Object detecting device, object detecting method, and program

Abstract

The present disclosure provides an object detecting device comprising: a transmitting and receiving unit that repeatedly transmits radio waves toward the ground surface while moving, and receives reflected signals; a pattern identifying unit that identifies the shape of a peak pattern formed by reflection intensity peaks in each received signal; and a position identifying unit that identifies the position of an object from the shape of the peak pattern. According to the present disclosure, object detection on the ground surface can be performed using a smaller amount of calculation than in the past.

Description

Object detection device, object detection method, and program

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

[0002] One of the purposes of earth surface observation from artificial satellites (spacecraft) is to detect the position of ships on the sea. Synthetic aperture radar using microwaves is used for earth surface observation from artificial satellites. The use of microwaves makes it possible to observe the earth surface even through clouds and at night. Synthetic aperture radar is a radio wave sensor that observes the earth surface by irradiating radio waves (microwaves) onto the earth 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 a large antenna even with a small antenna.

[0003] JP 2003-90880 A

[0004] Imaging using synthetic aperture processing requires a huge amount of computational processing, which consumes a large amount of the satellite's valuable computing resources. Specifically, azimuth compression processing is required for each position (pixel), and obtaining high-resolution (multi-pixel) images requires an extremely large amount of computation.

[0005] Another option is to transmit the received wave data to the ground and perform synthetic aperture processing there, but while this would save computational resources, it would consume a huge amount of communication resources, which are 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 conventional techniques.

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

[0008] Specifically, one aspect of the present invention is an object detection device that includes a transmitter / receiver 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 traces a specific shape in a data space whose axes are the transmission position (transmission time) and the time difference between transmission and reception, and is recorded approximately on a parabola. The shape of the parabola is determined by the relative position of the radio wave transmission position and the point in question, the moving speed of the object detection device, and other factors. Conversely, once the shape of the reflection intensity peak is determined, the corresponding point on the ground can be determined. If the ground surface is in an area with low reflectivity, it can be assumed that the object to be detected is present at a 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 computationally intensive task of performing synthetic aperture processing on 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 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 system 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 allows the object to be imaged, 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 existing on the ground surface with low reflectivity of the radio waves, and in particular, may be a ship existing on the sea.

[0014] In one aspect of the present invention, the object detection device may be mounted on a spacecraft (artificial satellite) moving in 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 aspect 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 transmitting / receiving 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 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 the steps of the above method.

[0017] According to the present invention, object detection on the ground surface can be performed with a smaller amount of calculation than conventional methods.

[0018] The present invention relates to a spacecraft having a structure in which a received reflected wave is received, and a spacecraft having a reflected intensity peak is detected.

[0019] One embodiment of the present invention is a spacecraft (artificial satellite) that travels in low Earth orbit (LEO) and has the function of identifying the position of a ship at 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 that content. For example, the object detection device may be a moving 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] [Configuration] Figure 1 shows the configuration of a spacecraft 100 according to this embodiment. The spacecraft 100 has a radar transceiver and antenna that emits microwave pulses, and transmits and receives microwave pulses while moving in orbit, identifying the position of a ship at sea and obtaining images 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 programs stored in the auxiliary storage device into the main storage device and executes them to control each part of the spacecraft 100. The control unit 101 particularly controls orbit changes and Earth surface observations. Note that some or all of the functions provided by the control unit 101 may be realized by dedicated hardware circuits.

[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 generated by a chemical reaction of fuel.

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

[0025] The calculation unit 105 is a computer including a processor, a main memory device, and an auxiliary memory device, and the processor loads a program stored in the auxiliary memory device into the main memory device and executes it to detect and image the object to be detected. The details of the processing performed by the calculation unit 105 will be explained 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 unit, a transmitting unit, and a receiving unit. The signal processing unit has a pulse generation control unit, an AD conversion unit, a received wave recording unit, etc. The transmitting and receiving unit includes a frequency oscillator, a chirp oscillator, and a high-output amplifier, and feeds the generated chirp signal to the antenna 107. The receiving unit includes a low-noise amplifier, a gain control unit, and a detection unit, and performs detection from the received signal obtained by the antenna 107.

[0027] [Outline of Synthetic Aperture Radar] An overview of synthetic aperture radar (prior art) will be described with reference to Figure 2. As shown in Figure 2(A), a spacecraft 100 uses a method called side-looking, in which a microwave beam is emitted in a diagonally downward direction when observing the Earth's surface. The direction in which the microwave beam is emitted 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 will be 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 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 beamwidth of the microwave beam, which in turn depends on the antenna width. The wider the antenna width, the higher the resolution in the azimuth direction. While high resolution can be achieved by using microwaves with short wavelengths such as X-band, achieving the same resolution using microwaves with relatively long wavelengths such as L-band would require an antenna of unrealistic size to mount on the spacecraft 100. Therefore, synthetic aperture technology is a process that uses a virtual long antenna to obtain high-resolution observation results from signals received using a moving antenna. Of course, high resolution can be achieved using synthetic aperture processing regardless of the microwave wavelength, and synthetic aperture processing is also effective when using X-band. As shown in Figure 2(B), the spacecraft 100 repeatedly transmits and receives pulses while moving in the azimuth direction (direction of travel). The difference between the pulse transmission time and the reception time from the target varies 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. Signals received from the same point appear at a predetermined transmission and reception time difference in the respective received reflected waves. Therefore, by combining signals received from the same location, an image with high azimuth resolution can be obtained, a process known as azimuth compression.

[0030] The observation modes of a 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 Position Identification Processing] The above-mentioned azimuth compression processing is generally performed as a convolution integral in the frequency domain to speed up the processing, but performing the azimuth compression processing for the entire observation area still requires a large amount of calculation, which results in a problem of consuming valuable computational resources of the spacecraft 100.

[0032] For example, in response to a request 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 required, and azimuth compression processing, which involves a high computational load, is not necessarily required. In this embodiment, the position of the object is identified by a simple calculation. The object position identification process in this embodiment will be described below 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 while 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 reflected signals. The microwave transceiver 106 generates a microwave beam consisting of chirp pulses, transmits it via the antenna 107, and receives the reflected signals from the Earth's surface via the antenna 107. The microwave transceiver 106 sends the received signals to the calculation unit 105, and the calculation unit 105 stores the received signals in a memory unit. The calculation unit 105 may perform the above-mentioned range compression processing on the received signals.

[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] 4A is a diagram showing a schematic representation of a received signal 401 after range compression of the 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 location 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, since a similar unique pattern is obtained for the reflection intensity peak even before range compression, processing 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 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 intensity peaks is approximately parabolic, so the calculation unit 105 identifies a parabola connecting the peaks. The shape of a parabola is specified by its vertex, curvature, and axis, so the calculation unit 105 identifies these. The calculation unit 105 may perform this process using a curve fitting algorithm such as least-squares approximation, for example.

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

[0041] 4A to 4C 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 identifies the shape of each peak pattern under the constraint that the peak pattern must be a parabola.

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

[0043] In step S14, the calculation unit 105 identifies the location of the object corresponding to the peak pattern from the shape of the peak pattern identified in step S13. The shape of the peak pattern corresponding to a specific location on the Earth's surface is determined by the relative positional relationship with the spacecraft 100, the moving speed of the spacecraft 100, and the like. Conversely, the location of the Earth's surface corresponding to this peak pattern can be determined from the shape of the peak pattern. For example, the calculation unit 105 determines the observation position of the spacecraft 100 from highly accurate time and position information acquired from a positioning satellite system such as GPS, and also determines the distance between the observation position and the Earth's surface from the time difference between transmission and reception, and from these, the location on the Earth's surface can be determined. The calculation unit 105 can identify the Earth's surface location of the object based on the apex position of a parabola, or it may perform limited azimuth compression as described below and identify the Earth's surface location of the object based on the result. Alternatively, the calculation unit 105 may identify the Earth's surface location of the object based on the Earth's surface location corresponding to the observation position of the spacecraft, the apex position of the parabola in the t-T 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 on the vicinity of the position of the object identified in step S14, thereby obtaining 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 system requirements, but it is preferable to set it to a size that includes the entire object so that the type and condition of the object can be identified. For example, when targeting a ship, a 500m square is considered sufficient. Note that if only the position of the object is required and no image is necessary, 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 the communication unit 104. The transmission destination is not particularly limited.

[0048] [Advantageous Effects of the Present Embodiment] According to the present embodiment, the position of an object can be determined from the peak pattern of the received signal without performing computationally intensive synthetic aperture processing, thereby reducing the amount of calculation required to identify the position of the object. Furthermore, 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. Furthermore, since an image limited to the area of ​​the object can be obtained, there is also the effect of reducing the amount of data transmitted from the spacecraft.

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

Claims

1. An object detection device comprising: 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.

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, characterized in that the pattern identification section performs range compression processing on the received signal and then identifies the shape of the peak pattern.

4. The object detection device according to claim 1, further comprising 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.

5. The object detection device according to claim 1, characterized in that the object is an object present on the ground surface that has a low reflectivity for the radio waves.

6. The object detection device according to claim 1, characterized in that the object is a ship on the sea.

7. A method for detecting an object, comprising: a transmitting / receiving 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 identifying step of identifying the shape of a peak pattern constituted by reflection intensity peaks in each received signal; and a position identifying step of identifying the position of the object from the shape of the peak pattern.

8. The object detection method according to claim 7, wherein in said pattern identifying step, said peak pattern is identified as a parabola.

9. The object detection method according to claim 7, characterized in that the pattern identification step performs range compression processing on the received signal and then identifies the shape of the peak pattern.

10. The object detection method according to claim 7, further comprising 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.

11. The object detection method according to claim 7, wherein the object is an object present on the ground surface having a low reflectivity for the radio waves.

12. The object detection method according to claim 7, wherein the object is a ship located on the sea.

13. A program for causing a computer to execute each step of the method according to any one of claims 7 to 12.

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