Signal processing device, signal processing method, and program

Abstract

This signal processing device comprises a transforming unit, a reference point generating unit, a calculating unit, an interpolation formula generating unit, and an interpolation processing unit. The transforming unit subjects a reflected signal with respect to a radar emitted from a flying object to a Fourier transform to obtain first signal data in the frequency domain. The reference point generating unit generates a plurality of reference points arranged in a direction that is inclined obliquely relative to a direction perpendicular to a direction of progress of the flying object, within a plane defined by the direction of progress and an emission direction in which the flying object emits the radar. The calculating unit calculates phases of each item of second signal data in the frequency domain obtained after reflected signals generated when the radar is virtually emitted onto the plurality of reference points have been subjected to a Fourier transform. The interpolation formula generating unit generates an interpolation formula using the phases of each item of second signal data. The interpolation processing unit uses the interpolation formula to interpolate the first signal data transformed by the transforming unit and subjects the same to an inverse Fourier transform.

Description

Signal processing device, signal processing method, and recording medium

[0001] The present invention relates to a signal processing device, a signal processing method, and a recording medium.

[0002] Patent Document 1 discloses a technique relating to a radar sensor having multiple transmitting antennas, which is used to estimate the relative velocity of a target.

[0003] Patent Document 2 discloses a technique relating to a synthetic aperture radar device, which reduces errors that occur in the relative positional relationship between a fixed target and a moving target and superimposes the images of both.

[0004] Patent Document 3 discloses a technique relating to a synthetic aperture radar device that clarifies a method for determining the optimal polynomial degree N and obtains images with improved resolution.

[0005] Non-Patent Document 1 discloses a technique related to the Omega-K Algorithm in a method for generating SAR images.

[0006] Non-Patent Document 2 discloses a technique related to SVD-Stolt in the generation of SAR images.

[0007] Non-Patent Document 3 discloses a technique relating to the extended wavenumber domain in a method for generating SAR images.

[0008] JP-T-2020-503519 A JP-A-2017-106799 JP-A-2000-266845

[0009] YL Neo, and FH Wong, “Interpretations of the Omega-K Algorithm and Comparisons with other Algorithms”, [online], 2003 IEEE, [Retrieved January 12, 2023], Internet<http: / / geo.uzh.ch / microsite / rsl-documents / research / SARlab / GMTILiterature / Ver09 / PDF / CNW03.pdf> D. D'Aria, A. Monti Guarnieri, "High resolution spaceborne SAR focusing by SVD-STOLT", [online], [Retrieved January 12, 2023], Internet <https: / / www.researchgate.net / profile / Davide-Daria / publication / 3449863_High-resolution_spaceborne_SAR_focusing_by_SVD-stolt / links / 54e714e00cf277664ff7802f / High-resolution-spaceborne-SAR-focusing-by-SVD-stolt.pdf> A. Reigber, E. Alivizatos, A. Potsis and A. Moreira, “Extended wavenumber-domain synthetic aperture radar focusing with integrated motion compensation,” [online], IEE Proc.-Radar Sonar Navig., Vol. 153, No. 3, June 2006, [Retrieved January 12, 2023], Internet <https: / / www.researchgate.net / profile / Alberto-Moreira-2 / publication / 3357927_Extended_wavenumber-domain_synthetic_aperture_radar_focusing_with_integrated_motion_compen sation / links / 00b4951ccc4bb99e83000000 / Extended-wavenumber-domain-synthetic-aperture-radar-focusing-with-integrated-motion-compensation.pdf>.

[0010] The above-mentioned Patent Documents 2 and 3 disclose techniques related to synthetic aperture radar (SAR) images. In the SAR image generation techniques described therein, imaging performance may deteriorate if the squint angle of the flying object increases.

[0011] In view of the above-mentioned problems, an example of an 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.

[0012] According to one aspect of the present invention, there is provided a signal processing device comprising: a conversion means for Fourier transforming a reflected signal representing the reflection from a scatterer of a radar irradiated from a flying object into first signal data in the frequency domain; a reference point generation means for generating a plurality of reference points arranged in a plane defined by the traveling direction of the flying object and the irradiation direction in which the flying object irradiates the radar, the reference points being arranged in a direction obliquely inclined with respect to a direction perpendicular to the traveling direction; a calculation means for calculating the phase of second signal data in the frequency domain after Fourier transforming the reflected signal when the radar is virtually irradiated onto the plurality of reference points; an interpolation formula generation means for generating an interpolation formula using the phase of each of the second signal data calculated by the calculation means; and an interpolation processing means for interpolating and inverse Fourier transforming the first signal data converted by the conversion means using the interpolation formula.

[0013] According to one aspect of the present invention, there is provided a signal processing method in which one or more computers Fourier transform a reflected signal representing the reflection from a scatterer of a radar irradiated from a flying object into first signal data in the frequency domain, generate a plurality of reference points arranged in a plane defined by the flying object's direction of travel and the direction in which the flying object irradiates the radar, in a direction obliquely inclined with respect to a direction perpendicular to the flying object's direction of travel, calculate the phase of second signal data in the frequency domain after Fourier transforming the reflected signal when the radar is virtually irradiated to the plurality of reference points, generate an interpolation formula using the phase of each of the second signal data, and interpolate and inverse Fourier transform the first signal data using the interpolation formula.

[0014] According to one aspect of the present invention, there is provided a computer-readable recording medium having recorded thereon a program for causing a computer to execute the following steps: Fourier transforming a reflected signal representing the reflection from a scatterer of a radar irradiated from a flying object into first signal data in the frequency domain; generating a plurality of reference points arranged in a plane defined by the flying object's direction of travel and the direction in which the flying object irradiates the radar, the reference points being arranged in a direction obliquely inclined with respect to a direction perpendicular to the direction of travel; calculating the phase of second signal data in the frequency domain after Fourier transforming the reflected signal when the radar is virtually irradiated to the plurality of reference points; generating an interpolation formula using the phase of each of the second signal data; and interpolating the first signal data using the interpolation formula and performing an inverse Fourier transform on the first signal data.

[0015] According to one aspect of the present invention, a signal processing device, a signal processing method, and a program that can improve imaging performance are provided.

[0016] FIG. 1 is a schematic diagram of a satellite irradiating radar onto the Earth's surface. FIG. 1 is a schematic diagram of a satellite irradiating radar onto scatterers on the Earth's surface. FIG. 1 is a schematic diagram for explaining a signal related to a satellite. FIG. 2 is a block diagram showing an overview of a signal processing device according to a first embodiment. FIG. 3 is a schematic diagram for explaining a reference point F in the first embodiment. FIG. 4 is a diagram showing an example of the hardware configuration of a signal processing device. FIG. 5 is a diagram showing a conventional method for generating a plurality of reference points. FIG. 6 is a block diagram showing an overview of a signal processing device according to a second embodiment. FIG. 7 is a flow diagram of a process until an interpolation formula generation unit generates an interpolation formula. FIG. 8 is a flow diagram showing details of a method for generating an interpolation formula. FIG. 9 is a flow diagram of a process until an SAR image is generated. FIG. 10 is a flow diagram of a process until a reference point generation unit according to a third embodiment generates a reference point F. FIG. 11 is a block diagram showing an overview of a signal processing device according to a fourth embodiment. FIG. 12 is a flow diagram showing processing by a coordinate system conversion unit according to the fourth embodiment. FIG. 13 is a block diagram showing an overview of a signal processing device according to a fifth embodiment. FIG. 14 is a flow diagram showing processing by a position information conversion unit according to the fifth embodiment. FIG. 15 is a block diagram showing an overview of a signal processing device according to a sixth embodiment. FIG. 16 is a flow diagram of a process until an interpolation formula generation unit according to the sixth embodiment generates an interpolation formula. FIG. 17 is a flow diagram showing details of a method for generating an interpolation formula according to the sixth embodiment. FIG. 18 is a flow diagram of a process until an SAR image is generated in the sixth embodiment. FIG. 19 is a block diagram showing an overview of a signal processing device according to a seventh embodiment.

[0017] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings, like components are designated by like reference numerals, and the description thereof will be omitted as appropriate.

[0018] Synthetic Aperture Radar (SAR) technology is a technology in which an antenna mounted on a flying object (such as an artificial satellite or an airplane) transmits and receives electromagnetic waves (radar) while the flying object is moving, thereby artificially synthesizing an aperture so as to obtain an image (SAR image) equivalent to that obtained by an antenna with a large aperture. Hereinafter, an artificial satellite (SAR satellite) will be used as an example of the flying object.

[0019] (Definition of Terms) Fig. 1 is a schematic diagram of an artificial satellite 5 irradiating a radar onto the earth's surface E. Terms used in the embodiment will be defined using Fig. 1. The earth's surface E is actually a spherical surface with irregularities, but Fig. 1 simply shows the earth's surface as a flat surface.

[0020] 1 indicates the direction of travel of the artificial satellite 5. Each parallelogram drawn along the direction of travel DR1 represents an artificial satellite 5. These parallelograms indicate the movement trajectory of the artificial satellite 5.

[0021] In synthetic aperture radar, the reflection of electromagnetic waves emitted with a certain spread (width) from multiple satellite positions is synthesized to calculate the reflection of electromagnetic waves that would be emitted with a small spread (width) from a certain satellite position. The satellite position after synthesis is called azimuth, etc. In reality, there is no difference between azimuth and satellite position, so here we will not distinguish between azimuth and satellite position. In other words, the azimuth direction refers to the direction of travel DR1. The coordinate axis deployed in the azimuth direction is called the azimuth axis.

[0022] DR2 is a vertical direction DR2 that is perpendicular to the azimuth direction DR1 (= traveling direction DR1) within the plane P. The vertical direction DR2 is the direction in which the Doppler is 0. The direction in which the Doppler is 0 is also called the zero Doppler direction. Hereinafter, the vertical direction DR2 will also be called the zero Doppler direction DR2.

[0023] The plane P is a plane defined by the azimuth direction DR1 and the irradiation direction DR3. The irradiation direction DR3 is the direction in which the satellite 5 emits its radar. The irradiation direction DR3 is the direction in which the center of the radar of the satellite 5 travels. The irradiation direction DR3 is also the direction in which the antenna of the satellite 5 points. When the satellite is squint, the irradiation direction DR3 is inclined obliquely with respect to the zero Doppler direction DR2. Hereinafter, the irradiation direction DR3 will also be referred to as the range direction DR3.

[0024] The artificial satellite 5 emits radar (electromagnetic waves) toward the Earth's surface E and receives a reflected signal indicating the reflection of the radar. The radar emitted from the antenna of the artificial satellite 5 in the range direction DR3 hits the imaging area R and bounces back, and the phase delay, reflection strength, etc. are recorded. Then, an SAR image is formed using data related to the reflected signal. Because the radar has a certain width, the radar is emitted in a cone shape toward the Earth's surface E. The imaging area R in Figure 1 indicates the area 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 the squint angle θ ｓｑ The range direction DR3 is angled θ with respect to the zero Doppler direction DR2. ｓｑ It is inclined. Squint angle θ ｓｑ A typical squint angle is, for example, the squint angle of the artificial satellite 5 in a certain satellite orbit that the antenna is pointed at for the longest period of time.

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

[0027] (Explanation of the coordinate system) Fig. 2 is a schematic diagram of a satellite 5 irradiating a radar beam at a scatterer N on the Earth's surface E. Fig. 2 shows the state when the satellite 5 irradiates a radar beam at a scatterer N in an imaging region R on the Earth's surface E. The scatterer N represents a virtual point on the Earth's surface E, which is irradiated with the radar beam and reflects the radar beam in various directions. The coordinate system used in this specification will be described using Fig. 2.

[0028] (ξ-r system) The time when the satellite 5a comes in front of a certain scatterer N in the range direction DR3 (= irradiation direction DR3) is called the beam crossing azimuth time (ξ). ｐ The beam passing azimuth time ξｐ is the squint angle θ ｓｑ This is the time when the scatterer N comes directly in front of the antenna (radar) if the representative squint angle is not changed. ｐ is the time when the center of the radar emitted by the artificial satellite 5a passes through each scatterer N.

[0029] Beam passage azimuth time ξ ｐ In the above, the distance from the satellite 5a to the scatterer N in the range direction DR3 is defined as range r.

[0030] Hereinafter, one axis represents the range, and the other axis represents the beam passing azimuth time ξ ｐ The coordinate system represented by the axes is called the ξ-r system.

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

[0032] Nearest time η ｐ In this case, the distance from the satellite 5a to the scatterer N in the zero Doppler direction DR2 is defined as the zero Doppler range ρ.

[0033] Hereinafter, one axis represents the zero Doppler range ρ, and the other axis represents the nearest time η ｐ The coordinate system represented by the axes is called the η-ρ system.

[0034] While the terminology and coordinate system have been described based on a flat ground surface E and a straight orbit parallel to it as shown in Figure 1, the same applies to a spherical ground surface E and a curved satellite orbit. It is also well known that, for radar images acquired using a curved ground surface and a curved satellite orbit, it is possible to acquire radar images by approximating the ground surface to a plane and the satellite orbit to a straight line. The squint angle on this approximated geometry is referred to as the effective squint angle, but the following description will not distinguish between them.

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

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

[0037] The width of the ellipse in the left diagram of Figure 3 represents the intensity of the reflected signal. For example, the beam passing azimuth time ξ in the left diagram of Figure 3 ｐ At this point, the radar of the satellite 5 is pointing directly at the scatterer N, so the signal strength is strongest.

[0038] The right diagram of Fig. 3 shows data obtained by two-dimensional Fourier transform in the frequency domain from the data shown in the left diagram of Fig. 3. The horizontal axis of the right diagram of Fig. 3 is the azimuth frequency f obtained by converting the azimuth time η into the frequency domain. η The vertical axis of the right diagram in FIG. 3 is the range frequency f obtained by converting the range time τ into the frequency domain. τ The center frequency f ｃ is the center frequency of the frequency band of the radar (electromagnetic wave) emitted by the artificial satellite 5, and is determined in advance for each artificial satellite 5.

[0039] A SAR image is formed by aggregating data related to reflected signals from multiple scatterers N, as shown in the left and right diagrams of FIG. 3 . That is, data related to radar reflected signals, as shown in the left and right diagrams of FIG. 3 , can also be referred to as pixel data. The pixel data is a value associated with each two-dimensional grid. In the region on the left side of FIG. 3 , the two axes of the grid are the range time τ and the azimuth time η, and each two-dimensional grid records the phase and absolute value (also referred to as amplitude or signal strength) of each scatterer N as pixel data, associated with the range time τ and the azimuth time η. In the region on the right side of FIG. 3 , the two axes of the grid are the range frequency and the azimuth frequency, and each two-dimensional grid records the phase and absolute value of each scatterer N as pixel data, associated with the range frequency and the azimuth frequency.

[0040] 4 is a block diagram showing an overview of a signal processing device 1 according to a first embodiment. The signal processing device 1 includes 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 converter 10 performs a two-dimensional Fourier transform on the reflected signal to generate first signal data in the frequency domain. The reflected signal is a signal that represents the reflection from the scatterer N of the radar irradiated from the satellite 5. The reflected signal may be stored in the memory 2, or may be a signal immediately after being received from the satellite 5. The data format of the first signal data is as shown in the right diagram of Fig. 3.

[0042] 5 is a schematic diagram for explaining reference points F in the first embodiment. The reference point generation unit 20 generates a plurality of reference points F arranged in a direction obliquely inclined with respect to the zero Doppler direction DR2 (=vertical direction DR2). In the first embodiment, the obliquely inclined direction is the range direction DR3 (=illumination direction DR3). That is, the reference point generation unit 20 generates reference points F in the ξ-r system, not the η-ρ system. The reference point generation unit 20 generates reference points F in the ξ-r system, not in the η-ρ system, based on the squint angle θ ｓｑ The range direction DR3 is determined using data on

[0043] The reference point F corresponds to a virtual point selected from the imaging region R shown in FIG. 1. The reference point F is generated, for example, at the center of the imaging region R. The reference point F is a point generated to improve the imaging accuracy around the reference point F. The reference point F is selected, for example, by the user. For example, 30 reference points F are selected, but the number may be more or less than this. The multiple reference points F are selected based on the beam passing azimuth time ξ １ The range r is different in １ ~r ３ For example, a plurality of reference points F are generated within a range r of -30 km to 30 km, starting from the center of the imaging region R.

[0044] Returning to Fig. 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 signal data obtained by performing a two-dimensional Fourier transform on the reflected signal when a radar is virtually irradiated onto the reference point F. The second signal data in the frequency domain is linked to the range frequency and the azimuth frequency, as in the right diagram of Fig. 3. The calculation unit 30 calculates the phase of each of the second signal data in the frequency domain obtained by performing a Fourier transform on the reflected signal when a radar is virtually irradiated onto the 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 (described later) and thereby improve the imaging accuracy. Specific examples of the interpolation formula will be described in the second embodiment and thereafter.

[0046] The interpolation processing unit 50 interpolates and inverse Fourier transforms the first signal data converted by the conversion unit 10 using the interpolation formula generated by the interpolation formula generation unit 40. The data format of the signal data after the inverse Fourier transform is the same as the data described in the left diagram of FIG. 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 the signal data to an SAR image generation unit (not shown) that generates an SAR image. The SAR image is formed by collecting a large number of signal data after the inverse Fourier transform.

[0047] 6 is a diagram showing an example of the hardware configuration of the signal processing device 1. The signal processing device 1 has a bus 1010, a processor 1020, a memory 1030, a storage device 1040, an input / output interface 1050, and a network interface 1060.

[0048] The bus 1010 is a data transmission path for transmitting and receiving data among the processor 1020, memory 1030, storage device 1040, input / output interface 1050, and network interface 1060. However, the method of connecting the processor 1020 and the like to each other is not limited to bus connection.

[0049] The processor 1020 is implemented by a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), or the like.

[0050] The memory 1030 is a main storage device realized by a RAM (Random Access Memory) or the like.

[0051] The storage device 1040 is an auxiliary storage device realized by removable media such as a hard disk drive (HDD), a solid state drive (SSD), or a memory card, or a read-only memory (ROM), and has a recording medium. The recording medium of the storage device 1040 stores program modules that realize each function of the signal processing device 1 (e.g., 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 loads each of these program modules into the memory 1030 and executes them, thereby realizing each function corresponding to the program module. The storage device 1040 may also function as the memory 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 to various input / output devices.

[0053] The network interface 1060 is an interface for connecting the signal processing device 1 to a network. This network is, for example, a local area network (LAN) or a wide area network (WAN). The network interface 1060 may connect to the network wirelessly or by wire. The signal processing device 1 may communicate with the artificial satellite 5 via the network interface 1060.

[0054] 7 is a diagram showing a conventional method for generating multiple reference points. Conventionally, reference points are generated in the zero Doppler direction, but as the squint angle increases (for example, 5 degrees or more), the imaging performance deteriorates.

[0055] In the signal processing device 1 according to the first embodiment, the reference point F is generated based on a new coordinate system (ξ-r system) that takes the squint angle into account, thereby improving the imaging performance when generating a SAR image.

[0056] That is, according to this signal processing device, the signal processing device 1 capable of improving the imaging performance can be obtained.

[0057] 8 is a block diagram showing an outline of a signal processing device 1 according to a second embodiment. In the signal processing device 1 according to the second embodiment, the processing of the reference point generating unit 20, the calculating unit 30, the interpolation formula generating 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 orbit data of the artificial satellite 5, data relating to the azimuth frequency interval, and data relating to the range frequency interval to calculate the phase of each of the second signal data corresponding to the reference point F. The orbit 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 (the magnitude and direction of the velocity of the artificial satellite 5). The orbit data of the artificial satellite 5 may be stored in the storage unit 3 or may be acquired from the artificial satellite 5.

[0059] The data relating to the azimuth frequency interval is data relating to the interval at which data is acquired on the azimuth frequency axis (see the horizontal axis in the right diagram of FIG. 3). For example, if the interval is 100 [Hz], the phase of the second signal data is calculated at 100 [Hz] intervals on the azimuth frequency axis for a certain reference point F. The data relating to the azimuth frequency interval may be predetermined by the user.

[0060] The data relating to the range frequency interval is data relating to the interval at which data is acquired on the range frequency axis (see the vertical axis in the right diagram of FIG. 3). For example, if the interval is 10,000 [Hz], the phase of the second signal data is calculated at intervals of 10,000 [Hz] on the range frequency axis for a certain reference point F. The data relating to the range frequency interval may be determined in advance 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 process will be described later.

[0062] (Interpolation Formula Generation Flow) Fig. 9 is a flow diagram showing the process up to the generation of an interpolation formula by the interpolation formula generation unit 40. The process of generating an interpolation formula by the interpolation formula generation unit 40 will be described with reference to Fig. 9.

[0063] In step S100, reference point generation unit 20 acquires information regarding the representative squint angle from storage unit 4. Note that reference point generation unit 20 may also acquire information regarding the representative squint angle from satellite 5. To determine the representative squint angle, for example, the squint angle of satellite 5 at the time when the antenna is pointed most pointed may be used as the representative squint angle. As another example, in an imaging format in which the squint angle does not change during imaging (strip map mode), it is desirable to use the squint angle at that time. As another example, in a mode in which imaging is performed while moving the antenna slightly (by about 1 to 2 degrees) (such as called spotlight mode), 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 a plurality of reference points in the range direction DR3 (=illumination direction DR3) using the acquired representative squint angle. The reference point generation unit 20 generates a plurality of 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 generating unit 40 generates an interpolation formula using the phase of each piece of second signal data calculated by the calculating unit 30. Details of the method for generating the interpolation formula will be described in the following flow.

[0067] Fig. 10 is a flow diagram showing details of a method for generating an interpolation formula. The process for generating an interpolation formula will be described with reference to Fig. 10. It is assumed that an equation having the following relationship is obtained.

[0068] The left side of equation (1) is a relational expression related to the phase of the second signal data (two-dimensional spectrum). The data related to the phase of the second signal data for each reference point F calculated by the calculation unit 30 is calculated using the azimuth frequency intervals and the azimuth frequency intervals and range frequency intervals determined by the data related to the range frequency intervals. The relational expression related to the phase of the second signal data is given as equation (2).

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

[0070] In step S131, the interpolation formula generating unit 40 subtracts the phase related to the center of the imaging area R from the phase of the second signal data. The phase related to the center of the imaging area R is the phase of the second signal data when a reference point F is set at the center of the imaging area R. The phase related to the center of the imaging area R is the first term on the right side of equation (1) and is expressed as equation (3). Equation (3) is the phase calculated by the calculation unit 30 for the point corresponding to the center of the reference point F. Note that this phase is calculated using the data related to the azimuth frequency interval and the azimuth frequency and range frequency determined by the data related to the range frequency interval. Equation (3) is also a term based on the phase of the second signal data when a radar is virtually irradiated onto reference point F located at the center of the imaging area R.

[0071] r ｃ is the range relative to a reference point F set at the center of the imaging region R, η ｃ is the azimuth time (= beam passage azimuth time) for the reference point F set at the center of the imaging area R. All reference points are in the ξ-r system, η ｃ On the azimuth time (= beam passing azimuth time), different range r ｐ (See Figure 5.)

[0072] In step S131, a process of subtracting equation (3) from equation (2) is performed. That is, 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 are subtracted from the phases at the range frequency and azimuth frequency calculated by the calculation unit 30 for each reference point F. At this time, in the ξ-r system, η ｃ and η ｐ are equal, so η in Eq. (1) ｃ and η ｐ The term relating to the difference between ｎ (η ｐ -η ｃ ) can be ignored in step S132 described later. η (η ｐ -η ｃ ) is also a phase-based term that represents a shift in the traveling direction DR1 relative to each of a plurality of reference points F.

[0073] In step S132, the interpolation formula generating unit 40 calculates an interpolation formula that can be expressed by the following formula (4) using the subtraction result obtained in step S131. ｐ represents the range.

[0074] In addition, in the formula (4), u(f τ , f η The function shown by the formula (5) is expressed as follows: η For range frequency f τ It is a function that depends only on

[0075] In addition, in the formula (4), v(r ｐ , f η The function shown by the formula (6) is expressed as follows: η is a function that depends only on the distance between the satellite 5 and the reference point F (in this case, the range r).

[0076] In step S133, the interpolation formula generation unit 40 transmits information (interpolation formula) regarding the function represented by equation (5) to the second interpolation unit 54, and transmits information (interpolation formula) regarding the function represented by equation (6) 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 (described later). The third interpolation unit 57 transmits information about the function shown in equation (6) to the resampling processing unit 56 (described later).

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

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

[0080] (SAR Image Generation Flow) Fig. 11 is a flow diagram showing the process up to generation of an SAR image. The process up to generation of an SAR image will be described with reference to Fig. 11 .

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

[0082] In step S210, the bulk compression processor 52 performs bulk compression on the first signal data using data relating to the phase of the second signal data at the reference point F interpolated by the first interpolator 51. The bulk compression is similar to the Reference Function Multiply (bulk compression) described in Non-Patent Document 1.

[0083] The bulk compression process will be described in more detail. The bulk compression processing unit 52 multiplies the first signal data generated by the conversion unit 10 through two-dimensional Fourier transform by a reference signal. The reference signal is a complex conjugate obtained by Fourier transform of an ideal response (range time) from a scatterer N when the scatterer N is present at a reference point F (a reference point provided at the center of the imaging region R). The reference signal is calculated as a complex number with an absolute value of 1 and a phase obtained by adding a negative sign to the phase obtained from the first interpolation unit 51.

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

[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 executes resampling processing. The resampling processing is processing for changing the coordinate system. The resampling processing unit 56 performs resampling in accordance with equation (6) received from the third interpolation unit 57. That is, in the mapping processing performed in step S220, mapping is performed so that equation (5) becomes the new range frequency, and therefore, in the range inverse Fourier transform of the mapping result, v(r ｐ ,f η ) position rｐ This is expressed as r by the inverse function of v in equation (6). ｐ The resampling processing unit 56 performs resampling processing 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 (see the left diagram in Figure 3) where the vertical axis represents range time τ and the horizontal axis represents azimuth time η, and a large amount of this data is collected to generate a SAR image. Note that while the signal data is depicted as an ellipse in the left diagram in Figure 3, by undergoing the processing of steps S200 to S250, the signal data will be depicted as a dot (or rectangle).

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

[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 satellites 5, the data relating to the azimuth frequency interval, and the data relating to the range frequency interval to calculate the phase of each piece of second signal data corresponding to the reference point F. This makes it possible to accurately calculate the phase of the second signal data for each reference point F.

[0091] The interpolation processing unit 50 also 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 precision.

[0092] (Third embodiment)

[0093] 12 is a flow diagram showing the process performed by the reference point generation unit 20 according to the third embodiment when generating a reference point F. In the signal processing device 1 according to the third embodiment, the process performed by the reference point generation unit 20 is different from the process performed by the reference point generation unit 20 according to the first embodiment.

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

[0095] Hereinafter, the process of generating the reference point F by the reference point generating unit 20 according to the third embodiment will be described with reference to FIG.

[0096] In step S300, the reference point generating unit 20 acquires position data relating to the position of the satellite 5 from the satellite 5 (or may acquire it from a storage unit). The position data is the beam passage azimuth time ξ when the radar of the satellite 5 irradiates a representative point in the imaging area R (for example, the center point of the imaging area R). ｐ , and the beam passing azimuth time ξ ｐ The coordinate system includes position information of the satellite 5 in the Earth (position information of the satellite 5 relative to the Earth, preferably in a geocentric, Earth-fixed coordinate system).

[0097] In step S310, the reference point generating unit 20 acquires irradiation data relating to the irradiation direction DR3 from the satellite 5. The irradiation data includes information about the direction in which the antenna of the satellite 5 is pointing, and the squint angle θ ｓｑ Contains information about.

[0098] In step S320, the reference point generation unit 20 acquires position information (geographical position information) of a location in the ξ-r system that is r [km] away from the position of the satellite 5 in the range direction DR3 (illumination direction DR3). r [km] is the distance assigned as the location where the reference point is to be placed. Note that r [km] may be specified in advance by the user.

[0099] In step S330, the reference point generating unit 20 calculates the beam passing azimuth time ξ ｐis a straight line parallel to the azimuth direction DR1 (= traveling direction DR1) at the beam passing azimuth time ξ ｐ The trajectory is obtained by rotating the location r [km] away around the axis of a straight line passing through the position of the satellite at the point.

[0100] In step S340, the reference point generation unit 20 acquires position information at the intersection of the rotated trajectory and the Earth using the terrain data. Of the acquired intersections, the reference point generation unit 20 acquires position information at the intersection that is closer to the range direction DR3. The terrain data includes data on the Earth's terrain, and the terrain data has coordinate data in three-dimensional space for unevenness on the Earth's surface (including buildings, mountain ranges, etc.).

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

[0102] According to the third embodiment, the reference point generating unit 20 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 topographical data. This allows the reference points F to be generated taking into consideration the effects of the actual topography of the Earth's surface (presence or absence of unevenness, etc.) and the Earth's curvature, etc. This allows for improved imaging accuracy.

[0103] 13 is a block diagram showing an outline of a signal processing device 1 according to a fourth embodiment. The signal processing device 1 according to the fourth embodiment differs from the signal processing device 1 according to the second embodiment in that it further includes a coordinate system conversion unit 60.

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

[0105] The first coordinate system is a ξ-r system. That is, one axis of the first coordinate system 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 irradiated by the satellite 5 passes through the scatterer N.

[0106] The coordinate system conversion unit 60 converts the third signal data into fourth signal data. The fourth signal data is data based on a second coordinate system different from the first coordinate system. The fourth signal data is data obtained by converting the coordinate system of the third signal data. The second coordinate system is the η-ρ system. That is, the second coordinate system has one axis representing the distance (zero Doppler range ρ) from the satellite 5 to the scatterer N in the vertical direction DR2 (= zero Doppler direction DR2), and the other axis representing the time when the satellite 5 and the scatterer N are closest to each other (= closest time). The coordinate system conversion unit 60 converts the ξ-r system into the η-ρ system for the third signal data.

[0107] 14 is a flowchart showing the processing of the coordinate system conversion unit 60 according to the fourth embodiment. The processing of the coordinate system conversion unit 60 will be described in detail with reference to FIG. 14. 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 conversion unit 60 acquires the third signal data from the interpolation processing unit 50 (= the azimuth inverse Fourier transform unit 58).

[0109] In step S410, the coordinate system conversion unit 60 acquires each pixel data linked to the third signal data, and acquires position information (ξ-r system) of each pixel data with respect to the artificial satellite 5. The position information of the pixel data with respect 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 conversion unit 60 calculates the closest approach distance and closest approach time η between each pixel data and the artificial satellite 5. ｐThe closest approach distance is the distance when the scatterer N corresponding to a certain pixel data and the satellite 5 are closest to each other. In other words, the closest approach 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 closest approach distance is calculated based on the closest approach time η ｐ It is also the distance from the satellite 5 to the scatterer N at the closest time η ｐ is also the time when the artificial satellite 5 is at its closest distance.

[0111] In step S430, the coordinate system conversion unit 60 converts the coordinate system of the third signal data into fourth signal data based on the second coordinate system. ｐ The coordinate system converter 60 converts the ξ-r system related to the third signal data into the η-ρ system, with the axis related to the ξ-r system being the azimuth axis and the axis related to the closest distance being the range axis.

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

[0113] As described above, the signal processing device 1 according to the fourth embodiment further includes the coordinate system conversion unit 60. By converting the coordinate system back to the conventional coordinate system before imaging, conventional imaging techniques can be used as they are, and an SAR image can be easily generated.

[0114] 15 is a block diagram showing an outline of a signal processing device 1 according to a fifth embodiment. The signal processing device 1 according to the fifth embodiment differs from the signal processing device 1 according to the second embodiment in that it further includes a position information conversion unit 70.

[0115] The position information converter 70 converts the position information of the pixel data associated with the data obtained by performing an inverse Fourier transform on the first signal data by the interpolation processor 50 into position information on the Earth. The pixel data contains position information. The position information includes information on the distance r between the scatterer N corresponding to the pixel data and the artificial satellite 5.

[0116] The data obtained by performing the inverse Fourier transform on the first signal data by the interpolation processing unit 50 may be the third signal data according to the fourth embodiment. Hereinafter, the data obtained by performing the inverse Fourier transform on the first signal data by the interpolation processing unit 50 will be referred to as the third signal data.

[0117] 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 described in detail using FIG. 16. It is assumed that the imaging results in the ξ-r system, orbit data, topographical data, and squint angle are known.

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

[0119] In step S510, the position information conversion unit 70 converts the beam passing azimuth time ξ corresponding to each pixel data in the ξ-r system. ｐ and information about the velocity of the artificial satellite 5 (velocity vector). The position information conversion unit 70 may obtain the information about the position of the artificial satellite 5 and the information about the velocity of the artificial satellite 5 from the orbit data stored in the storage unit 3, or may obtain the information directly from the artificial 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 converts the beam passing azimuth time ξ ｐ The location corresponding to each pixel data is rotated around a line that is parallel to the velocity vector of the satellite 5 and passes through the azimuth position of the satellite 5 (the radius of rotation is the range r), and the trajectory drawn by the location corresponding to each pixel data is calculated.

[0122] In step S540, the position information conversion unit 70 determines the point where the trajectory intersects with the Earth as the ground position of the pixel data. The position data on the Earth's surface may be extracted from the topographical 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 the 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, the position information of pixel data can be directly converted into position information on the Earth without undergoing a process of converting from the ξ-r system to the η-ρ system. This makes it possible to easily generate a SAR image.

[0124] 17 is a block diagram showing an outline of a signal processing device 1 according to a 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] The reference point generating unit 20 according to the sixth embodiment does not generate multiple reference points in the ξ-r system, but may generate multiple reference points in the η-ρ system.

[0126] The interpolation formula generator 40 calculates the center frequency f of the radar irradiated from the artificial satellite 5. ｃ The interpolation formula is generated using a phase-based term for the center frequency f ｃ The phase of the second signal data in the frequency domain relative to the reference point F is the phase of the second signal data in the frequency domain relative to the reference point F. ｃ The phase of the part related to the center frequency f ｃ is the center frequency of the frequency band of the radar (electromagnetic wave) emitted by the artificial satellite 5. The specific processing will be described later.

[0127] The correction processing unit 59 calculates the center frequency f ｃ A correction process is performed to correct the influence of the phase-based term on the .times. ...

[0128] 18 is a flow diagram showing the process up to when the interpolation formula generating unit 40 according to the sixth embodiment generates an interpolation formula. The process of generating an interpolation formula by the interpolation formula generating unit 40 according to the sixth embodiment will be described with reference to FIG.

[0129] In step S111, the reference point generating unit 20 generates a plurality of reference points. In the sixth embodiment, the reference point generating unit 20 generates a plurality of 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 generating unit 40 generates an interpolation formula using the phase of each piece of second signal data calculated by the calculating unit 30. Details of the method for generating the interpolation formula will be described in the following flow.

[0132] 19 is a flow diagram showing details of a method for generating an interpolation formula according to the sixth embodiment. The process for generating an interpolation formula according to the sixth embodiment will be described in detail with reference to FIG.

[0133] When generating an interpolation formula in the η-ρ system, it may be difficult to calculate the interpolation formula (formula (4)) with high accuracy (the accuracy of formula (1) is not good to begin with, but in the case of high squint, the accuracy of formula (1) becomes particularly poor). Therefore, the inventor has made efforts to find the center frequency f ｃ We found that the accuracy of the interpolation formula can be improved by adding a term based on the phase of f to equation (1) (at least f τ = f ｃ For u(f τ , f η ) = v(r ｐ , f η ) = 0). Center frequency f ｃ The equation obtained by adding a term based on the phase regarding to equation (1) is expressed as follows:

[0134] The following explanation is based on the assumption that equation (7) has been found. The coordinate system is the η-ρ system. The interpolation equation is generated using equation (7). The interpolation equation includes a term based on the phase of the second signal data when a reference point F located at the center of the imaging area R is virtually irradiated with a radar, and a term based on the center frequency f ｃ a phase-based term expressed as the product of a function that depends only on the range frequency for each azimuth 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), and a phase-based term that represents a shift in the direction of travel DR1 for each of the multiple reference points F, which is generated using an equation (see equation (7)) that matches the phase-based term of the second signal data for each of the multiple reference points F.

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

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

[0137] In step S135, the interpolation formula generator 40 calculates the center frequency f ｃ The phase for the center frequency f is further subtracted from the relation obtained by subtracting equation (3) from equation (2). ｃ The phase-based term for the center frequency f is the second term on the right side of equation (7) and is expressed as equation (8). ｃThe phase-based term (Equation (8)) for the center frequency f ｃ Radar (center frequency f ｃ The phase of the reflected signal data obtained when a radar with only one frequency is virtually irradiated and a reference point F placed at the center of the imaging area R is calculated using a central frequency f ｃ This term is based on the difference in phase between the reflected signal data obtained when a radar is virtually irradiated at the target.

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

[0139] After generating the interpolation formula (9), u(f τ , f η ) and v(r ｐ , f η ) to each interpolation unit is the same as in the second embodiment.

[0140] 20 is a flow diagram showing the process up to generation of an SAR image in the sixth embodiment. The process up to generation of an SAR image in the sixth embodiment will be described with reference to FIG.

[0141] The sixth embodiment differs from the second embodiment in that correction processing is performed by a correction processing unit 59. The processes of steps S200, S210, S220, S230, S240, and S250 are the same as those of the second embodiment, and therefore descriptions thereof will be omitted.

[0142] In the sixth embodiment, in step S241 between steps S240 and S250, the correction processing unit 59 calculates the center frequency f ｃ This correction process is similar to the "azimuth focusing" process described in Non-Patent Document 3. More specifically, the correction processing unit 59 performs correction by multiplying the resampled data by the following equation (10):

[0143] In this embodiment, when generating the interpolation formula, the center frequency f ｃ The new part is the part where a term based on the phase regarding is added to equation (1) to generate a new relational expression.

[0144] In the signal processing device 1 according to the sixth embodiment, the center frequency f ｃ By considering the phase-based terms for , the accuracy of the interpolation formula is improved, which can improve imaging performance in generating SAR images.

[0145] In the sixth embodiment, the signal processing device 1 can also be realized with the configuration shown in Fig. 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 an example of the configuration of the interpolation processing unit 50.

[0146] In the sixth embodiment, when the signal processing device 1 is realized as shown in FIG. 4, the functions of the conversion unit 10 and the calculation unit 30 are the same as those of the first embodiment. The reference point generation unit 20 generates a plurality of reference points, but unlike the first embodiment, the reference points are not generated in the ξ-r system. In addition, the interpolation formula generation unit 40 generates a center frequency f ｃ The interpolation processing unit 50 (correction processing unit 59) generates an interpolation formula using a term based on the phase of the center frequency f ｃ The second embodiment differs from the first embodiment in that a correction process is performed to correct the influence caused by the term based on the phase of

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

[0148] Similar to the first embodiment, the reference point generation unit 20 according to the seventh embodiment generates multiple reference points in a direction obliquely inclined with respect to a vertical direction DR2 perpendicular to the traveling direction DR1 within a plane P defined by the traveling direction DR1 and the irradiation direction DR3. Note that the obliquely inclined direction may 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 achieves the same effects as those of the first, second, and sixth embodiments.

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

[0151] The signal processing device 1 may be realized by a single computer, or may be realized by multiple computers each having the functions of the signal processing device 1 (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) installed therein.

[0152] The configurations according to the third to fifth embodiments may be applied to the sixth and seventh embodiments.

[0153] In the fourth embodiment, when the orbit of the satellite 5 does not deviate significantly from a straight line, the range in the SAR image may be determined by multiplying the distance r from the satellite 5 to the pixel data by cos θ, and the azimuth time in the SAR image may be determined by multiplying the distance r by sin θ and dividing the result by the satellite traveling speed.

[0154] In addition, although the flowcharts used in the above description show multiple steps (processes) in a sequential order, the order of steps executed in each embodiment is not limited to the order shown. In each embodiment, the order of the steps shown in the drawings can be changed as long as it does not cause any problems in terms of content. Furthermore, the above-described embodiments can be combined as long as the content is not contradictory.

[0155] Some or all of the above embodiments can be described as, but are not limited to, the following supplementary notes: 1. A signal processing device comprising: a transformation means for Fourier transforming a reflected signal representing reflection from a scatterer of a radar irradiated from a flying object into first signal data in the frequency domain; a reference point generation means for generating a plurality of reference points arranged in a plane defined by the traveling direction of the flying object and the irradiation direction in which the flying object irradiates the radar, the reference points being arranged in a direction obliquely inclined with respect to a direction perpendicular to the traveling direction; a calculation means for calculating the phase of second signal data in the frequency domain after Fourier transforming the reflected signal when the radar is virtually irradiated to the plurality of reference points; an interpolation formula generation means for generating an interpolation formula using the phase of each of the second signal data calculated by the calculation means; and an interpolation processing means for interpolating and inverse Fourier transforming the first signal data converted by the transformation means using the interpolation formula. 2. The signal processing device described in 1., wherein the obliquely inclined direction is the irradiation direction. 3.2. 4. The signal processing device according to claim 2, further comprising: coordinate system conversion means for acquiring third signal data based on a first coordinate system from data obtained by performing an inverse Fourier transform on the first signal data using the interpolation processing means, and converting the third signal data into fourth signal data based on a second coordinate system different from the first coordinate system, wherein one axis of the first coordinate system represents the distance from the flying object to the scatterer in the irradiation direction, and the other axis represents the time when the center of the radar irradiated by the flying object passes through the scatterer, and wherein one axis of the second coordinate system represents the distance from the flying object to the scatterer in a direction perpendicular to the traveling direction, and the other axis represents the time when the flying object and the scatterer are closest to each other. 4. The signal processing device according to claim 2 or 3, further comprising position information conversion means for converting position information of pixel data associated with data obtained by performing an inverse Fourier transform on the first signal data using the interpolation processing means into position information on the Earth.5. The signal processing device according to any one of 2. to 4., wherein the reference point generating means generates the plurality of reference points using position data relating to the position of the flying object, illumination data relating to the illumination direction, and terrain data. 6. The signal processing device according to any one of 1. to 5., wherein the calculating means calculates the phase of each of the second signal data corresponding to the reference point further using trajectory data of the flying object, data relating to the azimuth frequency interval, and data relating to the range frequency interval. 7. The signal processing device according to any one of 1. to 6., wherein the interpolation processing means includes bulk compression processing means that performs bulk compression processing, mapping processing means that performs mapping processing, and resampling processing means that performs resampling processing. 8. A signal processing method in which one or more computers Fourier transform a reflected signal representing the reflection from a scatterer of a radar irradiated from a flying object into first signal data in the frequency domain, generate a plurality of reference points arranged in a plane defined by the flying object's direction of travel and the direction in which the flying object irradiates the radar, the reference points being arranged in a direction obliquely inclined with respect to a direction perpendicular to the flying object's direction of travel, calculate the phase of second signal data in the frequency domain after Fourier transforming the reflected signal when the radar is virtually irradiated to the plurality of reference points, generate an interpolation formula using the phase of each of the second signal data, and interpolate and inverse Fourier transform the first signal data using the interpolation formula.9. A program causing a computer to execute the following steps: Fourier transforming reflected signals representing reflections from scatterers of radar irradiated from a flying object into first signal data in the frequency domain; generating a plurality of reference points arranged in a plane defined by the traveling direction of the flying object and the irradiation direction in which the flying object irradiates the radar, the reference points being aligned in a direction obliquely inclined with respect to the traveling direction; calculating the phase of second signal data in the frequency domain after Fourier transforming the reflected signals when the radar is virtually irradiated to the plurality of reference points; generating an interpolation formula using the phase of each of the second signal data; and interpolating and inverse Fourier transforming the first signal data using the interpolation formula.

[0156] This application claims priority based on Japanese Patent Application No. 2023-002920, filed January 12, 2023, the disclosure of which is incorporated herein by reference in its entirety.

[0157] REFERENCE SIGNS LIST 1 Signal processing device 2 Memory unit 3 Memory unit 4 Memory unit 5 Artificial satellite 10 Conversion unit 20 Reference point generation unit 30 Calculation unit 40 Interpolation formula generation unit 50 Interpolation processing unit 60 Coordinate system conversion unit 70 Position information conversion unit DR1 Direction of travel (azimuth direction) DR2 Vertical direction (zero Doppler direction) DR3 Illumination direction (range direction) F Reference point R Shooting area

Claims

1. a transforming means for Fourier transforming a reflected signal representing a reflection from a scatterer of a radar beam emitted from the flying object into first signal data in the frequency domain; a reference point generating means for generating a plurality of reference points arranged in a direction obliquely inclined with respect to a direction perpendicular to the traveling direction of the flying object within a plane defined by the traveling direction of the flying object and the irradiation direction in which the flying object irradiates the radar; a calculation means for calculating a phase of second signal data in a frequency domain after Fourier transforming the reflected signal when the radar is virtually irradiated onto the plurality of reference points; an interpolation equation generating means for generating an interpolation equation using the phase of each of the second signal data calculated by the calculating means; and an interpolation processing means for interpolating and inverse Fourier transforming the first signal data converted by the conversion means using the interpolation formula.

2. 2. The signal processing device according to claim 1, The signal processing device, wherein the obliquely inclined direction is the irradiation direction.

3. 3. The signal processing device according to claim 2, a coordinate system conversion means for acquiring third signal data based on a first coordinate system from data obtained by performing an inverse Fourier transform on the first signal data by the interpolation processing means, and converting 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 flying object to the scatterer in the irradiation direction, and the other axis representing the time when the center of the radar irradiated by the flying object passes through the scatterer; The second coordinate system has one axis representing the distance from the flying object to the scattering object in a direction perpendicular to the direction of travel, and the other axis representing the time when the flying object and the scattering object are closest to each other.

4. 4. The signal processing device according to claim 2, The signal processing device further comprises a position information conversion means for converting position information of pixel data associated with data obtained by performing an inverse Fourier transform on the first signal data by the interpolation processing means into position information on the Earth.

5. 4. The signal processing device according to claim 2, The reference point generating means generates the plurality of reference points using position data relating to the position of the flying object, illumination data relating to the illumination direction, and topographical data.

6. 4. The signal processing device according to claim 1, The calculation means further uses trajectory data of the flying object, data relating to an interval of azimuth frequency, and data relating to an interval of range frequency to calculate the phase of each of the second signal data corresponding to the reference point.

7. 4. The signal processing device according to claim 1, The signal processing device, wherein the interpolation processing means includes bulk compression processing means for performing bulk compression processing, mapping processing means for performing mapping processing, and resampling processing means for performing resampling processing.

8. One or more computers Fourier transforming a reflected signal representing a reflection from a scatterer of a radar beam emitted from the flying object into first signal data in the frequency domain; generating a plurality of reference points arranged in a direction obliquely inclined with respect to a direction perpendicular to the traveling direction of the flying object within a plane defined by the traveling direction of the flying object and the irradiation direction in which the flying object irradiates the radar; calculating a phase of second signal data in the frequency domain after Fourier transforming the reflected signal when the radar is virtually irradiated to the plurality of reference points; generating an interpolation equation using a phase of each of the second signal data; a signal processing method for performing an inverse Fourier transform and interpolating the first signal data using the interpolation formula;

9. On the computer, a step of Fourier transforming a reflected signal representing a reflection from a scatterer of a radar irradiated from the flying object into first signal data in the frequency domain; a step of generating a plurality of reference points arranged in a plane defined by the traveling direction of the flying object and the irradiation direction in which the flying object irradiates the radar, the reference points being arranged in a direction inclined obliquely with respect to a direction perpendicular to the traveling direction; calculating a phase of second signal data in a frequency domain after Fourier transforming the reflected signal when the radar is virtually irradiated onto the plurality of reference points; generating an interpolation equation using a phase of each of the second signal data; a program for executing a procedure of interpolating the first signal data using the interpolation formula and performing an inverse Fourier transform on the first signal data.