Object detection device, object detection method, and program

Abstract

The present disclosure provides an object detection device including a transceiver unit that repeatedly transmits radio waves toward the ground surface while moving and receives reflected signals, a pattern identification unit that identifies the shape of a peak pattern formed by reflection intensity peaks in each received signal, and a position identification unit that identifies the position of an object from the shape of the peak pattern. According to the present disclosure, it is possible to detect objects on the ground surface with less calculations than conventional methods.

Description

[Technical field]

[0001] The present invention relates to an object detection device, an object detection method, and a program. [Background technology]

[0002] One of the purposes of surface observation from artificial satellites (spacecraft) is to detect the position of ships on the sea. Synthetic aperture radar using microwaves is used for surface observation from artificial satellites. The use of microwaves makes it possible to observe the earth's surface even if there are clouds or at night. Synthetic aperture radar is a radio wave sensor that observes the earth's surface by irradiating radio waves (microwaves) onto the earth's surface and receiving the reflected waves. Synthetic aperture radar repeatedly irradiates the ground with radio waves while moving in orbit and synthesizes the reflected waves from the same position, making it possible to obtain high-resolution images similar to those obtained using large antennas even with a small antenna. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP2003-90880A Summary of the Invention [Problem to be solved by the invention]

[0004] Imaging using synthetic aperture processing requires a huge amount of calculations, which consumes a large amount of the satellite's valuable computing resources. Specifically, azimuth compression processing is required for each position (pixel), and an extremely large number of calculations are required to obtain a high-resolution (multiple-pixel) image.

[0005] It is also possible to transmit the received wave data to the ground and perform synthetic aperture processing there, but although this would save computational resources, it would consume a large amount of communication resources, which is just as valuable as computational resources, so this method is not an effective solution.

[0006] An object of the present invention is to provide a technique for detecting objects on the ground surface with a smaller amount of calculation than in the past. [Means for solving the problem]

[0007] One aspect of the present invention is based on the idea that if it is known in advance that a detection target is present on the earth's surface, which has a low reflectivity of radio waves, strong reflected waves can be obtained only from the target, and the target's position can be identified without imaging using synthetic aperture processing. Note that the term "earth's surface" or "earth's surface" in this disclosure refers not only to the surface of the land on Earth, but also to the water surface in areas such as the sea, rivers, and lakes.

[0008] Specifically, one aspect of the present invention is an object detection device that includes a transceiver unit that repeatedly transmits radio waves toward the ground surface while moving and receives reflected signals, a pattern identification unit that identifies the shape of a peak pattern formed by reflection intensity peaks in each received signal, and a position identification unit that identifies the position of an object from the shape of the peak pattern.

[0009] The peak of the reflected signal from any point on the ground is recorded approximately on a parabola, tracing a specific shape in a data space with the transmission position (transmission time) and the time difference between transmission and reception as axes. The shape of the parabola is determined by the relative position of the radio wave transmission position and the relevant point, the moving speed of the object detection device, etc. Conversely, once the shape of the reflection intensity peak is determined, the corresponding point on the ground is determined. If the ground surface is an area with low reflectance, it can be assumed that the object to be detected exists at the point where a strong reflected wave is obtained. Therefore, the position of the object to be detected can be identified with simple processing, without the need for the highly computationally demanding synthetic aperture processing for all pixels.

[0010] In one aspect of the present invention, the pattern identification unit may identify the peak pattern as a parabola. The shape of a parabola can be identified by an apex, an axis, and a curvature, but since the axis substantially coincides with the range direction, the pattern identification unit may obtain only the apex and the shape.

[0011] In one aspect of the present invention, the pattern identification unit may identify the shape of the peak pattern after performing range compression processing on the received signal. Range compression improves the resolution in the range direction. In another aspect of the present invention, the pattern identification unit may identify the shape of the peak pattern based on the received signal before range compression processing. Even without range compression, the intensity peak pattern exhibits a unique shape, and the position of the detection target can be identified from the shape of this intensity peak pattern.

[0012] In one aspect of the present invention, the present invention may further include a synthetic aperture processing unit that performs synthetic aperture processing on a partial region including the position of the object identified by the position identification unit to generate an image of the partial region. The synthetic aperture processing can image the object, making it possible to visually determine the type and state of the object. In this case, by imaging only the partial region where the object exists, rather than imaging the entire radio wave irradiation range, it is possible to significantly reduce calculation processing.

[0013] In one aspect of the present invention, the target object may be an object present on the ground surface that has a low reflectivity for the radio waves, and in particular may be a ship present on the sea.

[0014] In one embodiment of the present invention, the object detection device may be mounted on a spacecraft (artificial satellite) moving in the Earth's orbit, or may be configured as an integral part of the spacecraft. The orbit of the spacecraft is not particularly limited, but is typically a low orbit at an altitude of 200 to 1000 km. In another embodiment of the present invention, the object detection device may be mounted on a manned or unmanned aircraft, or may be configured as an integral part of the aircraft.

[0015] Another aspect of the present invention is an object detection method including a transmission / reception step of repeatedly transmitting radio waves from an object detection device toward the ground surface while moving and receiving reflected signals by the object detection device, a pattern identification step of identifying the shape of a peak pattern constituted by reflection intensity peaks in each received signal, and a position identification step of identifying the position of the object from the shape of the peak pattern.

[0016] Another aspect of the present invention is a computer program for causing a computer to execute each step of the above method. Effect of the Invention

[0017] According to the present invention, object detection on the ground surface can be performed with a smaller amount of calculation than in the past. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a configuration diagram of a spacecraft according to an embodiment. [Diagram 2] (A) and (B) are diagrams explaining the transmission and reception of radio waves by a spacecraft. [Diagram 3] 4 is a flowchart showing the flow of an object detection process performed by a spacecraft. [Figure 4] (A) shows the received reflected wave, (B) shows the reflection intensity peak, and (C) shows the peak pattern. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] One embodiment of the present invention is a spacecraft (artificial satellite) that moves in low Earth orbit (LEO) and has a function of identifying the position of a ship on the sea. That is, the spacecraft of this embodiment is an example of an object detection device according to the present invention. Note that the following description is based on one embodiment and is not intended to limit the present invention to the contents thereof. For example, the object detection device may be a moving body or flying object other than a spacecraft, the object to be detected may be an object other than a ship, and the radio waves used may have a wavelength other than microwaves.

[0020] [composition] 1 is a diagram showing the configuration of a spacecraft 100 according to this embodiment. The spacecraft 100 has a radar transceiver and an antenna for emitting microwave pulses, and transmits and receives microwave pulses while moving on orbit, and identifies the position of a ship on the sea and obtains an image of the ship from the received waves. The spacecraft 100 includes a control unit 101, an attitude control unit 102, a propulsion unit 103, a communication unit 104, a calculation unit 105, a microwave transceiver 106, and an antenna 107.

[0021] The control unit 101 is a computer including a processor, a main storage device, and an auxiliary storage device, and the processor loads a program stored in the auxiliary storage device into the main storage device and executes it to control each part of the spacecraft 100. In particular, the control unit 101 performs control when changing the orbit and control of the earth's surface observation. Note that some or all of the functions provided by the control unit 101 may be realized by a dedicated hardware circuit.

[0022] The attitude control unit 102 uses a reaction wheel to control the attitude of the spacecraft 100. Specifically, the attitude of the spacecraft 100 is controlled by changing the angular momentum of a flywheel. The attitude control unit 102 may also control the attitude of the spacecraft 100 by using an attitude control thruster.

[0023] The propulsion unit 103 generates thrust to finely correct the trajectory of the spacecraft 100. The propulsion unit 103 is, for example, a rocket thruster, and obtains thrust by ejecting gas produced by a chemical reaction of fuel.

[0024] The communication unit 104 is a functional unit for communicating with a communication device on the ground. Through the communication unit 104, the spacecraft 100 transmits the results of the Earth's surface observation, for example, the position of the ship and an image of the ship, to the communication device on the ground.

[0025] The calculation unit 105 is a computer including a processor, a main storage device, and an auxiliary storage device, and the processor loads a program stored in the auxiliary storage device into the main storage device and executes it to detect and image the object to be detected. The contents of the processing performed by the calculation unit 105 will be described later with reference to a flowchart. Note that some or all of the functions provided by the calculation unit 105 may be realized by a dedicated hardware circuit.

[0026] The microwave transceiver 106 transmits microwave pulses and receives their reflected waves (reflected signals). The microwave transceiver 106 has a signal processing section, a transmitting section, and a receiving section. The signal processing section has a pulse generation control section, an AD conversion section, a received wave recording section, etc. The transmitting and receiving section includes a frequency oscillator, a chirp oscillator, and a high-output amplifier, and feeds the generated chirp signal to the antenna 107. The receiving section includes a low-noise amplifier section, a gain control section, and a detection section, and performs detection from the received signal obtained by the antenna 107.

[0027] [Synthetic Aperture Radar Overview] An overview of synthetic aperture radar (conventional technology) will be described with reference to Fig. 2. As shown in Fig. 2(A), a spacecraft 100 uses a method called side-looking, which irradiates a microwave beam in an obliquely downward direction when observing the Earth's surface. The irradiation direction of the microwave beam is called the slant range direction, the Earth's surface component of the slant range direction is called the ground range direction (hereinafter, the ground range direction is simply called the range direction), and the azimuth direction of the traveling direction of the spacecraft 100. The irradiation range of one microwave beam is trapezoidal, as shown in area 200.

[0028] The resolution in the range direction depends on the pulse width of the microwave beam, and the shorter the pulse width, the higher the resolution. Therefore, the spacecraft 100 achieves high resolution by using a technique called pulse compression processing. The spacecraft 100 transmits a chirp pulse, which is a relatively long pulse that has been frequency-modulated, and shortens the pulse width by performing a convolution integral of the received signal and a reference signal (transmitted signal).

[0029] The resolution in the azimuth direction depends on the beam width of the microwave beam, which in turn depends on the antenna width. The larger the antenna width, the higher the resolution in the azimuth direction. High resolution can be achieved by using microwaves with short wavelengths such as X-band, but to obtain the same resolution using microwaves with relatively long wavelengths such as L-band, an antenna of an unrealistic size would be required to be mounted on the spacecraft 100. Synthetic aperture technology is a process that obtains high-resolution observation results using a virtual long antenna from signals received using a moving antenna. Of course, high resolution is possible by synthetic aperture processing regardless of the wavelength of the microwave, and synthetic aperture processing is effective even when using X-band. As shown in Figure 2(B), the spacecraft 100 repeatedly transmits and receives pulses while moving in the azimuth direction (traveling direction). The difference between the pulse transmission time and the reception time from the target changes approximately in proportion to the square of the distance depending on the azimuth position (transmission time), and the spacecraft 100 records this change as the phase of the received signal. Received signals from the same point appear at a position with a predetermined transmission and reception time difference in each received reflected wave. Therefore, by combining signals received from the same point, an image with high azimuth resolution can be obtained, a process known as azimuth compression.

[0030] The observation modes of the synthetic aperture radar include a stripe map mode, a spotlight mode, and a scanning mode. The stripe map mode is a mode in which the irradiation direction of the microwave beam relative to the spacecraft 100 is fixed, and the beam irradiation area moves as the spacecraft 100 moves. The spotlight mode is a mode in which the irradiation direction relative to the spacecraft 100 is changed so that the beam is irradiated to the same area. The scanning mode is a mode in which the beam irradiation direction moves in the range direction as the spacecraft 100 moves. The spacecraft 100 according to this embodiment may use any of the observation modes.

[0031] [Object location identification process] The above-mentioned azimuth compression process is generally performed as a convolution integral in the frequency domain in order to speed up the process, but performing the azimuth compression process for the entire observation area still requires a large amount of calculation, which is a problem in that it consumes valuable computational resources of the spacecraft 100.

[0032] For example, in order to observe the sea and identify the position of an object (such as a ship) on the sea, an image of the entire observation area is not necessary, and azimuth compression processing, which has a high calculation load, is not necessarily required. In this embodiment, the position of the object is identified by a simple calculation. Hereinafter, the object position identification processing in this embodiment will be described with reference to Figs. 3 and 4.

[0033] 3 is a flowchart showing the flow of the object position specifying process in this embodiment. This object position specifying process is performed when the spacecraft 100 is moving in a predetermined orbit such as a low earth orbit.

[0034] In step S11, the control unit 101 controls the microwave transceiver 106 to repeatedly transmit radio waves to the earth's surface and receive a reflected signal. The microwave transceiver 106 generates a microwave beam consisting of chirp pulses, transmits it via the antenna 107, and receives a reflected signal from the earth's surface via the antenna 107. The microwave transceiver 106 sends the received signal to the calculation unit 105, and the calculation unit 105 stores the received signal in a storage unit. The calculation unit 105 may perform the above-mentioned range compression processing on the received signal.

[0035] The microwave transceiver 106 and antenna 107 that execute the process of step S11 correspond to the transceiver unit in the present invention.

[0036] FIG. 4(A) is a diagram showing a schematic representation of a received signal 401 after range compression of a reflected signal. In this diagram, the vertical axis represents the transmission time t of the microwave beam (corresponding to the position of the spacecraft 100), the horizontal axis represents the time difference T between the transmission times of each microwave beam (the time until the reflected wave is received), and the Z axis represents the reflection intensity. At the position of a ship (or other object) where the microwave reflection intensity is greater than that of the surrounding sea, a strong peak appears in each received signal 401. This peak forms a characteristic pattern (approximately a parabola) on the plane defined by t and T. Note that a similar unique pattern is obtained for the reflection intensity peak even before range compression, so the process may proceed without performing range compression in step S11.

[0037] In step S12, the calculation unit 105 identifies the positions (time difference T) of the intensity peaks in the respective received signals. This process can be realized by using any existing peak detection algorithm for time-series data.

[0038] FIG. 4B is a diagram showing peaks 402 in each received signal detected in step S12.

[0039] In step S13, the calculation unit 105 identifies the shape of the pattern of the peaks detected in step S12. As described above, the pattern of the intensity peaks is approximately parabolic in shape, so the calculation unit 105 identifies a parabola connecting the peaks. The shape of a parabola is identified by a vertex, a curvature, and an axis, so the calculation unit 105 identifies these. The calculation unit 105 may perform this process using a curve fitting algorithm such as a least squares approximation method.

[0040] FIG. 4C is a diagram showing an example of a peak pattern 403 identified in step S13.

[0041] 4(A) to 4(C) show an example in which only one object exists in the observation area. When multiple objects exist in the observation area, a peak in which multiple peak patterns are superimposed is obtained. The calculation unit 105 extracts and specifies the shape of each peak pattern under the constraint that the peak pattern is a parabola.

[0042] The calculation unit 105 that executes the processes in steps S12 and S13 corresponds to the pattern identification unit in the present invention.

[0043] In step S14, the calculation unit 105 specifies the position of the object corresponding to the peak pattern from the shape of the peak pattern specified in step S13. The shape of the peak pattern corresponding to a specific position on the ground surface is determined by the relative positional relationship with the spacecraft 100, the moving speed of the spacecraft 100, and the like. Conversely, the position of the ground surface corresponding to this peak pattern can be obtained from the shape of the peak pattern. For example, the calculation unit 105 can obtain the observation position of the spacecraft 100 from highly accurate time and position information obtained from a positioning satellite system such as GPS, and can also obtain the distance between the observation position and the ground from the time difference between transmission and reception, and can specify the ground position from these. The calculation unit 105 can specify the ground surface position of the object based on the apex position of a parabola, or may specify the ground surface position of the object based on the result of performing limited azimuth compression as described later. Alternatively, the calculation unit 105 may specify the ground surface position of the object based on the position of the ground surface corresponding to the observation position of the spacecraft, the apex position of the parabola in the tT plane, and the curvature of the parabola.

[0044] The calculation unit 105 that executes the process of step S14 corresponds to the position identification unit in the present invention.

[0045] In step S15, the calculation unit 105 performs synthetic aperture processing only in the vicinity of the position of the object identified in step S14 to obtain an image of the area near the object. Specifically, the calculation unit 105 may limit the position to be processed to, for example, a rectangular area of ​​a predetermined size centered on the position of the object, and perform azimuth compression processing on the limited area. The predetermined size may be determined according to the system requirements, but it is preferable to set the size so that the entire object is included so that the type and state of the object can be identified. For example, when a ship is the object, a 500m square is considered sufficient. Note that, if only the position of the object is required and no image is required, the processing of step S15 may be omitted.

[0046] The calculation unit 105 that executes the process of step S15 corresponds to the synthetic aperture processing unit in the present invention.

[0047] In step S16, the position of the object measured in step S14 and the image of the vicinity of the object obtained in step S15 are transmitted to a ground or airborne device via communication unit 104. The transmission destination is not particularly limited.

[0048] [Advantageous Effects of the Present Embodiment] According to this embodiment, the position of the object can be obtained from the peak pattern of the received signal without performing computationally intensive synthetic aperture processing, so the amount of calculation required to identify the position of the object can be reduced. Also, when obtaining an image of the object, by performing synthetic aperture processing only on the vicinity of the object, the required image can be obtained with less computational effort than imaging the entire observation area. Also, since an image limited to the area of ​​the object can be obtained, the amount of data transmitted from the spacecraft can be reduced. [Explanation of symbols]

[0049] 100: Spacecraft 101: Control unit 102: Attitude control unit 103: Promotion Department 104: Communications Department 105: Calculation unit 106: Microwave transmission / reception unit 107: Antenna

Claims

1. A transceiver unit that repeatedly transmits radio waves toward the ground surface while moving and receives reflected signals; A pattern identification unit that identifies the shape of a peak pattern formed by the peak reflection intensities in each received signal; A position identification unit that identifies the position of an object from the shape of the peak pattern; A synthetic aperture processing unit that performs synthetic aperture processing on a partial region including the position of the object identified by the position identification unit to generate an image of the partial region; An object detection device comprising the above.

2. The object detection device according to claim 1, wherein the pattern identification unit identifies the peak pattern as a parabola.

3. The object detection device according to claim 1, wherein the pattern identification unit performs range compression processing on the received signal and then identifies the shape of the peak pattern. The object detection device according to claim 1, characterized by the above.

4. The object detection device according to claim 1, wherein the object is an object existing on the ground surface with a low reflectivity of the radio waves.

5. The object detection device according to claim 1, wherein the object is a ship existing on the sea.

6. A transceiver step of repeatedly transmitting radio waves from an object detection device toward the ground surface while moving and receiving reflected signals with the object detection device; A pattern identification step of identifying the shape of a peak pattern formed by the peak reflection intensities in each received signal; A position identification step of identifying the position of an object from the shape of the peak pattern; A synthetic aperture processing step of performing synthetic aperture processing on a partial region including the position of the object identified by the position identification step to generate an image of the partial region; An object detection method including the above.

7. In the pattern identification step, the peak pattern is identified as a parabola. The object detection method according to claim 6, characterized by the above.

8. The pattern identification step performs range compression processing on the received signal and then identifies the shape of the peak pattern. The object detection method according to claim 6, characterized by the above.

9. The object detection method according to claim 6, wherein the object is an object existing on the ground surface with a low reflectivity of the radio waves.

10. The object detection method according to claim 6, wherein the object is a ship existing on the sea.

11. A program for causing a computer to execute each step of the method according to any one of claims 6 to 10.

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