for example, the center point of the imaging area R). p , and the azimuth time ξ of the beam's passage. p This includes the position information of satellite 5 (position information of satellite 5 relative to the Earth, preferably in a geocentric Earth-fixed coordinate system).

[0097] In step S310, the reference point generation unit 20 acquires irradiation data related to the irradiation direction DR3 from the artificial satellite 5. This irradiation data includes information about the direction the antenna of the artificial satellite 5 is pointing, and the squint angle θ. sq Includes information about this.

[0098] In step S320, the reference point generation unit 20 acquires location information (geospatial location information) for a location r [km] away from the position of the artificial satellite 5 in the range direction DR3 (irradiation direction DR3) in the ξ-r system. r [km] is the distance given for the location to place the reference point. Note that r [km] may be specified in advance by the user.

[0099] In step S330, the reference point generation unit 20 generates the beam azimuth time ξ p A straight line parallel to the azimuth direction DR1 (=direction of travel DR1) at the beam passage azimuth time ξ p The trajectory is obtained when a point at a distance of r [km] is rotated around a straight line passing through the satellite's position.

[0100] In step S340, the reference point generation unit 20 acquires positional information at the intersection of the rotated trajectory and the Earth using terrain data. The reference point generation unit 20 acquires positional information at the intersection closest to the range direction DR3 among the multiple intersection points acquired. The terrain data includes data on the topography of the Earth, and the terrain data has coordinate data in three-dimensional space of the irregularities of the Earth's surface (including buildings and mountain ranges).

[0101] In step S350, the reference point generation unit 20 generates the position at the intersection (position in three-dimensional space) obtained in step S340 as a reference point F at that distance r.

[0102] According to the third embodiment, the reference point generation unit 20 generates multiple reference points F using positional data relating to the position of the artificial satellite 5, irradiation data relating to the irradiation direction DR3, and terrain data. This makes it possible to generate reference points F while taking into account the influence of the actual terrain of the Earth's surface (such as the presence or absence of irregularities) and the curvature of the Earth. Therefore, the imaging accuracy can be improved.

[0103] (Fourth Embodiment) Figure 13 is a block diagram illustrating the overview of the signal processing device 1 according to the fourth embodiment. Unlike the signal processing device 1 according to the second embodiment, the signal processing device 1 according to the fourth embodiment further includes a coordinate system transformation unit 60.

[0104] The coordinate system transformation unit 60 acquires third signal data from the interpolation processing unit 50. The third signal data is data based on the first coordinate system, obtained by the interpolation processing unit 50 performing an inverse Fourier transform on the first signal data (after predetermined processing such as bulk compression). The third signal data is data based on the first coordinate system, obtained by the azimuth inverse Fourier transform unit 58 performing a one-dimensional inverse Fourier transform in the processing of the interpolation processing unit 50.

[0105] The first coordinate system is the ξ-r system. That is, in the first coordinate system, one axis represents the distance (range) from the satellite 5 to the scatterer in the irradiation direction DR3 (=range direction DR3), and the other axis represents the time (=beam passage azimuth time) when the center of the radar emitted by the satellite 5 passes through the scatterer N.

[0106] The coordinate system transformation unit 60 transforms the third signal data into the fourth signal data. The fourth signal data is based on a second coordinate system different from the first coordinate system. The fourth signal data is the data obtained by transforming the coordinate system of the third signal data. The second coordinate system is the η-ρ system. That is, in the second coordinate system, one axis represents the distance from the artificial satellite 5 to the scatterer N (zero Doppler range ρ) in the vertical direction DR2 (= zero Doppler direction DR2), and the other axis represents the time when the distance between the artificial satellite 5 and the scatterer N is closest (= nearest tangency time). The coordinate system transformation unit 60 transforms the third signal data from the ξ-r system to the η-ρ system.

[0107] Figure 14 is a flowchart showing the processing of the coordinate system transformation unit 60 according to the fourth embodiment. The processing of the coordinate system transformation unit 60 will be explained in detail using Figure 14. As a prerequisite, it is assumed that the imaging result in the ξ-r system and the squint angle are known.

[0108] In step S400, the coordinate system transformation unit 60 acquires third signal data from the interpolation processing unit 50 (= inverse azimuth Fourier transform unit 58).

[0109] In step S410, the coordinate system transformation unit 60 acquires each pixel data associated with the third signal data and acquires the position information (ξ-r system) of each pixel data relative to the artificial satellite 5. The position information of the pixel data relative to the artificial satellite 5 is information regarding the distance (range) between the scatterer N corresponding to the pixel data and the artificial satellite 5.

[0110] In step S420, the coordinate system transformation unit 60 determines the closest proximity distance and nearest approach time η between each pixel data and the artificial satellite 5. pThe nearest-neighbor distance is calculated as the distance at which the scatterer N corresponding to a given pixel data and the satellite 5 come closest together. That is, the nearest-neighbor distance is the distance from the satellite 5 to the scatterer N in the η-ρ system, and is also the zero-Doppler range ρ. The nearest-neighbor distance is calculated as the nearest-neighbor time η p This is also the distance from satellite 5 to scatterer N. Nearest contact time η p This is also the time when satellite 5 is at its nearest-to-nearest distance.

[0111] In step S430, the coordinate system transformation unit 60 transforms the coordinate system of the third signal data into the fourth signal data which is based on the second coordinate system. The coordinate system transformation unit 60 then converts the nearest neighbor time η calculated in step S420. p The axis related to the azimuth axis and the axis related to the nearest neighbor distance are used as the range axis for data conversion. In this way, the coordinate system conversion unit 60 converts the ξ-r system related to the third signal data to the η-ρ system.

[0112] Furthermore, the SAR image generation unit (not shown) may generate a SAR image using the fourth signal data that has been coordinate-transformed by the coordinate system transformation unit 60.

[0113] As described above, the signal processing device 1 according to the fourth embodiment further includes a coordinate system transformation unit 60. By converting to the conventional coordinate system before imaging, conventional imaging methods can be used as is. Therefore, SAR images can be easily generated.

[0114] (Fifth embodiment) Figure 15 is a block diagram illustrating the overview of the signal processing device 1 according to the fifth embodiment. Unlike the signal processing device 1 according to the second embodiment, the signal processing device 1 according to the fifth embodiment further includes a position information conversion unit 70.

[0115] The position information conversion unit 70 converts the position information of the pixel data associated with the data obtained by inverse Fourier transforming the first signal data by the interpolation processing unit 50 into position information on Earth. The pixel data contains position information. This position information includes information about the distance r between the scatterer N corresponding to the pixel data and the artificial satellite 5.

[0116] Note that the data obtained by inverse Fourier transforming the first signal data by the interpolation processing unit 50 may also be the third signal data according to the fourth embodiment. Hereinafter, the data obtained by inverse Fourier transforming the first signal data by the interpolation processing unit 50 will be treated as the third signal data.

[0117] Figure 16 is a flowchart showing the processing of the position information conversion unit 70 according to the fifth embodiment. The processing of the position information conversion unit 70 will be explained in detail using Figure 16. It should be assumed that the imaging result, orbital data, terrain data, and squint angle in the ξ-r system are known.

[0118] In step S500, the position information conversion unit 70 acquires third signal data from the interpolation processing unit 50 (= inverse azimuth Fourier transform unit 58).

[0119] In step S510, the position information conversion unit 70 determines the beam passage azimuth time ξ corresponding to each pixel data in the ξ-r system. p The system acquires information regarding the position of the satellite 5 and information regarding the velocity of the satellite 5 (velocity vector). The position information conversion unit 70 may acquire the information regarding the position of the satellite 5 and the information regarding the velocity of the satellite 5 from the orbital data stored in the storage unit 3, or it may acquire them directly from the satellite 5.

[0120] In step S520, the position information conversion unit 70 calculates the relative position of each pixel data from the position of the artificial satellite 5 using the range r.

[0121] In step S530, the position information conversion unit 70 determines the beam passage azimuth time ξ pThe system calculates the trajectory traced by each pixel data point by rotating the location corresponding to the satellite 5 around a straight line parallel to the satellite 5's velocity vector and passing through the satellite 5's azimuth position (with a rotation radius of range r).

[0122] In step S540, the position information conversion unit 70 determines the ground position of the pixel data at the point where the trajectory intersects with the Earth. The Earth surface position data may be extracted from the terrain data according to the third embodiment. In this way, the position information conversion unit 70 converts the position information of the pixel data associated with the third signal data into position information on Earth.

[0123] As described above, the signal processing device 1 according to the fifth embodiment further includes a position information conversion unit 70. Unlike the fourth embodiment, it is possible to directly convert the position information of pixel data into position information on Earth without going through the process of converting from the ξ-r system to the η-ρ system. This makes it possible to easily generate SAR images.

[0124] (Sixth Embodiment) Figure 17 is a block diagram showing an overview of the signal processing device 1 according to the sixth embodiment. Unlike the second embodiment, the interpolation processing unit 50 according to the sixth embodiment includes a correction processing unit 59, and the processing of the reference point generation unit 20 and the interpolation formula generation unit 40 differs from that of the second embodiment.

[0125] In the sixth embodiment, the reference point generation unit 20 does not generate multiple reference points in the ξ-r system. The reference point generation unit 20 may generate multiple reference points in the η-ρ system.

[0126] The interpolation generation unit 40 generates the center frequency f of the radar emitted from the artificial satellite 5. c An interpolation equation is generated using a term based on the phase related to the center frequency f. c The phase with respect to the reference point F is the phase of the second signal data in the frequency domain relative to the reference point F, with respect to the center frequency f c This refers to the phase of the part related to the center frequency f. cThis is the central frequency of the radar (electromagnetic wave) frequency band emitted by satellite 5. Specific processing details will be described later.

[0127] The correction processing unit 59 controls the center frequency f c A correction process is performed to compensate for the effects caused by terms based on the phase related to [the relevant parameter]. The specific process will be described later.

[0128] Figure 18 is a flowchart showing the process by which the interpolation formula generation unit 40 according to the sixth embodiment generates an interpolation formula. The process by which the interpolation formula generation unit 40 according to the sixth embodiment generates an interpolation formula will be explained using Figure 18.

[0129] In step S111, the reference point generation unit 20 generates multiple reference points. In the sixth embodiment, the reference point generation unit 20 generates multiple reference points in the η-ρ system.

[0130] In step S121, the calculation unit 30 calculates the phase of the second signal data (two-dimensional spectrum) in the frequency domain for each reference point F.

[0131] In step S131, the interpolation formula generation unit 40 generates an interpolation formula using the phases of each second signal data calculated by the calculation unit 30. The details of the interpolation formula generation method are explained in the following flowchart.

[0132] Figure 19 is a flowchart showing the details of the interpolation formula generation method according to the sixth embodiment. The process for generating the interpolation formula according to the sixth embodiment will be explained in detail using Figure 19.

[0133] When generating interpolation equations in the η-ρ system, it can be difficult to calculate the interpolation equation (equation (4)) with high accuracy (equation (1) is not very accurate to begin with, but in the case of high squint, the accuracy of equation (1) is particularly poor). Therefore, the inventor, through diligent effort, has developed a center frequency f c We found that the accuracy of the interpolation equation can be improved by adding a term based on the phase with respect to (1) (at least f τ =f c Regarding u(fτ ,f η )=v(r p ,f η The idea is that ()=0). Center frequency f c The equation obtained by adding a term based on the topology with respect to (1) is expressed as follows:

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[0134] The following explanation assumes that equation (7) has been derived. Note that the coordinate system is the η-ρ system. The interpolation equation is generated using equation (7). The interpolation equation consists of a term based on the phase of the second signal data when the radar is virtually irradiated onto a reference point F located at the center of the imaging area R, and the center frequency f c The formula is the sum of a phase-based term relating to the function, a phase-based term expressed as the product of a function that depends only on the range frequency and a function that depends only on the distance between the satellite 5 and the reference point F (in this case, the zero-Doppler range ρ) for each azimuth frequency, and a phase-based term that represents the shift in the direction of travel DR1 for each of the multiple reference points F, and is generated using a formula (see formula (7)) that matches the phase-based term of the second signal data for each of the multiple reference points F.

[0135] The left-hand side of equation (7) is a relation relating to the phase of the second signal data (two-dimensional spectrum), and is the same as equation (2). The left-hand side of equation (7) is a term based on the phase of the second signal data for each of the multiple reference points F.

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[0136] In step S134, the interpolation equation generation unit 40 subtracts the phase with respect to the center of the imaging region R from the phase of the second signal data. The phase with respect to the center of the imaging region R is the phase of the second signal data when a reference point F is set at the center of the imaging region R. The phase with respect to the center of the imaging region R is the first term on the right-hand side of equation (7), and is the same as equation (3). In step S134, the process of subtracting equation (3) from equation (2) is performed. Equation (3) is a term based on the phase of the second signal data when the radar is virtually irradiated onto a reference point F located at the center of the imaging region R.

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[0137] In step S135, the interpolation generation unit 40 generates a center frequency f c The phase related to is subtracted from the relation obtained by subtracting equation (3) from equation (2). Center frequency f c The term based on the phase with respect to is the second term on the right-hand side of equation (7), and is expressed as equation (8). Center frequency f c The phase-based term (Equation (8)) refers to a reference point F located at a different position from the center of the imaging area R of satellite 5, with a center frequency f c Radar in (center frequency f c The phase of the reflected signal data obtained when virtually irradiated by a radar (which only has this capability) and the reference point F located at the center of the imaging area R, with a center frequency f c This term is based on the phase difference between the reflected signal data obtained when the radar virtually illuminates the area and the actual radar signal.

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[0138] In step S136, the interpolation formula generation unit 40 uses the phase data of the second signal data for each reference point F calculated by the calculation unit 30 (and the relational expression described above) to calculate an interpolation formula that can be expressed by the following equation (9). pρ represents the zero Doppler range. Equation (9) is a phase-based term expressed as the product of a function that depends only on the range frequency (a function of u) and a function that depends only on the distance between satellite 5 and reference point F (the zero Doppler range ρ) (a function of v) for each azimuth frequency.

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[0139] Note that after generating the interpolation formula (9), u(f τ ,f η The function shown by and v(r p ,f η The process of sending the function shown in ) to each interpolation unit is the same as in the second embodiment.

[0140] Figure 20 is a flowchart illustrating the process of generating a SAR image in the sixth embodiment. The process of generating a SAR image in the sixth embodiment will be explained using Figure 20.

[0141] The sixth embodiment differs from the second embodiment in that it includes a correction process by the correction processing unit 59. The processes in steps S200, S210, S220, S230, S240, and S250 are the same as in the second embodiment, so their explanation is omitted.

[0142] In the sixth embodiment, in step S241 between step S240 and step S250, the correction processing unit 59 determines the center frequency f c A correction process is performed to compensate for the effects caused by terms based on the phase related to . This correction process is the same as the "azimuth focusing" described in Non-Patent Document 3. More specifically, the correction processing unit 59 corrects the resampled data by multiplying it by the following equation (10).

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[0143] In this embodiment, when generating the interpolation formula, the center frequency f c The novel part is the section where a term based on the topology with respect to is added to equation (1) to generate a new relation.

[0144] In the signal processing device 1 according to the sixth embodiment, the center frequency f c By considering a phase-based term related to the interpolation formula, the accuracy of the interpolation formula is improved. This allows for improved imaging performance when generating SAR images.

[0145] In addition, in the sixth embodiment, the signal processing device 1 can also be realized with the configuration shown in Figure 4. The first interpolation unit 51, bulk compression processing unit 52, mapping processing unit 53, second interpolation unit 54, range inverse Fourier transform unit 55, resampling processing unit 56, third interpolation unit 57, azimuth inverse Fourier transform unit 58, and correction processing unit 59 are merely examples of the configuration of the interpolation processing unit 50.

[0146] In the sixth embodiment, when the signal processing device 1 is implemented as shown in Figure 4, the functions of the conversion unit 10 and the calculation unit 30 are the same as in the first embodiment. The reference point generation unit 20 generates multiple reference points, but unlike the first embodiment, it does not generate reference points in the ξ-r system. Also, the interpolation generation unit 40 generates a center frequency f c The first embodiment differs in that it generates an interpolation formula using a term based on the phase related to f. The interpolation processing unit 50 (correction processing unit 59) generates the interpolation formula using a term based on the center frequency f. c This differs from the first embodiment in that it performs a correction process to compensate for the effects caused by terms based on the phase related to .

[0147] (Seventh Embodiment) Figure 21 is a block diagram showing an overview of the signal processing device 1 according to the seventh embodiment. The processing of the reference point generation unit 20 according to the seventh embodiment differs from that of the sixth embodiment.

[0148] The reference point generation unit 20 according to the seventh embodiment generates multiple reference points in a direction that is inclined diagonally with respect to the vertical direction DR2, which is perpendicular to the direction of travel DR1, within the plane P defined by the direction of travel DR1 and the irradiation direction DR3, similar to the first embodiment. The direction that is inclined diagonally may also be the irradiation direction DR3. The reference point generation unit 20 according to the seventh embodiment generates multiple reference points in the ξ-r system.

[0149] The signal processing device 1 according to the seventh embodiment also provides the same effects and advantages as the first, second, and sixth embodiments.

[0150] The embodiments of the present invention have been described above with reference to the drawings, but these are merely examples of the present invention, and various other configurations can also be adopted.

[0151] The signal processing device 1 may be implemented by a single computer. Alternatively, the signal processing device 1 may be implemented by arbitrarily installing each of its functions (conversion unit 10, reference point generation unit 20, calculation unit 30, interpolation formula generation unit 40, and interpolation processing unit 50) in multiple computers.

[0152] Furthermore, the configurations of the third to fifth embodiments may be applied to the sixth and seventh embodiments.

[0153] In the fourth embodiment, if the orbit of the satellite 5 does not deviate significantly from a straight line, the range in the SAR image may be obtained by multiplying the distance r from the satellite 5 to the pixel data by cosθ, and the azimuth time in the SAR image may be obtained by multiplying the distance r by sinθ and dividing by the satellite's velocity.

[0154] Furthermore, while the flowcharts used in the above description show multiple steps (processes) in sequence, the execution order of the steps performed in each embodiment is not limited to the order in which they are described. In each embodiment, the order of the illustrated steps can be changed to the extent that it does not impede the content. Also, the above embodiments can be combined to the extent that their contents do not conflict.

[0155] Some or all of the above embodiments may also be described as follows, but are not limited to the following: 1. A transformation means for Fourier transforming a reflected signal representing the reflection from a scatterer to a radar emitted from a flying object into a first signal data in the frequency domain, A reference point generation means for generating multiple reference points, A calculation means for calculating the phase of each of the second signal data in the frequency domain obtained by Fourier transforming the reflected signals when the radar is virtually irradiated to the plurality of reference points, An interpolation formula generation means generates an interpolation formula using the phase of each of the second signal data calculated by the calculation means, The system comprises an interpolation processing means that interpolates the first signal data converted by the conversion means using the interpolation formula and performs an inverse Fourier transform on it, The interpolation formula generation means generates the interpolation formula using a term based on the phase with respect to the center frequency of the radar emitted from the flying object. The interpolation processing means is a signal processing device that performs a correction process to correct the effects caused by the term based on the phase with respect to the center frequency. 2. In the signal processing device described in 1., The term based on the phase with respect to the center frequency is a term based on the difference between the phase of the reflected signal data obtained when the reference point, located at a position different from the center of the imaging area of ​​the flying object, is virtually illuminated by the radar at the center frequency, and the phase of the reflected signal data obtained when the reference point, located at the center of the imaging area, is virtually illuminated by the radar at the center frequency, in a signal processing device. 3. In the signal processing device described in 2. The term based on the phase with respect to the center frequency is expressed by the following equation, which is a signal processing device.

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[0156] This application claims priority based on Japanese Patent Application No. 2023-002921, filed on 12 January 2023, and incorporates all of its disclosures herein. [Explanation of symbols]

[0157] 1. Signal Processing Device 2 Storage section 3 Storage section 4 Storage section 5 Satellite 10 Conversion section 20 Reference point generator 30 Calculation Section 40 Interpolation formula generation unit 50 Interpolation Processing Unit 60 Coordinate System Transformation Unit 70 Location Information Conversion Unit DR1 Traveling direction (azimuth direction) DR2 Vertical direction (zero Doppler direction) DR3 Irradiation direction (range direction) F reference point R imaging area

Claims

1. A signal processing device that generates a SAR image based on a reflection signal representing the reflection from a scatterer to a radar that is irradiated from a flying object into a shooting area, A transformation means for performing a Fourier transform on the reflected signal to obtain first signal data in the frequency domain, A reference point generation means that generates a plurality of reference points in a plane defined by the direction of travel of the flying object and the direction of illumination from which the flying object illuminates the radar, A calculation means for calculating the phase of each of the second signal data in the frequency domain obtained by Fourier transforming the reflected signals when the radar is virtually irradiated to the plurality of reference points, An interpolation formula generation means generates an interpolation formula using the phase of each of the second signal data calculated by the calculation means, An interpolation processing means that interpolates the first signal data converted by the conversion means using the interpolation formula and performs an inverse Fourier transform on it, The system includes a SAR image generation means that generates a SAR image based on the signal data after the inverse Fourier transform, The interpolation formula generation means generates the interpolation formula using a term based on the phase with respect to the center frequency of the radar emitted from the flying object. The interpolation processing means is a signal processing device that performs a correction process to correct the effects caused by the term based on the phase with respect to the center frequency.

2. In the signal processing apparatus according to claim 1, The term based on the phase with respect to the center frequency is a term based on the difference between the phase of the reflected signal data obtained when the reference point, located at a position different from the center of the imaging area of ​​the flying object, is virtually illuminated by the radar at the center frequency, and the phase of the reflected signal data obtained when the reference point, located at the center of the imaging area, is virtually illuminated by the radar at the center frequency, in a signal processing device.

3. In the signal processing apparatus according to claim 2, The term based on the phase with respect to the center frequency is expressed by the following equation, which is a signal processing device. [Math 1]

4. In the signal processing apparatus according to claim 2 or 3, The interpolation formula is an expression that is generated using an expression that matches the phase-based term of the second signal data for each of the multiple reference points, wherein the interpolation formula is expressed as the sum of a term based on the phase of the second signal data when the radar is virtually irradiated onto the reference point located at the center of the imaging area, a term based on the phase with respect to the center frequency, a term based on the phase expressed as the product of a function that depends only on the range frequency and a function that depends only on the distance between the flying object and the reference point for each azimuth frequency, and a term based on the phase that represents the shift of the flying object in the direction of travel for each of the multiple reference points.

5. In the signal processing apparatus according to claim 4, The interpolation formula is generated using the following formula in a signal processing device. [Math 2]

6. In the signal processing apparatus according to any one of claims 1 to 3, The reference point generation means is a signal processing device that generates a plurality of reference points in a plane defined by the direction of travel of the flying object and the direction in which the flying object illuminates the radar, in a direction that is inclined obliquely with respect to the direction perpendicular to the direction of travel.

7. In the signal processing apparatus according to claim 6, The aforementioned diagonally inclined direction is the direction of irradiation, in the signal processing device.

8. In the signal processing apparatus according to any one of claims 1 to 3, The system further includes coordinate system transformation means that obtains third signal data based on a first coordinate system from the data obtained by inverse Fourier transforming the first signal data using the interpolation processing means, and transforms the third signal data into fourth signal data based on a second coordinate system different from the first coordinate system. The first coordinate system has one axis representing the distance from the projectile to the scatterer in the irradiation direction, and the other axis representing the time when the center of the radar irradiated by the projectile passes through the scatterer. The second coordinate system is a signal processing device in which one axis represents the distance from the projectile to the scattering object in a direction perpendicular to the direction of the projectile's movement, and the other axis represents the time when the distance between the projectile and the scattering object is closest.

9. One or more computers, A signal processing method for generating a SAR image based on a reflection signal representing the reflection from a scatterer to a radar beam that is projected onto a shooting area from a flying object, The reflected signal is Fourier transformed into a first signal data in the frequency domain. In a plane defined by the direction of travel of the flying object and the direction of illumination from which the flying object illuminates the radar, a plurality of reference points are generated. The phase of the second signal data in the frequency domain obtained by Fourier transforming the reflected signals when the radar is virtually irradiated to the plurality of reference points is calculated for each of the above. An interpolation equation is generated using the phase of each of the aforementioned second signal data. The first signal data is interpolated using the interpolation formula and then subjected to an inverse Fourier transform. This includes generating a SAR image based on the signal data after the inverse Fourier transform, In generating the interpolation formula, the interpolation formula is generated using a term based on the phase with respect to the center frequency of the radar emitted from the flying object. A signal processing method that performs a correction process to correct the effects caused by the term based on the phase with respect to the center frequency by interpolating using the interpolation formula.

10. On the computer, A program that generates a SAR image based on a reflection signal representing the reflection from a scatterer to a radar beam that is projected onto a shooting area from a flying object, The reflected signal is Fourier transformed into a first signal data in the frequency domain. In a plane defined by the direction of travel of the flying object and the direction of illumination from which the flying object illuminates the radar, a plurality of reference points are generated. The phase of the second signal data in the frequency domain obtained by Fourier transforming the reflected signals when the radar is virtually irradiated to the plurality of reference points is calculated for each of the above. An interpolation equation is generated using the phase of each of the aforementioned second signal data. The first signal data is interpolated using the interpolation formula and then subjected to an inverse Fourier transform. The computer is instructed to generate a SAR image based on the signal data after the inverse Fourier transform. In generating the interpolation formula, the interpolation formula is generated using a term based on the phase with respect to the center frequency of the radar emitted from the flying object. A program that performs a correction process to compensate for the effects caused by the term based on the phase with respect to the center frequency by interpolating using the aforementioned interpolation formula.

Images