JP7621643B2 - Underwater Positioning System - Google Patents

Description

This disclosure relates to an underwater positioning system.

Conventionally, underwater vehicles such as autonomous underwater vehicles (AUVs) equipped with sensors have been used in marine surveys and the like (see, for example, Non-Patent Documents 1 and 2).

Tamaki Ura et al., "Sperm Whale Tracking Observation Experiment Using Two Hydrophone Arrays," Production Research, Institute of Industrial Science, University of Tokyo, 2004, Vol. 56, No. 2, pp. 27-30 Shingo Yoshizawa, "Acoustic Positioning Technology for Small Underwater Robots," Underwater Robot Symposium, November 6, 2020 <URL: http://islab.elec.kitami-it.ac.jp/yoshizawa/slide/Underwater＿Robot ＿Symposium2020 ＿Yoshizawa.pdf >

However, in the conventional underwater vehicles, the round-trip propagation time of the acoustic signal is used to measure the distance from the reference station to the underwater vehicle, making it difficult to simultaneously determine the positions of multiple underwater vehicles.

To efficiently explore the ocean over a wide area, it is desirable to simultaneously deploy multiple underwater vehicles equipped with sensors, and to achieve high-quality exploration, the positions of these multiple underwater vehicles must be determined in real time with high accuracy. However, as mentioned above, since the round-trip propagation time of acoustic signals is used, it is necessary to determine the position of each reference station and underwater vehicle on a one-to-one basis, and there is a limit to the number of underwater vehicles that can be deployed simultaneously.

However, in a method called OWTT (One Way Travel Time) that has been attracting attention in recent years, the acoustic signal transmitters and receivers of the reference station and underwater vehicle are perfectly synchronized, and the distance from the reference station to the underwater vehicle is measured based on the time it takes for the underwater vehicle to receive an acoustic signal transmitted by the reference station at a specified timing. However, the OWTT requires that the clocks of all underwater vehicles and reference stations be synchronized with high accuracy at the level of atomic clocks, which results in a problem of extremely high costs.

The present disclosure aims to provide an underwater positioning system that can resolve the above-mentioned conventional problems and determine the position of an underwater moving object with high accuracy and low cost by using the angle of arrival of an acoustic signal transmitted from a reference station and the depth information of the underwater moving object to determine the position.

To this end, the underwater positioning system has a propulsion device and is equipped with an underwater moving body having a reference station that transmits an acoustic signal underwater, a transducer that transmits and receives the acoustic signal, a depth sensor, an attitude sensor, and a control device, in which the acoustic signal includes position information of the reference station, the transducer measures the angle of arrival of the acoustic signal, the depth sensor measures the depth of the underwater moving body, the attitude sensor measures the attitude of the underwater moving body, and the control device corrects the angle of arrival of the acoustic signal measured by the transducer based on the attitude of the underwater moving body measured by the attitude sensor. The horizontal distance from the reference station to the underwater moving body is calculated based on the depth of the underwater moving body measured by the depth sensor and the angle of arrival of the corrected acoustic signal, and the position information of the underwater moving body is calculated based on the calculated horizontal distance, the angle of arrival of the corrected acoustic signal, and the position information of the reference station received by the transducer. If the elevation/depression angle of the angle of arrival of the acoustic signal is less than a threshold value, the transducer transmits the position information of the underwater moving body to the reference station. When the reference station receives the position information of the underwater moving body, it activates the propulsion device and approaches the underwater moving body until the elevation/depression angle is equal to or greater than the threshold value.

In another underwater positioning system, the reference station further broadcasts the acoustic signal, there are multiple underwater moving bodies, and a control device for each underwater moving body calculates the position information of each underwater moving body itself.

In yet another underwater positioning system, the reference station is multiple, and the underwater vehicle's transducer receives acoustic signals emitted by the multiple reference stations.

In yet another underwater positioning system, the control device further estimates an underwater sound speed profile and corrects the angle of arrival of the acoustic signal measured by the transducer, taking into account the underwater sound speed obtained from the estimated underwater sound speed profile.

In yet another underwater positioning system, the reference station and/or the underwater moving body further includes a water temperature sensor and a salinity sensor, and the control device estimates the underwater sound speed profile using the water temperature measured by the water temperature sensor, the salinity measured by the salinity sensor, and the water pressure measured by the depth sensor.

This disclosure makes it possible to measure the position of underwater moving objects with high accuracy and low cost.

2 is a schematic diagram showing the operation of the underwater positioning system in this embodiment projected onto a vertical plane. FIG. 2 is a schematic diagram showing the operation of the underwater positioning system in the present embodiment projected onto a horizontal plane. FIG. 1 is a graph showing the relationship between the speed of sound in water and the water temperature and salinity concentration. 1 is a graph showing the relationship between the angle of the underwater sound speed relative to the layer boundary surface and the angle change due to refraction. 1 is a photograph of a demonstrator of an underwater vehicle according to the present embodiment. 1 is a photograph of a demonstrator of a reference station in the present embodiment. 1 is a photograph of a demonstrator of an underwater vehicle and a demonstrator of a reference station in the present embodiment.

The following describes the implementation form in detail.

Figure 1 is a schematic diagram of the operation of the underwater positioning system in this embodiment projected onto a vertical plane, Figure 2 is a schematic diagram of the operation of the underwater positioning system in this embodiment projected onto a horizontal plane, Figure 3 is a graph showing the relationship between the underwater sound speed and the water temperature and salinity concentration, and Figure 4 is a graph showing the relationship between the angle of the underwater sound speed relative to the layer boundary surface and the angle change due to refraction.

In the figure, 10 is an underwater vehicle included in the underwater positioning system of this embodiment, which is a device that navigates or moves underwater in a submerged state in the sea or river, and performs various investigations and observations, for example, by taking images and measuring the state of the water with a thermometer, chemical sensor, etc., but may be used for any purpose. In addition, the underwater vehicle 10 is, for example, an unmanned underwater vehicle, an unmanned exploration vehicle, an autonomous underwater robot (AUV), etc., but is not limited to these, and may be, for example, a device held by a diver swimming underwater to perform various investigations and observations, or an underwater scooter or manned submersible operated by a diver, or any other device that moves underwater. Here, for convenience of explanation, the underwater vehicle 10 will be explained as an AUV that moves autonomously in the sea.

20 is a reference station, which in this embodiment is a floating device such as a buoy or float located on the water surface 31, has a thruster 25 as a propulsion device, and is capable of navigation.

In addition to the thruster 25, the reference station 20 has a transducer 21 that transmits and receives acoustic signals underwater, a GNSS receiver 22 as a position sensor that receives signals from GNSS (Global Navigation Satellite System) satellites to measure its position, a direction sensor 23 as a direction measuring device that measures the direction of the reference station 20, and a control device 24 that controls the operation of each part of the reference station 20.

The GNSS receiver 22 is similar to position sensors such as GPS receivers used in smartphones, car navigation devices, etc., and measures and outputs the position of the reference station 20 on the XY coordinate axis, i.e., the absolute position of the reference station 20, based on the signals received from the GNSS satellites.

The transducer 21 also functions as a transmitter that broadcasts acoustic signals periodically or at any timing at a predetermined interval, and as a receiver that receives acoustic signals from the underwater moving body 10. The transducer 21 may have separate transmitter and receiver parts, but for the sake of convenience, it is described here as a single transducer. The absolute position information (position information) of the reference station 20 measured by the GNSS receiver 22 is also transmitted on the acoustic signal (broadcast signal) from the transducer 21. The part of the transducer 21 that functions as a receiver does not necessarily need to measure the angle of arrival of the received acoustic signal, so it may be composed of, for example, a single hydrophone (underwater microphone).

The orientation sensor 23 further includes a geomagnetic sensor, a MEMS gyro, a gas rate gyro, an optical fiber gyro, etc., and detects the yaw direction of the reference station 20 based on the detection results of these. Note that if the GNSS receiver 22 is capable of detecting orientation and can function as an orientation measurement device, the orientation sensor 23 can be omitted.

Furthermore, the control device 24 is a type of computer that includes an arithmetic unit such as a CPU, a storage device such as a memory, a communication device, etc., and controls the operation of the thruster 25 to move the reference station 20 to the desired position.

The underwater moving body 10 also has a transducer 11 that transmits and receives acoustic signals underwater, a depth sensor 12 that measures the depth of the underwater moving body 10 underwater (the distance in the Z-axis direction from the water surface 31), an attitude sensor 13 that measures the attitude of the underwater moving body 10 underwater, and a control device 14 that performs calculations to determine the position of the underwater moving body 10 underwater.

The depth sensor 12 measures water pressure and, based on the measured water pressure, calculates and outputs the depth of the underwater moving body 10 to which the depth sensor 12 is attached. Since the depth of the transducer 21 at the reference station 20 and the difference in depth (height) between the depth sensor 12 and the transducer 11 at the underwater moving body 10 are known, the depth difference dz between the transducer 21 and the transducer 11 can be calculated if the depth of the underwater moving body 10 is known.

The attitude sensor 13 also uses, for example, a three-axis gyro sensor, a three-axis acceleration sensor, etc. to measure and output the attitude in the yaw, roll, and pitch directions of the underwater moving body 10 to which the attitude sensor 13 is attached.

The transducer 11 has a function as a transmitter that transmits an acoustic signal to the reference station 20, a function as a receiver that receives the acoustic signal transmitted by the transducer 21, and a function to measure and output the direction from which the acoustic signal arrives (angle of arrival). The transducer 11 may have separate transmitter and receiver parts, but for the sake of convenience, the transmitter and receiver parts are described as being integrated. The receiver part is preferably a hydrophone array made up of multiple hydrophones, has three or more hydrophones, and can measure the angle of arrival of the received acoustic signal three-dimensionally. The angle of arrival of the acoustic signal is measured based on the time difference or phase difference between the acoustic signals arriving at multiple hydrophones, and the method is publicly known. Non-Patent Documents 1 and 2 explain a method for measuring the angle of arrival of the received acoustic signal two-dimensionally.

The control device 14 is a type of computer including an arithmetic device such as a CPU, a storage device such as a memory, a communication device, etc., and calculates and outputs the position of the underwater moving body 10 based on the angle of arrival of the acoustic signal output by the transducer 11, the depth difference dz output by the depth sensor 12, and the attitude output by the attitude sensor 13. As shown in Fig. 1, if the water depth of the transducer 21 of the reference station 20 is _z1 and the water depth of the transducer 11 of the underwater moving body 10 is _z2 , the depth difference dz can be expressed by the following formula (1).
dz=z ₂ -z ₁ ...Formula (1)

Specifically, the control device 14 calculates the distance from the reference station 20 to the underwater moving body 10 in the horizontal plane (X-Y plane), i.e., the horizontal distance r, using trigonometric functions based on the depth difference dz and the elevation angle θ _r of the arrival angle of the acoustic signal, calculates the X-axis component dx and the Y-axis component dy of the horizontal distance r using trigonometric functions based on the horizontal distance r, a yaw angle θ _y indicating the azimuth angle which is the direction of the underwater moving body 10 relative to a predetermined direction (e.g., north), and an azimuth angle θ _a of the arrival angle of the acoustic signal, and calculates the position of the underwater moving body 10 based on the X-axis component dx and the Y-axis component dy of the horizontal distance r, the depth difference dz, and information on the absolute position of the reference station 20.

Next, we will explain in detail how to calculate the position of the underwater moving body 10.

1, the reference station 20 is located at the water surface 31, the water depth of the transducer 21 is _z1 , and the underwater moving body 10 is located below the water surface 31 in the vertical direction (Z-axis direction) and at a water depth of the transducer 11 is _z2 . The depth sensor 12 measures and outputs the depth.

The transducer 11 also receives an acoustic signal, which is a broadcast signal transmitted from the transducer 21 of the reference station 20 periodically at a predetermined interval or at any timing. The acoustic signal also contains information on the position of the reference station 20 on the X-Y-Z coordinate axes measured by the GNSS receiver 22. The transducer 11 then measures and outputs the angle of arrival of the received acoustic signal. The attitude sensor 13 also measures and outputs the attitude of the underwater moving body 10.

Then, the control device 14 corrects the value of the angle of arrival of the acoustic signal output by the transducer 11 according to the attitude of the underwater moving body 10 measured by the attitude sensor 13, and obtains the value of the elevation/depression angle _θr as shown in Fig. 1 from the angle of arrival of the acoustic signal with respect to the underwater moving body 10. Since the depth _z1 of the transducer 21 at the reference station 20 and the difference in depth (height) between the depth sensor 12 and the transducer 11 at the underwater moving body 10 are known, the control device 14 can calculate the water depth _z2 of the transducer 11 from the output of the depth sensor 12, and calculate the depth difference dz between the transducer 21 and the transducer 11 according to the above formula (1). As is clear from Fig. 1, the horizontal distance r from the reference station 20 to the underwater moving body 10 can be expressed by the following formula (2).
r=dz/tanθ _r ...Formula (2)

Then, the control device 14 performs a calculation according to the formula (2) to obtain the value of the horizontal distance r.

Next, the control device 14 obtains a yaw angle _θy indicating an azimuth angle with respect to an arrow N indicating north as shown in Fig. 2 from the attitude of the underwater moving body 10 measured by the attitude sensor 13, corrects the value of the angle of arrival of the acoustic signal output by the transducer 11 by the attitude of the underwater moving body 10 measured by the attitude sensor 13, and obtains the value of an azimuth angle _θa as shown in Fig. 2 from the angle of arrival of the acoustic signal with respect to the underwater moving body 10. In Fig. 2, an arrow A indicates the straight direction (traveling direction) of the underwater moving body 10. As is clear from Fig. 2, the X-axis component dx and the Y-axis component dy of the horizontal distance r can be expressed by the following equations (3) and (4).
dx=-r×cos(θ _a +θ _y )...Equation (3)
dy=-r×sin(θ _a +θ _y )...Equation (4)

Then, as shown in Figure 2, if the values on the X coordinate axis and the Y coordinate axis of the reference station 20 and the underwater vehicle 10 in the horizontal plane (X-Y plane) are ( _x1 , _y1 ) and ( _x2 , _y2 ), respectively, the values on the X coordinate axis and the Y coordinate axis of the underwater vehicle 10 can be expressed by the following equations (5) and (6).
x ₂ =x ₁ -r×cos(θ _a +θ _y )...Equation (5)
y ₂ =y ₁ -r×sin(θ _a +θ _y )...Equation (6)

Then, the control device 14 performs calculations according to the formulas (5) and (6) to obtain a value _x2 on the X coordinate axis and a value _y2 on the Y coordinate axis of the underwater moving body 10. The value _z2 on the Z coordinate axis of the underwater moving body 10 is the depth.

In this way, the underwater moving body 10 can determine the horizontal distance r and azimuth angle from the reference station 20 using only the acoustic signal emitted from the reference station 20. Therefore, even if there are multiple underwater moving bodies 10, each underwater moving body 10 can determine the horizontal distance r and azimuth angle from the reference station 20 independently of each other. In other words, multiple underwater moving bodies 10 can simultaneously measure their relative positions from the reference station 20 using the acoustic signal, which is a broadcast signal emitted from a single reference station 20.

In addition, unlike the conventional underwater vehicles described in the section "Problems to be Solved by the Invention," which use the round-trip propagation time of an acoustic signal, the underwater vehicle 10 simply receives an acoustic signal emitted from the reference station 20, making it possible to shorten the positioning interval (time interval), and even if there are multiple underwater vehicles 10, positioning is possible.

In addition, information on the absolute position (position information) of the reference station 20 measured by the GNSS receiver 22 is transmitted on an acoustic signal (broadcast signal) from the transducer 21, making it possible to measure the absolute position of the underwater vehicle 10, which is expected to be extremely useful for underwater surveys, mapping, etc.

The reference station 20 does not necessarily have to be a single station, but may be multiple stations. In this case, the underwater moving body 10 can measure its relative position from multiple reference stations 20, thereby improving the measurement accuracy of its absolute position. Furthermore, if multiple reference stations 20 exist over a wide range, the underwater moving body 10 can also measure its absolute position while moving over a wide range.

Incidentally, the underwater sound speed changes depending on the water temperature, salinity, and pressure as shown in Fig. 3 (see, for example, Non-Patent Document 3). It is known as Snell's law that sound waves traveling through water are refracted due to differences in sound speed. In general, in the ocean, the water temperature changes significantly depending on the water depth, and the difference in underwater sound speed also increases. Therefore, for highly accurate acoustic positioning with small positioning error, it is desirable to obtain an underwater sound speed profile, which is the vertical distribution of underwater sound speed, and to consider the refraction of sound due to the profile.
Paul Pinet, "Oceanography: Fourth Edition," Tokai University Press, 2010, p.177

Therefore, in this embodiment, a sound speed sensor, a water temperature sensor, a salinity concentration sensor, etc. (not shown) are attached to the underwater moving body 10 and/or the reference station 20 as necessary to obtain an underwater sound speed profile. This makes it possible to take sound refraction into consideration when calculating the horizontal distance r from the reference station 20 to the underwater moving body 10, thereby reducing positioning errors and improving positioning accuracy. Specifically, the water temperature is measured by a water temperature sensor, and the underwater sound speed is obtained based on the measured water temperature using known relational expressions such as the Medwin formula and the UNESCO formula (see, for example, Section 7.3 of Non-Patent Document 4), and the sound refraction is calculated using Snell's law (see, for example, Section 3.4 of Non-Patent Document 4).
"Fundamentals and Applications of Marine Acoustics" edited by the Marine Acoustics Society, Seizando Shoten, 2004

When calculating the speed of sound in water, it is more desirable to also measure the salinity concentration, which improves accuracy. However, since salinity has a smaller effect on the speed of sound in water compared to water temperature, it can be assumed that the salinity is a constant value. Pressure (water pressure) is measured by the depth sensor 12.

In this embodiment, the underwater sound speed profile can be estimated as follows. First, the water temperature is measured by a water temperature sensor (not shown) that both the reference station 20 and the underwater moving body 10 have. The water temperature measured by the reference station 20 is transmitted on an acoustic signal. The control device 14 of the underwater moving body 10 can acquire both the water temperature measured by the reference station 20 and the water temperature measured by the underwater moving body 10 in real time, so it determines the water temperature profile by assuming that the change between the two water temperatures is linear, and further estimates the underwater sound speed profile by known relational expressions such as the Medwin formula and the UNESCO formula using the values of the salinity and pressure. In this case, the salinity is measured in the same way as the water temperature or is determined by assuming that it is a constant value, and the pressure is determined by the depth of the underwater moving body 10.

Furthermore, by measuring the strength of the acoustic signal received by the transducer 11, the accuracy of the horizontal distance r from the reference station 20 to the underwater vehicle 10 can be further improved.

In addition, since the underwater sound speed is distributed in layers and the horizontal change is very small depending almost only on the water depth, the smaller the elevation/depression angle _θr , the larger the positioning error becomes. Therefore, in this embodiment, when the elevation/depression angle _θr is less than a predetermined threshold _θmin , that is, when the condition _θr < _θmin is satisfied, the control device 14 controls the transducer 11 to transmit the position information of the underwater moving body 10 calculated as described above, that is, the value ( _x2 , _y2 , _z2 ) on the X-Y-Z coordinate axis, on an acoustic signal to the reference station 20, and when the reference station 20 receives the acoustic signal, it operates the thruster 25 to approach the underwater moving body 10 until _θr ≧ _θmin is satisfied. This makes it possible to reduce the positioning error and improve the positioning accuracy.

Specifically, the direction α in which the reference station 20 moves is an angle that satisfies the following expressions (7) and (8).
sin (α) = (y ₂ - y ₁ )/r...Equation (7)
cos(α)=(x ₂ -x ₁ )/r...Formula (8)

Here, the horizontal distance r is expressed by the following equation (9).
r={(x ₂ - x ₁ ) ² + (y ₂ - y ₁ ) ² } ^1/2 ...Formula (9)

The reference station 20 moves until the elevation/depression angle _θr when the underwater vehicle 10 measures its relative position from the reference station 20 becomes equal to or larger than the threshold value _θmin , that is, until the following formula (10) is satisfied.
r≦(z ₂ - _{z 1} )/tanθ _min ...Formula (10)

To achieve this, the reference station 20 needs to know its own orientation, so the reference station 20 measures its own orientation using an orientation sensor 23. If the GNSS receiver 22 is capable of detecting the orientation, the orientation sensor 23 can be omitted. The control device 24 performs these calculations and also controls the operation of the thrusters 25 to move the reference station 20 until the above formula (10) is satisfied.

When there are multiple underwater vehicles 10, it is possible that _θr < _θmin will be satisfied simultaneously for two or more underwater vehicles 10. In this case, it is desirable for the reference station 20 to approach the underwater vehicle 10 with the most stringent conditions (i.e., the elevation/depression angle _θr is smallest). However, this cannot be generalized because the importance of positioning may differ depending on the underwater vehicle 10, for example, when manned submersibles and unmanned exploration vehicles are mixed.

As mentioned above, the smaller the elevation/depression angle _θr , the larger the positioning error becomes. This is because the underwater sound speed is distributed in layers, and the horizontal change is very small, depending almost only on the water depth. When a sound wave enters a layer with a different sound speed, it is refracted according to Snell's law. Snell's law can be expressed by the following formula (11) according to formula 3.70 shown in section 3.4 of Non-Patent Document 4.
ν _A /cosθ=ν _B /cos(θ+Δθ)...Formula (11)

where νA and νB are the sound velocities in layers A and B, respectively, and θ is the angle of the sound wave incident from layer A to layer B with respect to the boundary surface between layers A and B, which is equal to the elevation/depression angle θr when the boundary surface is a horizontal plane. Also, Δθ is the angle change of the sound wave due to refraction.

According to the above formula (11) and the definition of the derivative of cos θ, the angle change Δθ can be approximated by the following formula (12) when the value of Δθ is small.
Δθ=(ν _A - _{ν B} )/ν _A /tanθ...Equation (12)

It can be seen from the above formula (12) that the smaller the value of θ, that is, the smaller the elevation/depression angle _θr , the larger the angle change Δθ becomes. Therefore, it can be seen that the smaller the elevation/depression angle _θr, the larger the positioning error becomes.

The sound speed distribution in the actual sea or river varies in time and space due to various factors such as water flow and rainfall, and there are also measurement errors in various sensors, so even if the underwater sound speed is measured and the elevation/depression angle _θr is corrected, an error will inevitably remain. From the above consideration, it can be seen that the influence of this error on the elevation/depression angle _θr and the horizontal distance r expressed by the above formula (2) becomes larger as the elevation/depression angle _θr becomes smaller.

Fig. 4 is a graph showing the relationship between θ and Δθ obtained from the above formula (12) when ν _A =1530 [m/s] and ν _B =1520 [m/s], with reference to Fig. 8.7 shown in Section 8.4 of Non-Patent Document 4. In Fig. 4, the horizontal axis indicates the value of θ [deg], and the vertical axis indicates the value of Δθ [deg].

4, it can be seen that Δθ increases rapidly when θ becomes equal to or less than 30 degrees, which suggests that the threshold value θ _min should be set to approximately 30 degrees.

Next, we will explain the actual underwater vehicle 10 and reference station 20 demonstration model.

Figure 5 is a photograph of a demonstration machine for an underwater vehicle in this embodiment, Figure 6 is a photograph of a demonstration machine for a reference station in this embodiment, and Figure 7 is a photograph of a demonstration machine for an underwater vehicle and a demonstration machine for a reference station in this embodiment.

The inventors of the present invention have actually constructed a demonstrator of the underwater vehicle 10 and a demonstrator of the reference station 20 as shown in Figures 5 to 7.

As shown in Figure 5, the demonstration model of the underwater vehicle 10 is an AUV equipped with a propulsion device that moves autonomously in the sea, and is equipped with a hydrophone array called SSBL (Super Short Base Line) or USBL (Ultra Short Base Line) as a transmitter/receiver 11 that receives acoustic signals. Specifically, the hydrophone array is a product called X150 USBL Transponder Beacon manufactured by Blueprint subsea <URL: https://www.blueprintsubsea.com/pages/product.php?PN=BP00795>. The hydrophone array also incorporates a sensor that functions as an attitude sensor 13. Furthermore, the demonstration model is equipped with a Blue Robotics product called Bar30 as the depth sensor 12: <https://bluerobotics.com/store/sensors-sonars-cameras/sensors/bar30-sensor-r1/>.

As shown in Figure 6, the demonstration model of the reference station 20 is a floating device like a buoy or float that floats on the water surface, but is capable of autonomous movement, and is equipped with a GNSS receiver 22, a GPS (Global Positioning System) receiver that receives signals from GPS satellites to determine position.

Then, as shown in FIG. 7, the underwater vehicle 10 demonstration model receives acoustic signals from the reference station 20 demonstration model and determines its position.

Thus, in this embodiment, the underwater positioning system has a thruster 25 and comprises a reference station 20 that transmits acoustic signals underwater, an underwater moving body 10 having a transmitter/receiver 11 that transmits and receives acoustic signals, a depth sensor 12, an attitude sensor 13, and a control device 14. The acoustic signal includes position information of the reference station 20, the transducer 11 measures the angle of arrival of the acoustic signal, the depth sensor 12 measures the depth of the underwater moving body 10, the attitude sensor 13 measures the attitude of the underwater moving body 10, the control device 14 corrects the angle of arrival of the acoustic signal measured by the transducer 11 based on the attitude of the underwater moving body 10 measured by the attitude sensor 13, calculates a horizontal distance r from the reference station 20 to the underwater moving body 10 based on the depth of the underwater moving body 10 measured by the depth sensor 12 and the corrected angle of arrival of the acoustic signal, calculates position information of the underwater moving body 10 based on the calculated horizontal distance r, the corrected angle of arrival of the acoustic signal, and the position information of the reference station 20 received by the transducer 11, and determines whether or not an elevation/depression angle _θr of the angle of arrival of the acoustic signal is greater than or equal to a threshold value θ If the elevation angle θr is less than the threshold value _θmin , the transmitter/receiver 11 transmits position information of the underwater vehicle 10 to the reference station 20, and when the reference station 20 receives the position information of the underwater vehicle 10, it activates the thrusters 25 and approaches the underwater vehicle 10 until the elevation/depression angle _θr becomes equal to or greater than the threshold value _θmin .

This allows the positioning of the underwater moving body 10 to be performed with high accuracy and at low cost. Since the relative positional relationship from the reference station 20 to the underwater moving body 10 can be accurately measured without using an expensive device such as an atomic clock, the position information of the underwater moving body 10 can be obtained accurately at low cost based on the position information of the reference station 20 and the direction from the reference station 20. Furthermore, when the elevation/depression angle _θr is less than the threshold value _θmin , the reference station 20 approaches the underwater moving body 10 until the elevation/depression angle _θr becomes equal to or greater than the threshold value _θmin , thereby reducing the positioning error and improving the positioning accuracy.

The reference station 20 broadcasts an acoustic signal, and there are multiple underwater moving bodies 10, and the control device 14 of each underwater moving body 10 calculates the position information of each underwater moving body 10 itself. Therefore, the position information of multiple underwater moving bodies 10 can be obtained simultaneously and individually.

Furthermore, the threshold value θ _min is 30 degrees, which prevents the angle change Δθ of the sound wave caused by refraction from becoming large, thereby making it possible to reduce positioning errors.

Furthermore, there are multiple reference stations 20, and the transducer 11 of the underwater moving body 10 receives acoustic signals emitted by the multiple reference stations 20. This allows the underwater moving body 10 to measure its relative position from the multiple reference stations 20, thereby improving the measurement accuracy of its own absolute position. Furthermore, if multiple reference stations 20 exist over a wide range, the underwater moving body 10 can also measure its own absolute position while moving over a wide range.

Furthermore, the control device 14 estimates the underwater sound speed profile and corrects the angle of arrival of the acoustic signal measured by the transducer 11, taking into account the underwater sound speed obtained from the estimated underwater sound speed profile. This makes it possible to take into account sound refraction when calculating the horizontal distance r from the reference station 20 to the underwater moving body 10, thereby improving positioning accuracy.

Furthermore, the reference station 20 and/or the underwater moving body 10 have a water temperature sensor and a salinity concentration sensor, not shown, and the control device 14 estimates the underwater sound speed profile using the water temperature measured by the water temperature sensor, the salinity measured by the salinity concentration sensor, and the water pressure measured by the depth sensor 12. Therefore, it is possible to improve the positioning accuracy while keeping the cost low.

As described above, according to the disclosure of this specification, the present invention can realize underwater positioning of multiple underwater moving bodies 10 without using expensive devices such as atomic clocks, and therefore contributes greatly to the realization of low-cost and large-scale underwater exploration by multiple moving bodies. Currently, simultaneous deployment technology of multiple underwater robots (especially autonomous underwater robots: AUVs) is attracting attention, and large-scale development by national projects is also underway (see, for example, Non-Patent Document 5). On the other hand, underwater exploration moving bodies that are lower in cost than conventional ones, such as "underwater drones," are being realized (see, for example, Non-Patent Document 6), and are expected to be applied to various applications such as underwater construction, fishing, resource exploration, offshore energy development, and infrastructure inspection, so the demand for the present invention that can realize low-cost multi-moving body exploration is thought to be large.
Overview of the second SIP marine project "Innovative deep sea resource exploration technology"<URL: https://www.kantei.go.jp/jp/singi/kaiyou/sanyo/dai41/shiryou5.pdf > Toshihiro Maki, Underwater Drones: Are Low-Cost AUVs Useful? Newsletter "Umi", No. 33, Oxytec, May 2017, pp. 14-18

The disclosure of this specification describes features of preferred and exemplary embodiments. Various other embodiments, modifications, and variations within the scope and spirit of the claims appended hereto will naturally occur to those skilled in the art upon review of the disclosure of this specification.

This disclosure can be applied to underwater positioning systems.

10 Underwater moving body 11 Transmitter/receiver 12 Depth sensor 13 Attitude sensor 14 Control device 20 Reference station 21 Transmitter/receiver 22 GNSS receiver 23 Orientation sensor 24 Control device 25 Thruster 31 Water surface

Claims (5)

a reference station having a propulsion device and transmitting an acoustic signal underwater;
an underwater moving body having a transducer for transmitting and receiving the acoustic signal, a depth sensor, an attitude sensor, and a control device;
An underwater positioning system comprising:
the acoustic signal includes position information of the reference station;
The transducer measures an angle of arrival of the acoustic signal;
The depth sensor measures the depth of the underwater moving object,
The attitude sensor measures an attitude of the underwater moving body,
the control device corrects the angle of arrival of the acoustic signal measured by the transducer based on the attitude of the underwater moving body measured by the attitude sensor, calculates a horizontal distance from the reference station to the underwater moving body based on the depth of the underwater moving body measured by the depth sensor and the corrected angle of arrival of the acoustic signal, calculates position information of the underwater moving body based on the calculated horizontal distance, the corrected angle of arrival of the acoustic signal, and position information of the reference station received by the transducer, and transmits the position information of the underwater moving body from the transducer to the reference station if an elevation/depression angle of the angle of arrival of the acoustic signal is less than a threshold value;
An underwater positioning system characterized in that, when the reference station receives position information of the underwater vehicle, it activates the propulsion device and approaches the underwater vehicle until the elevation/depression angle becomes equal to or greater than a threshold value. The underwater positioning system of claim 1, wherein the reference station broadcasts the acoustic signal, there are multiple underwater moving bodies, and a control device of each underwater moving body calculates the position information of each underwater moving body itself. The underwater positioning system according to claim 1 or 2, wherein the reference stations are multiple, and the underwater vehicle's transducer receives acoustic signals transmitted by the multiple reference stations. The underwater positioning system according to claim 1 or 2, wherein the control device estimates an underwater sound speed profile and corrects the angle of arrival of the acoustic signal measured by the transducer, taking into account the underwater sound speed obtained from the estimated underwater sound speed profile. The underwater positioning system of claim 4, wherein the reference station and/or the underwater moving body has a water temperature sensor and a salinity sensor, and the control device estimates the underwater sound speed profile using the water temperature measured by the water temperature sensor, the salinity measured by the salinity sensor, and the water pressure measured by the depth sensor.