Passive bistatic radar system

Abstract

To enable target measurement to be easily carried out.SOLUTION: A passive bistatic radar system 1 comprises: a transmission device 20 capable of transmitting radio waves; a first antenna 22 capable of receiving direct waves transmitted from the transmission device 20; a second antenna 24 capable of receiving reflected waves transmitted from the transmission device 20 and reflected by a target 10 that is the object to be measured; and a radar device 26 that measures the position of at least the target 10 on the basis of the electric power of the direction wave received by the first antenna 22 and the electric power of the reflected wave received by the second antenna 24. The transmission device 20 is a public mobile station that is usable while being carried and transmits radio waves used by the public.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a passive bistatic radar system. [Background technology]

[0002] For example, Patent Document 1 discloses a ground-penetrating radar device for searching for objects buried underground. Such a ground-penetrating radar device comprises a transmitting unit that transmits radio waves into the ground and a receiving unit that receives radio waves reflected by the object. In other words, such a ground-penetrating radar device actively transmits radio waves for searching. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-100793 [Overview of the project] [Problems that the invention aims to solve]

[0004] Conventional ground-penetrating exploration technology, an example of a technique that uses radar to measure targets (objects), actively transmits radio waves. However, due to restrictions under the Radio Law, transmitting radio waves into the air is prohibited, and to suppress radio wave leakage, the system must be configured to transmit radio waves from near the ground towards the ground. For this reason, conventional ground-penetrating exploration technology may have limitations and constraints on the use of radar equipment.

[0005] Therefore, the present invention aims to provide a passive bistatic radar system that can easily measure targets. [Means for solving the problem]

[0006] To solve the above problems, a passive bistatic radar system according to one embodiment of the present invention comprises a transmitting device capable of transmitting radio waves, a first antenna capable of receiving direct waves transmitted from the transmitting device, a second antenna capable of receiving reflected waves transmitted from the transmitting device and reflected by a target to be measured, and a radar device that measures at least the position of a target based on the power of the direct waves received by the first antenna and the power of the reflected waves received by the second antenna, wherein the transmitting device is portable and usable. Frequencies used in public wireless systems A public mobile station that transmits radio waves The transmitting device is positioned within a range in which the radio waves to be transmitted can reach the estimated position of the target, the first antenna is positioned near the transmitting device, and the second antenna is positioned within a range in which it can receive reflected waves reflected at the estimated position of the target, and is positioned further away from the first antenna than the distance between the first antenna and the transmitting device. .

[0007] The transmitting device may be a smartphone, mobile phone, tablet, mobile router, or laptop computer.

[0008] The target is at least one of buried objects and underground cavities, and the frequency of the radio waves transmitted from the transmitting device may be less than 1 GHz.

[0009] The target is the ground, and the frequency of the radio waves transmitted from the transmitting device may be 1 GHz or higher.

[0010] The radar system may derive a correlation function between the power of the direct wave received by the first antenna and the power of the reflected wave received by the second antenna, and determine the position of the target according to the derived correlation function.

[0012] The first antenna may be positioned near the transmitting device and connected to the radar device by a wire.

[0014] The second antenna may be a cross-Yagi antenna having a first element and a second element that are orthogonal to each other, and may be configured to receive the horizontal and vertical polarizations of the reflected wave.

[0015] The second antenna is configured to be capable of receiving reflected waves while shifting its reception position, and the radar device may perform synthetic aperture processing based on the power of the reflected waves for each reception position received by the second antenna, and identify characteristics of a target based on a result of the synthetic aperture processing.

[0016] The passive bistatic radar system may further include a shielding body, and the shielding body may be arranged between the transmitting device and the second antenna.

Effects of the Invention

[0017] According to the present invention, it becomes possible to easily perform measurement of a target.

Brief Description of Drawings

[0018] [Figure 1] FIG. 1 is a schematic diagram showing a configuration of a passive bistatic radar system according to the present embodiment. [Figure 2] FIG. 2 is a diagram for explaining the relationship between a cross Yagi-Uda antenna, which is an example of the second antenna, and a target. [Figure 3] FIG. 3 is a schematic diagram showing another configuration of a passive bistatic radar system. [Figure 4] FIG. 4 is a flowchart for explaining a target measurement method. [Figure 5] FIG. 5 is a flowchart for explaining a flow of signal processing performed by a signal processing unit 50. [Figure 6] FIG. 6 is a diagram showing an example of experimental results. [Figure 7] FIG. 7 is a diagram for explaining the frequency of radio waves transmitted from a transmitting device. [Figure 8] FIG. 8 is a schematic diagram showing a configuration of a modified passive bistatic radar system. [Figure 9] FIG. 9 is a flowchart for explaining a flow of signal processing performed by a signal processing unit in a modified passive bistatic radar system. [Modes for carrying out the invention]

[0019] Embodiments of the present invention will be described in detail below with reference to the attached drawings. The specific dimensions, materials, numerical values, etc., shown in these embodiments are merely examples to facilitate understanding of the invention and do not limit the present invention unless otherwise specified. In this specification and drawings, elements having substantially the same function and configuration are denoted by the same reference numerals to avoid redundant explanations, and elements not directly related to the present invention are omitted from the illustrations.

[0020] Figure 1 is a schematic diagram showing the configuration of the passive bistatic radar system 1 according to this embodiment.

[0021] The passive bistatic radar system 1 is configured to measure a target 10. The target 10 is, for example, an underground structure or underground cavity. Examples of underground structures include water pipes, gas pipes, power lines, and communication lines, but are not limited to these. The passive bistatic radar system 1 can measure various characteristics of the target 10, such as its position, shape, size, and orientation. The target 10 may also be exposed above ground, such as the ground 12 or a wall. For example, the passive bistatic radar system 1 can also measure the surface displacement of the earth.

[0022] The passive bistatic radar system 1 comprises a transmitter 20, a first antenna 22, a second antenna 24, and a radar device 26. The first antenna 22, the second antenna 24, and the radar device 26 constitute a radar unit 30. The transmitter 20 is a device independent of the radar unit 30.

[0023] The transmitting device 20 is a device capable of transmitting radio waves. In this embodiment, the transmitting device 20 is a public mobile station. A public mobile station is a radio station that is portable and usable, and transmits radio waves for public use. Radio waves for public use are, for example, radio waves at frequencies used in public wireless systems such as LTE (Long Term Evolution). The radio waves transmitted by the public mobile station will be described in detail later.

[0024] The transmitting device 20 specifically includes smartphones, mobile phones, tablets, mobile routers, and laptop computers. However, the transmitting device 20 is not limited to the exemplified devices; it may be any device that functions as a public mobile station. In other words, the transmitting device 20 is a mobile device that can be used by users without registering it as a radio station, or without a radio operator's license.

[0025] The transmitting device 20 is positioned within a predetermined range from which radio waves transmitted from the transmitting device 20 can reach the estimated location where the target 10 is presumed to exist. This predetermined range is, for example, within 10m, but can be appropriately set depending on the radio waves used. Since the transmitting device 20 is portable, it can be positioned at any location within this predetermined range.

[0026] The first antenna 22 is configured to receive the direct waves of the radio waves transmitted from the transmitting device 20. The first antenna 22 is, for example, a chip antenna or a loop antenna. Note that the first antenna 22 is not limited to the types of antennas exemplified, but may be any type of antenna.

[0027] The first antenna 22 is positioned in the vicinity of the transmitter 20. Here, "nearby" means that the distance between the transmitter 20 and the first antenna 22 is sufficiently short compared to the distance between the transmitter 20 and the estimated position of the target 10. Furthermore, "nearby" here allows the first antenna 22 to be positioned in contact with the transmitter 20. By positioning the first antenna 22 in the vicinity of the transmitter 20, the first antenna 22 can properly receive the direct wave transmitted from the transmitter 20.

[0028] The first antenna 22 is wired to the radar device 26. This ensures that the signal transmission time from the first antenna 22 to the radar device 26 remains constant, regardless of the position of the first antenna 22. Therefore, by fixing the positions of the transmitter 20 and the first antenna 22 during measurement, the transmission time from the direct wave transmitted from the transmitter 20 to the radar device 26 via the first antenna 22 can be kept constant. As a result, fluctuations in the acquisition timing of the direct wave, which serves as the reference for measurement, can be suppressed, and measurement accuracy can be improved.

[0029] The second antenna 24 is configured to receive reflected waves from the target 10 that have been reflected off the radio waves transmitted from the transmitting device 20. The second antenna 24 is positioned at a distance from the first antenna 22. Specifically, the second antenna 24 is positioned such that the distance between the first antenna 22 and the second antenna 24 is greater than the distance between the first antenna 22 and the transmitting device 20. For example, it is preferable that the distance between the first antenna 22 and the second antenna 24 be five times or more the distance between the first antenna 22 and the transmitting device 20. Note that the comparison value of the distances is not limited to the example values ​​and can be set as appropriate.

[0030] Furthermore, the second antenna 24 is positioned within a predetermined range capable of receiving reflected waves reflected at the estimated position of the target 10. This predetermined range may be appropriately set considering various conditions such as the frequency of the radio waves transmitted from the transmitting device 20, the estimated position of the target 10, and the installation environment of the target 10.

[0031] In the passive bistatic radar system 1, "bistatic" means that the transmitter 20 is independent of the first antenna 22 and the second antenna 24.

[0032] Furthermore, conventional ground-penetrating radar systems were active-type radar systems that transmitted the radio waves for exploration themselves. On the other hand, in the passive bistatic radar system 1 of this embodiment, "passive" means that the radar unit 30 itself does not transmit radio waves. In other words, the radar unit 30 of this embodiment is a passive bistatic radar because both the first antenna 22 and the second antenna 24 are composed of receiving antennas.

[0033] The second antenna 24 is wired to the radar device 26. This ensures that the signal transmission time from the second antenna 24 to the radar device 26 remains constant, regardless of the position of the second antenna 24.

[0034] The second antenna 24 is, for example, a cross-Yagi antenna. However, the second antenna 24 is not limited to a cross-Yagi antenna; it may be any type of antenna.

[0035] Figure 2 illustrates the relationship between a cross-Yagi antenna 24a, which is an example of a second antenna 24, and the target 10. Target 10a in Figure 2 is an example of a target 10 positioned horizontally with respect to the ground 12. Target 10b is an example of a target 10 positioned perpendicularly with respect to the ground 12. Target 10c is an example of a target 10 positioned at an angle with respect to the ground 12.

[0036] A cross-Yagi antenna 24a, which is an example of a second antenna 24, has a plurality of first elements 40 and a plurality of second elements 42. The first elements 40 and the second elements 42 are orthogonal to each other. The cross-Yagi antenna 24a is positioned such that the first elements 40 are horizontal to the ground 12 and the second elements 42 are perpendicular to the ground 12. With the cross-Yagi antenna 24a positioned in this manner, the first elements 40 receive the horizontal polarization of the reflected wave reflected by the target 10, and the second elements 42 receive the vertical polarization reflected by the target 10.

[0037] The first element 40 is parallel to target 10a, which is positioned horizontally with respect to the ground 12, and perpendicular to target 10b, which is positioned perpendicular to the ground 12.

[0038] The first element 40 cannot properly receive reflected waves reflected by target 10b perpendicular to the first element 40, but it can receive reflected waves reflected by target 10a parallel to the first element 40. In other words, by using the cross Yagi antenna 24a, the power received by the first element 40 can be used to measure the horizontally positioned target 10a.

[0039] On the other hand, the second element 42 is parallel to the target 10b which is positioned perpendicular to the ground 12, and perpendicular to the target 10a which is positioned horizontally to the ground 12.

[0040] The second element 42 cannot properly receive reflected waves reflected by target 10a perpendicular to the second element 42, but it can receive reflected waves reflected by target 10b parallel to the second element 42. In other words, by using the cross Yagi antenna 24a, the power received by the second element 42 can be used to measure the vertically positioned target 10b.

[0041] Furthermore, the first element 40 can receive the horizontal component (horizontal polarization component) of the reflected wave reflected by the tilted target 10c. The second element 42 can receive the vertical component (vertical polarization component) of the reflected wave reflected by the diagonally positioned target 10c. By deriving the ratio of the power received by the first element 40 to the power received by the second element 42, the tilted target 10c can be measured. In other words, by using the cross Yagi antenna 24a, the tilted target 10c can also be measured.

[0042] In this way, by using a cross-Yagi antenna 24a as the second antenna 24, measurements of the target 10 can be performed regardless of the attitude of the target 10.

[0043] Let's return to Figure 1 for explanation. The radar device 26 has a signal processing unit 50 and a display unit 52. The signal processing unit 50 is composed of, for example, an AD converter and a DSP (Digital Signal Processor).

[0044] The signal processing unit 50 acquires the power of the direct wave received by the first antenna 22 (received power of the direct wave) and the power of the reflected wave received by the second antenna 24 (received power of the reflected wave). Based on the received power of the direct wave and the received power of the reflected wave, the signal processing unit 50 measures at least the position of the target 10.

[0045] For example, by taking the difference between the timing of acquiring the received power of the direct wave and the timing of acquiring the received power of the reflected wave, the time it takes for radio waves to propagate from the transmitting device 20 through the target 10 to the second antenna 24 can be determined. Converting this time into distance allows the distance of the path from the transmitting device 20 through the target 10 to the second antenna 24 to be determined. Then, by performing a predetermined distance adjustment based on the relationship between the positions of the transmitting device 20 and the second antenna 24, the distance from the second antenna 24 to the target 10 can be determined. In other words, the position of the target 10 can be determined.

[0046] The display unit 52 is, for example, a liquid crystal display or an organic EL display. The display unit 52 is configured to display the processing results from the signal processing unit 50.

[0047] Figure 3 is a schematic diagram showing another configuration of the passive bistatic radar system 1. As shown in Figure 3, the passive bistatic radar system 1 may further include a shielding body 60.

[0048] The shielding body 60 is made of a conductive metal. For example, the shielding body 60 can be made of wire mesh or iron plate intended for electromagnetic shielding. The shielding body 60 is placed between the transmitting device 20 and the second antenna 24. Note that the shielding body 60 is not limited to being made of a conductive metal; for example, it may be made of a material that absorbs radio waves.

[0049] By placing the shielding body 60, it is possible to prevent direct waves transmitted from the transmitting device 20 from being received by the second antenna 24. Since direct waves are prevented from mixing with the received power of the second antenna 24, it is possible to suppress a decrease in the accuracy of the measurement results of the target 10 in the signal processing unit 50.

[0050] Figure 4 is a flowchart illustrating the measurement method for target 10. To measure target 10, first, the transmitter 20 is made to transmit radio waves (S10).

[0051] For example, if the transmitting device 20 is a smartphone, it can transmit radio waves by making a call on the smartphone. Furthermore, it can transmit radio waves not only through calls, but also, for example, by performing data communication on the smartphone. While a smartphone is used as an example here, even if the transmitting device 20 is another type of public mobile station, it can transmit radio waves by performing wireless communication.

[0052] When the transmitting device 20 transmits radio waves, the first antenna 22 receives direct waves from the transmitting device 20 (S11).

[0053] Furthermore, the second antenna 24 receives the reflected wave transmitted from the transmitting device 20 and reflected off the target 10 (S12).

[0054] The signal processing unit 50 performs signal processing (S13) based on the received direct and reflected waves. This signal processing yields measurement results such as the position of the target 10. Signal processing (S13) will be described in detail later.

[0055] Once signal processing (S13) is complete and the measurement result is obtained, the signal processing unit 50 displays the measurement result on the display unit 52 (S14).

[0056] Figure 5 is a flowchart illustrating the signal processing (S13) performed by the signal processing unit 50.

[0057] The signal processing unit 50 first obtains the received power "f(t)" of the direct wave received by the first antenna 22 (S20). "f(t)" is a function of time of the received power.

[0058] Furthermore, the signal processing unit 50 obtains the received power "g(t)" of the reflected wave received by the second antenna 24 (S21). "g(t)" is a function of time of the received power.

[0059] The signal processing unit 50 performs a Fourier transform (for example, a fast Fourier transform) on the acquired received power "f(t)" of the direct wave to derive the frequency spectrum "F(f)" of the received power of the direct wave (S22).

[0060] Furthermore, the signal processing unit 50 performs a Fourier transform (for example, a fast Fourier transform) on the received power "g(t)" of the reflected wave to derive the frequency spectrum "G(f)" of the received power of the reflected wave (S23).

[0061] The signal processing unit 50 takes the complex conjugate of the frequency spectrum "F(f)" of the received power of the direct wave and obtains the complex conjugate "F" of "F(f)". * Derive (f) (S24).

[0062] Next, the signal processing unit 50 calculates the complex conjugate number "F" of the frequency spectrum of the received power of the direct wave. * Multiply "(f)" by the frequency spectrum of the received power of the reflected wave "G(f)" to get the cross spectrum "F * Derive (f)G(f) (S25).

[0063] The signal processing unit 50 performs cross-spectrum "F * We perform an inverse Fourier transform on (f)G(f) to derive the correlation function "C(t)" (S26). The correlation function "C(t)" is a function of time.

[0064] The signal processing unit 50 identifies the position of the target 10 based on the correlation function "C(t)" (S27) and terminates the signal processing (S13). The position of the target 10 may also include the distance from the second antenna 24 to the target 10 and the distance from the ground 12 to the target 10 (burial depth).

[0065] For example, the signal processing unit 50 identifies the point in time when the correlation value in the correlation function "C(t)" is maximum. The signal processing unit 50 converts the time from a reference point (e.g., 0) in the correlation function "C(t)" to the identified point in time into distance. The distance obtained in this way corresponds to the distance of the path from the transmitting device 20 through the target 10 to the second antenna 24. The signal processing unit 50 can derive the distance from the second antenna 24 to the position of the target 10 by multiplying the distance obtained in this way by a predetermined adjustment coefficient, such as 1 / 2.

[0066] The positions of the transmitter 20 and the second antenna 24 can be arbitrarily set by the user. For example, the user can pre-enter the positions of the transmitter 20 and the second antenna 24 into the radar device 26 to determine their positions. The signal processing unit 50 can determine the position of the target 10 from the position of the second antenna 24 and the derived distance from the second antenna 24 to the target 10.

[0067] Furthermore, some of the processes in the flowchart of Figure 5 may be executed in parallel. For example, the signal processing unit 50 may perform in parallel the process of acquiring the received power "f(t)" of the direct wave and deriving the frequency spectrum "F(f)" of the received power of the direct wave, and the process of acquiring the received power "g(t)" of the reflected wave and deriving the frequency spectrum "G(f)" of the received power of the emitted wave.

[0068] Figure 6 shows an example of experimental results. In the experiment, an aluminum plate was used as target 10 and was placed on the ground. In the experiment, a router was used as the transmitter 20, a loop antenna was used as the first antenna 22, and a Yagi antenna was used as the second antenna 24. In the experiment, a wire mesh shield 60 was placed between the transmitter 20 and the second antenna 24.

[0069] Figure 6(a) shows an example of the received power "f(t)" of the direct wave acquired by the signal processing unit 50 through the first antenna 22. Figure 6(b) shows an example of the received power "g(t)" of the reflected wave acquired by the signal processing unit 50 through the second antenna 24. As shown in Figures 6(a) and 6(b), the reflected wave contains information about the position of the target 10, and therefore the time variation of the received power is larger compared to the direct wave.

[0070] Figure 6(c) is an example of the frequency spectrum "F(f)" of the received power of the direct wave, obtained by performing a Fourier transform on the waveform in Figure 6(a). As shown in Figure 6(c), the frequency spectrum "F(f)" of the received power of the direct wave reflects the frequency characteristics of the radio waves transmitted from the transmitting device 20.

[0071] Figure 6(d) is an example of the frequency spectrum "G(f)" of the received power of the reflected wave, obtained by performing a Fourier transform on the waveform in Figure 6(b). As shown in Figure 6(d), the frequency spectrum "G(f)" of the received power of the reflected wave reflects the frequency characteristics including the radio waves reflected by target 10.

[0072] Figure 6(e) is an example of a correlation function "C(t)" derived based on the frequency spectrum "F(f)" in Figure 6(c) and the frequency spectrum "G(f)" in Figure 6(d). In Figure 6(e), time 0 indicates a reference time corresponding to the position of the transmitter 20. Time Tc indicates the time at which the correlation value of the correlation function "C(t)" is maximum. Time TL is the time from time 0 to time Tc.

[0073] Time TL is the time corresponding to the distance of the path from the transmitting device 20 through the target 10 to the second antenna 24. That is, the signal processing unit 50 identifies this time TL and determines the distance from the second antenna 24 to the target 10 by multiplying time TL by a predetermined conversion factor or a predetermined adjustment factor. As a result, the position of the target 10 is determined.

[0074] Figure 7 illustrates the frequencies of radio waves transmitted from the transmitter 20. In the passive bistatic radar system 1, it is preferable to use different frequencies of radio waves transmitted from the transmitter 20 depending on the type of target 10.

[0075] As shown in Figure 7, when the target 10 is at least one of an object buried underground or an underground cavity, it is preferable to set the frequency of the radio waves transmitted from the transmitting device 20 to less than 1 GHz. Here, if the frequency is 1 GHz or higher, the attenuation of radio waves underground increases significantly. Taking this into consideration, by using radio waves with a frequency of less than 1 GHz to measure the underground target 10, the attenuation of radio waves underground can be suppressed. As a result, the decrease in the measurement accuracy of the underground target 10 can be suppressed.

[0076] An example of a frequency below 1 GHz is the 703-915 MHz frequency range used in LTE. Note that the frequency of the radio waves transmitted from the transmitting device 20 is not limited to the example frequencies; it may be any frequency below 1 GHz.

[0077] Furthermore, as shown in Figure 7, if the target 10 is something that is visible above ground, such as the ground, it is preferable to set the frequency of the radio waves transmitted from the transmitting device 20 to 1 GHz or higher. Here, if the target 10 is above ground, it is not necessary to consider the attenuation of radio waves underground. Also, the higher the frequency, the higher the accuracy of signal processing in the signal processing unit 50 can be. Based on this, by using radio waves with a frequency of 1 GHz or higher to measure the target 10 on the ground, the measurement accuracy can be improved. For example, by using radio waves with a frequency of 1 GHz or higher, the surface displacement of the ground can be measured with high accuracy.

[0078] Examples of frequencies above 1 GHz include the configurations shown in Figure 7 (1) to (4). The first example (1) is the frequency range of 5470 to 5730 MHz, used in 5.6 GHz band WLANs. The second example (2) is the frequency range of 3.6 to 4.2 GHz and 4.4 to 4.9 GHz, used in 5G (fifth-generation mobile communication system). The third example (3) is the frequency range of 57 to 64 GHz, used in 60 GHz band mobile object detection sensors, etc. The fourth example (4) is the frequency range of 77 to 81 GHz, used in 79 GHz band automotive radar. Note that the frequency transmitted from the transmitting device 20 is not limited to the exemplified frequencies, but may be any frequency above 1 GHz.

[0079] As described above, the passive bistatic radar system 1 of this embodiment comprises a transmitting device 20 capable of transmitting radio waves, a first antenna 22, a second antenna 24, and a radar device 26. The first antenna 22 is capable of receiving direct waves transmitted from the transmitting device. The second antenna 24 is capable of receiving reflected waves transmitted from the transmitting device 20 and reflected by a predetermined target 10. The radar device 26 measures the target 10 based on the power of the direct waves received by the first antenna 22 and the power of the reflected waves received by the second antenna 24. Furthermore, in the passive bistatic radar system 1 of this embodiment, the transmitting device 20 is a portable public mobile station that transmits radio waves for public use.

[0080] Thus, in the passive bistatic radar system 1 of this embodiment, the radar unit 30, which consists of the first antenna 22, the second antenna 24, and the radar device 26, does not actively transmit radio waves, and the transmitting device 20 is a public mobile station, so registration as a radio station is not required. In other words, the passive bistatic radar system 1 of this embodiment omits the components that would require obtaining licenses such as radio station permits. Therefore, since there are no restrictions on radio station registration in the passive bistatic radar system 1 of this embodiment, users can easily utilize the passive bistatic radar system 1 of this embodiment.

[0081] Furthermore, since public mobile stations such as smartphones can be installed at a shorter distance from the target 10 compared to fixed stations such as cell phone base stations, the intensity of the radio waves irradiated onto the target 10 is relatively high. In the passive bistatic radar system 1 of this embodiment, since the transmitting device 20 is a public mobile station, the target 10 can be measured using radio waves with relatively high intensity. For this reason, the passive bistatic radar system 1 of this embodiment can suppress a reduction in the measurable distance and improve measurement accuracy.

[0082] Furthermore, in the passive bistatic radar system 1 of this embodiment, since the transmitting device 20 is a public mobile station, it is not limited to transmitting radio waves from near the ground 12 towards the ground. For example, in the passive bistatic radar system 1 of this embodiment, radio waves can be transmitted from a position sufficiently far from the ground 12, and the target 10 can be measured even if the radio waves are not directional. For this reason, the passive bistatic radar system 1 of this embodiment is more convenient than conventional ground-penetrating radar devices.

[0083] Furthermore, the passive bistatic radar system 1 of this embodiment can be used not only for ground-penetrating exploration but also for surface measurements such as ground surface displacement. For this reason, the passive bistatic radar system 1 of this embodiment is more versatile than conventional ground-penetrating radar devices that can only perform ground-penetrating exploration.

[0084] Furthermore, in the passive bistatic radar system 1 of this embodiment, since the transmitting device 20 is a public mobile station, radio waves at frequencies normally used by public mobile stations can be used to measure the target 10. Therefore, the passive bistatic radar system 1 of this embodiment can utilize radio waves at frequencies suitable for measuring the target 10.

[0085] Furthermore, in the passive bistatic radar system 1 of this embodiment, a public mobile station owned by the user can be used as the transmitting device 20. Therefore, the manufacturing cost of the radar unit 30 can be reduced in the passive bistatic radar system 1 of this embodiment.

[0086] Furthermore, in the passive bistatic radar system 1 of this embodiment, since the transmitting device 20 is a public mobile station, the user can carry and move the transmitting device 20 to the measurement location. For this reason, the passive bistatic radar system 1 of this embodiment can reliably transmit radio waves to the estimated position of the target 10 compared to a fixed station such as a radio tower.

[0087] Furthermore, in the passive bistatic radar system 1 of this embodiment, since the transmitting device 20 is a public mobile station, the user can freely determine the position of the transmitting device 20, that is, the radio wave transmission location. For this reason, in the passive bistatic radar system 1 of this embodiment, the radio wave transmission location can be easily determined compared to a fixed station such as a radio tower.

[0088] Therefore, according to the passive bistatic radar system 1 of this embodiment, it becomes possible to easily measure the target 10.

[0089] Figure 8 is a schematic diagram showing the configuration of a modified passive bistatic radar system 100. The passive bistatic radar system 100 is an application of the passive bistatic radar system 1 shown in Figure 1 to synthetic aperture radar (SAR).

[0090] In the passive bistatic radar system 100, the second antenna 24 is configured to receive reflected waves while shifting its receiving position, as shown by the double arrow A10 in Figure 8. For example, the second antenna 24 is configured to be movable in a direction parallel to the ground. By sequentially receiving reflected waves while shifting the receiving position, the same effect as if multiple second antennas 24 were lined up in the direction of movement of the second antenna 24 can be obtained. In other words, the same effect as receiving reflected waves with an antenna with a virtually large aperture can be obtained.

[0091] In Figure 8, the second antenna 24 is configured to be movable in its receiving position. However, for example, the entire radar unit 30, including the second antenna 24, may be configured to be movable, thereby resulting in the second antenna 24 being configured to be movable in its receiving position.

[0092] Furthermore, in the passive bistatic radar system 100, the signal processing unit 50 of the radar device 26 performs synthetic aperture processing based on the power of the reflected waves for each receiving position received by the second antenna 24.

[0093] By acquiring reflected waves while shifting the receiving position, reflected waves with different reflection paths are received at each receiving position. Therefore, distance information for the path from the transmitting device 20 through the target 10 to the second antenna 24 can be obtained for each receiving position. In the synthetic aperture processing, the distance information for each receiving position is combined to generate a distance information map in which distance information is associated with each position in a two-dimensional plane.

[0094] The signal processing unit 50 identifies the characteristics of the target 10 based on the results of the synthetic aperture processing. The characteristics of the target 10 here may include not only the position (distance) of the target 10, but also the shape of the target 10, the orientation of the target 10, the size of the target 10, etc. For example, when a distance information map is generated by the synthetic aperture processing, the distance from the second antenna 24 to the target 10 can be identified over a wide area of ​​the target 10, and as a result, the shape of the target 10 can also be identified.

[0095] Figure 9 is a flowchart illustrating the signal processing (S13) of the signal processing unit 50 in a modified passive bistatic radar system 100. In the passive bistatic radar system 100, reception by the first antenna 22 and reception by the second antenna 24 are performed for each receiving position of the second antenna 24.

[0096] The signal processing unit 50 first obtains the received power "f(t)0 to f(t)n" of the direct wave received by the first antenna 22 as the second antenna 24 moves, in sequence (S30). The 0 in f(t)0 and the n in f(t)n indicate the number of receiving positions of the second antenna 24, meaning that there are multiple receiving positions, or in other words, the number of times the first antenna 22 receives signals according to the receiving positions of the second antenna 24. That is, "f(t)0 to f(t)n" means that multiple received power values ​​of the direct wave are obtained.

[0097] Further, the signal processing unit 50 sequentially receives reflected waves while moving the second antenna 24, thereby acquiring the received powers "g(t)0 to g(t)n" of the reflected waves respectively received for each reception position by the second antenna 24 (S31). Here, the 0 in g(t)0 and the n in g(t)n indicate the number of reception positions, meaning that there are a plurality of reception positions. That is, "g(t)0 to g(t)n" means that a plurality of received powers of reflected waves are acquired.

[0098] The signal processing unit 50 performs Fourier transform (e.g., fast Fourier transform) on the acquired received powers "f(t)0 to f(t)n" of the direct wave, and derives frequency spectra "F(f)0 to F(f)n" of the received power of the direct wave (S32).

[0099] Further, the signal processing unit 50 performs Fourier transform (e.g., fast Fourier transform) on each of the acquired received powers "g(t)0 to g(t)n" of the reflected wave, and derives frequency spectra "G(f)0 to G(f)n" of the received power of the reflected wave (S33).

[0100] The signal processing unit 50 obtains the complex conjugate of the frequency spectra "F(f)0 to F(f)n" of the received power of the direct wave, and obtains the complex conjugate "F * (f)0 to F * (f)n" (S34).

[0101] Next, the signal processing unit 50 multiplies the complex conjugate "F * (f)0 to F * (f)n" of the frequency spectra of the received power of the direct wave by the frequency spectra "G(f)0 to G(f)n" of the received power of the reflected wave for each frequency spectrum of the received power of the reflected wave, and derives cross spectra "F * (f)0G(f)0 to F * (f)nG(f)n" (S35).

[0102] The signal processing unit 50 processes the cross spectrum "F * (f)0G(f)0 to F *An inverse Fourier transform is performed on each of (f)nG(f)n to derive the correlation function "C(t)0~C(t)n" for each receiving position (S36).

[0103] The signal processing unit 50 determines the distance "L0 to Ln" from the second antenna 24 to the target 10 based on the correlation function "C(t)0 to C(t)n" (S37). The distance "L0 to Ln" represents the distance for each receiving position.

[0104] The signal processing unit 50 performs a composite aperture process on the distances "L0 to Ln" to generate a distance information map in which distance information is associated with each position in the two-dimensional plane (S38).

[0105] The signal processing unit 50 identifies features such as the shape of the target 10 based on the distance information map (S39), and then terminates the signal processing (S13).

[0106] Furthermore, some of the processes in the flowchart of Figure 9 may be executed in parallel. For example, the signal processing unit 50 may perform in parallel the following processes: acquiring the received power of the direct wave "f(t)0~f(t)n" and deriving the frequency spectrum of the received power of the direct wave "F(f)0~F(f)n", and acquiring the received power of the reflected wave "g(t)0~g(t)n" and deriving the frequency spectrum of the received power of the emitted wave "G(f)0~G(f)n".

[0107] In the modified passive bistatic radar system 100, similar to the passive bistatic radar system 1 of the above embodiment, it is possible to easily measure the target 10.

[0108] Furthermore, in the modified passive bistatic radar system 100, it becomes possible to measure not only the position of the target 10, but also various characteristics of the target 10, such as the shape of the target 10.

[0109] Embodiments of the present invention have been described above with reference to the attached drawings, but it goes without saying that the present invention is not limited to these embodiments. It is clear to those skilled in the art that various modifications or alterations can be conceived within the scope of the claims, and these will naturally also fall within the technical scope of the present invention. [Explanation of Symbols]

[0110] 1.100 Passive Bistatic Radar System 10 Targets 20 Transmitter 22 Antenna No. 1 24 Second Antenna 26 Radar equipment 60 Shield

Claims

1. A transmitting device capable of transmitting radio waves, A first antenna capable of receiving direct waves transmitted from the aforementioned transmitting device, A second antenna capable of receiving reflected waves transmitted from the aforementioned transmitting device and reflected by the target to be measured, A radar device that measures at least the position of the target based on the power of the direct wave received by the first antenna and the power of the reflected wave received by the second antenna, Equipped with, The aforementioned transmitting device is a portable public mobile station that transmits radio waves at frequencies used in public wireless systems. The transmitting device is positioned within a range in which the radio waves to be transmitted can reach the estimated position of the target. The first antenna is positioned near the transmitting device. The second antenna is positioned within a range capable of receiving the reflected wave reflected at the estimated position of the target, and is positioned at a distance from the first antenna that is greater than the distance between the first antenna and the transmitting device. Passive bistatic radar system.

2. The passive bistatic radar system according to claim 1, wherein the transmitting device is one of a smartphone, mobile phone, tablet, mobile router, and laptop computer.

3. The target is at least one of an object buried underground and a cavity underground. The passive bistatic radar system according to claim 1, wherein the frequency of the radio waves transmitted from the transmitting device is less than 1 GHz.

4. The aforementioned target is the ground, The passive bistatic radar system according to claim 1, wherein the frequency of the radio waves transmitted from the transmitting device is 1 GHz or higher.

5. The passive bistatic radar system according to claim 1, wherein the radar device derives a correlation function between the power of the direct wave received by the first antenna and the power of the reflected wave received by the second antenna, and identifies the position of the target according to the derived correlation function.

6. The passive bistatic radar system according to claim 1, wherein the first antenna is located near the transmitting device and is wired to the radar device.

7. The passive bistatic radar system according to claim 1, wherein the second antenna is a cross-Yagi antenna having a first element and a second element that are orthogonal to each other, and receives the horizontal polarization and vertical polarization of the reflected wave.

8. The second antenna is configured to be able to receive the reflected wave while shifting the receiving position. The passive bistatic radar system according to claim 1, wherein the radar device performs synthetic aperture processing based on the power of the reflected waves for each receiving position received by the second antenna, and identifies the characteristics of the target based on the result of the synthetic aperture processing.

9. Equipped with additional shielding, The passive bistatic radar system according to claim 1, wherein the shielding body is disposed between the transmitting device and the second antenna.

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