Underwater lighthouse system and position measuring method employing underwater lighthouse system

Abstract

Provided is a synchronous pinger type underwater lighthouse system comprising at least three underwater lighthouses installed for use on the seabed at predetermined positions, each underwater lighthouse including a first GPS receiver that surfaces at predetermined intervals, receives a radio wave from a GNSS satellite, and outputs a GPS signal, a first synchronization signal generating unit that includes a first high-precision oscillator and generates a first synchronization signal having a predetermined period with a phase synchronized to the received GPS signal, and a transmitting unit that transmits a transmission signal having a predetermined period and comprising a PN sequence code into the water as an ultrasonic wave, in synchronization with the first synchronization signal generated by the first synchronization signal generating unit, wherein: an underwater robot having a receiving unit that receives the transmission signal from the underwater lighthouse includes a second GPS receiver that surfaces at predetermined intervals, receives a radio wave from a GNSS satellite, and outputs a GPS signal, a second synchronization signal generating unit that includes a second high-precision oscillator and generates a second synchronization signal having the same period as the first synchronization signal with a phase synchronized to the received GPS signal, and a correlator for identifying the transmission signal transmitted from the underwater lighthouse; and the underwater robot receives the transmission signal from the underwater lighthouse, obtains an arrival time from the second synchronization signal generated by the second synchronization signal generating unit and the reception time of the transmission signal, obtains the distance to the identified underwater lighthouse from the arrival time and the underwater speed of sound, and performs position measurement on the basis of the distance.

Description

Underwater lighthouse system and position measurement method using the underwater lighthouse system

[0001] The present invention relates to an underwater lighthouse system for underwater positioning and a position measurement method using the underwater lighthouse system.

[0002] It is also called an underwater lighthouse or an acoustic lighthouse, and as the name suggests, it is a concept that refers to the role of a lighthouse underwater. As is well known, light can only reach a few meters underwater, so even if a lighthouse on land is brought underwater, it cannot function as a lighthouse on land. Only sound waves can travel long distances underwater, so sound waves are the only way that a lighthouse can function underwater.

[0003] In the near future, underwater lighthouses will become an essential underwater infrastructure for autonomous underwater robots (hereinafter referred to as AUVs (Autonomous Underwater Vehicles)) to perform autonomous navigation. Underwater positioning is an important technology not only for AUVs, but also for remotely operated robots (hereinafter referred to as ROVs (Remotely Operated Vehicles)) and manned submersibles (hereinafter referred to as HOVs (Human Occupied Vehicles)). When we simply refer to an underwater robot, we mean either an AUV or an ROV.

[0004] Figure 1 is a conceptual diagram of an underwater lighthouse, which emits acoustic waves at regular intervals and allows an AUV to determine its own position by receiving these acoustic waves. To do this, the acoustic waves emitted by the underwater lighthouse must be synchronized. The present invention relates to the synchronization of underwater lighthouses.

[0005] One conventional underwater position measurement method is the LBL (Long Base Line) positioning method (transponder method), as shown in Figure 2. In this method, an underwater robot, such as an ROV, emits a sound wave, which is received by a device called a transponder, which returns a sound wave in response, and the distance is measured from the round-trip time. At least three transponders are installed at predetermined positions (coordinates) P1, P2, and P3, and the three distances between these transponders and the underwater robot are measured. The position of the underwater robot can be calculated by solving the following equation (see, for example, Patent Document 1):

[0006]

[0007] where x ROV , y ROV , z ROV are the xyz coordinates of the underwater robot's position, P1 (x1, y1, z1), P2 (x2, y2, z2), and P3 (x3, y3, z3) are the position coordinates of the three transponders, and r1, r2, and r3 are the distances between the underwater robot and the three transponders, respectively.

[0008] Japanese Patent Application Laid-Open No. 2004-138589

[0009] Another example of a conventional underwater position detection method, as shown in FIG. 3 , involves using floating buoys B1, B2, and B3 on the water surface to receive radio waves from the Global Positioning System (GPS) and link the underwater position to the latitude and longitude on land. In this case, the positions of the buoys B1, B2, and B3 on the water surface change depending on meteorological and hydrographic conditions. Therefore, in order to accurately measure the underwater position of the AUV, the position information of the buoys B1, B2, and B3 must be communicated to the AUV by some means. Note that although the term GPS is used in the following description, satellite positioning systems other than GPS can also be used. In other words, the present invention can use a Global Navigation Satellite System (GNSS), including the Quasi-Zenith Satellite System (QZSS), which is a general term for satellite positioning systems other than GPS.

[0010] The LBL positioning method shown in Figure 2 requires that the positions of multiple transponders be measured in some way when they are first submerged. In shallow waters, GPS is often used, with transponders suspended almost directly below the water. Furthermore, since each transponder responds to a call signal from an underwater robot during operation, the greater the number of underwater robots, the greater the risk that a transponder may not be able to return a response signal due to interference or overlap. Furthermore, the underwater robot may receive so many response signals that it becomes difficult to distinguish which response signal is a response to its own call, resulting in the inability of many robots to detect their positions simultaneously.

[0011] In the case of Figure 3, GPS signals can be used, so ultrasonic waves can be transmitted from the buoy at the same time as the precise 1-second pulse (hereinafter referred to as the 1 pps signal) formed by receiving signals from GPS. This allows the AUV to use the synchronous pinger method by occasionally surfacing to the sea surface to receive GPS signals, allowing the AUV to measure its position using only reception.

[0012] GNSS satellites such as GPS are equipped with ultra-high performance atomic oscillators, the accuracy of which is controlled and maintained by ground stations, and they provide highly accurate and precise time information. The reference oscillator receives GPS signals, controls the oscillator based on the precise time determined by GPS positioning, and outputs a highly accurate and precise 1 pps signal and reference frequency.

[0013] The synchronous pinger method is a method that can measure the distance between yourself and a buoy using only the received signal. For example, if buoys B1, B2, B3, and B4 in Figure 3 are arranged in a square, you can detect that you are in the center of the square if you assume that you receive the transmitted signals from all four buoys at the same time. This method can measure the distance between you and a buoy by measuring your own reception time based on the GPS 1 pps signal.

[0014] Figure 4 is a timing chart to explain AUV position measurement using the synchronous pinger method, showing an example of an AUV's received signal synchronized with a GPS 1 pps signal. For example, if three sound waves with different codes are transmitted at the same time from three buoys on the sea, the AUV will receive the GPS 1 pps signal and synchronize itself with the time transmitted by the buoys. This will allow the times when the signals from the three buoys are received, i.e., the times t1, t2, and t3 from transmission to reception, to be accurately measured. The respective distances r1, r2, and r3 can be calculated using the following formulas.

[0015]

[0016] Here, c is the speed of sound in water. Once r1, r2, and r3 can be calculated, the rest is the same as the LBL position detection method.

[0017] The disadvantage of this method is that, as mentioned earlier, the buoy moves, making it difficult to transmit that information to the AUV. Other disadvantages include the risk of the buoy being washed away due to weather conditions, and the risk of collision with a moving vessel or theft of the buoy.

[0018] The problems that need to be solved with conventional underwater position measurement methods are as follows: (1) With the transponder method, it is difficult to measure the positions of multiple underwater robots simultaneously. (2) With the marine buoy method, GPS can be received, but the position of the buoy itself moves, so this position information must be communicated to the underwater robot. (3) Marine buoys are at risk of collision with ships and theft. (4) With marine buoys, there is concern that under severe sea conditions, such as bubble breakdown, can have a negative impact on underwater ultrasonic wave propagation.

[0019] The present invention aims to provide an underwater lighthouse system and a position measurement method using an underwater lighthouse system that can overcome the drawbacks of conventional underwater position measurement and serve as an underwater lighthouse for future autonomous navigation of underwater robots.

[0020] The present invention provides a synchronous pinger type underwater lighthouse system having at least three underwater lighthouses that are installed on the bottom of the water at predetermined locations, each of which comprises: a first GPS receiver that surfaces at set times, receives radio waves from GNSS satellites, and outputs GPS signals; a first synchronization signal generation unit that has a first high-precision oscillator and generates a first synchronization signal of a predetermined period in phase synchronized with the received GPS signal; and a transmission unit that synchronizes with the first synchronization signal generated by the first synchronization signal generation unit and emits a transmission signal of a predetermined period consisting of a PN series code as ultrasonic waves into the water; and an underwater robot that has a reception unit that receives the transmission signal from the underwater lighthouse comprises: a second GPS receiver that surfaces at set times, receives radio waves from GNSS satellites, and outputs GPS signals; a second synchronization signal generation unit that has a second high-precision oscillator and generates a second synchronization signal of the same period as the first synchronization signal in phase synchronized with the received GPS signal; and a correlator for identifying the transmission signal transmitted from the underwater lighthouse. The underwater robot is an underwater lighthouse system that receives a transmission signal from an underwater lighthouse, calculates the arrival time from the second synchronization signal generated by the second synchronization signal generation unit and the reception time of the transmission signal, calculates the distance between the underwater lighthouse identified from the arrival time and the underwater sound speed, and measures its position based on the distance.

[0021] According to the present invention, it is possible to solve the problems of the transponder method, such as being able to measure underwater positions even when multiple underwater robots are present in the same area. The effects described here are not necessarily limited to those described herein, and may be any of the effects described in this specification.

[0022] FIG. 1 is a schematic diagram illustrating the concept of an underwater lighthouse. FIG. 2 is a schematic diagram illustrating a conventional underwater position measurement method, the LBL positioning method. FIG. 3 is a schematic diagram illustrating another conventional underwater position detection method. FIG. 4 is a timing chart illustrating AUV position measurement using the synchronous pinger method. FIGS. 5A and 5B are schematic diagrams of an underwater lighthouse according to an embodiment of the present invention. FIG. 6 is a schematic diagram illustrating temporal changes in the surfacing operation of a GPS receiver 2. FIG. 7 is a schematic diagram illustrating an example of an AUV. FIG. 8 is a block diagram of a partial configuration of a circuit included in a signal processing unit according to an embodiment of the present invention. FIG. 9 is a timing chart illustrating synchronization signal generation and the synchronous pinger method. FIGS. 10A and 10B show plan and side views of an example of an underwater lighthouse installation. FIGS. 11A and 11B illustrate a case in which multiple AUVs are in the same sea area. FIG. 12 is a block diagram of a receiving device installed on an AUV for performing positioning using the synchronous pinger method. FIG. 13 is a graph showing how changes in water temperature affect the underwater sound speed. FIG. 14 is a graph showing seasonal changes in water temperature near the surface. FIGS. 15A and 15B are plan and side views of a system according to another embodiment of the present invention. FIG. 16 is a timing chart showing processing by an offshore underwater lighthouse. FIG. 17 is a circuit block diagram of the underwater lighthouse. FIGS. 18A and 18B are plan and side views showing the concept of a further embodiment of the present invention relating to underwater sound speed measurement between underwater lighthouses. FIG. 19 is a timing chart used to explain a method for measuring underwater sound speed between underwater lighthouses. FIG. 20 is a timing chart used to explain a method for transmitting underwater sound speed between underwater lighthouses. FIGS. 21A and 21B are plan and side views used to explain processing for determining an accurate distance using the underwater sound speed transmitted from an underwater lighthouse. FIG. 22 is a flowchart showing the processing flow performed by an offshore underwater lighthouse. FIG. 23 is a flowchart showing the processing flow performed by an AUV. 24A and 24B are plan and side views for explaining the process of generating a synchronization signal by sequentially transmitting a synchronization signal from a reference lighthouse, and Fig. 25 is a timing chart for explaining the process of generating a synchronization signal by sequentially transmitting a synchronization signal from a reference lighthouse.Figure 26 is a timing chart for explaining a method for receiving AUV synchronization signals and adjusting them to synchronize timing. Figure 27 is a circuit block diagram inside an underwater lighthouse without a GPS receiver. Figures 28A and 28B are plan and side views of an image of an AUV test site.

[0023] Hereinafter, embodiments of the present invention will be described. Note that the embodiments described below are preferred specific examples of the present invention, and various technically preferable limitations are attached, but the scope of the present invention is not limited to these embodiments unless otherwise stated in the following description to the effect that the present invention is limited.

[0024] 5A and 5B are schematic diagrams of an underwater lighthouse 1 according to one embodiment of the present invention. This underwater lighthouse 1 is installed on the seabed and has a buoyant GPS receiver 2 mounted on its top portion. Since it is normally underwater, it cannot receive GPS signals. As shown in FIG. 5B, the GPS receiver 2 surfaces periodically, for example, once a day, to receive GPS signals at sea. When it surfaces, the GPS receiver 2 is connected to the main body 3 by a cable 7.

[0025] A GNSS antenna is connected to the GPS receiver 2. The GNSS antenna receives positioning signals from GNSS satellites and outputs a 1 pps signal (referred to as a GPS signal) formed from this positioning signal via a cable to the main body 3. The GPS signal is transmitted to a signal processing unit 4, including a reference signal generator and a positioning calculation unit, built into the main body 3 of the underwater lighthouse.

[0026] The reference signal generator includes a high-precision oscillator, such as a temperature-compensated crystal oscillator, and generates a reference signal based on a GPS signal. The main body 3 also houses a mechanism for raising and lowering the GPS receiver 2 and a battery unit 5. A fixing unit 6 is provided at the base of the main body 3. GNSS satellites include GPS satellites, QZSS satellites, and GLONASS satellites.

[0027] Figure 6 shows the time course of the GPS receiver 2 rising. When the locked state is released, the GPS receiver 2, connected to the main body 3 of the underwater lighthouse 1 by a cable 7, rises and surfaces. Once it surfaces, it receives GPS signals, acquires the necessary data, and transmits the GPS signals (1 pps signals) to the signal processing unit of the main body 3. After a certain period of time (for example, about one minute) has elapsed and the necessary data has been acquired, the winding motor in the main body 3 operates and the GPS receiver 2 is submerged again.

[0028] As shown in Figure 7, the AUV has a similar structure to a conventional one, but with the addition of a GPS receiver 8 and an acoustic wave receiving unit 9. Like an underwater lighthouse, the AUV periodically surfaces to receive radio waves to generate a reference signal, and then generates a synchronization signal synchronized with the synchronization signal generated by the underwater lighthouse. The same applies to ROVs and HOVs. Furthermore, the present invention can also be applied to detecting the position of divers.

[0029] The underwater lighthouse according to this embodiment solves problems (2), (3), and (4) that were issues with conventional underwater position measurement. To solve problem (1), this underwater lighthouse method, like the GPS reception method on land, requires the object to be measured (an underwater robot, such as an AUV) to only have a receiving function. The underwater lighthouse emits an ultrasonic transmission signal synchronized with the GPS 1 pps signal, and the underwater object, such as an AUV, simply receives the ultrasonic signal from the underwater lighthouse to measure its position. The positioning method uses the synchronous pinger method shown in Figure 4. Because it uses the synchronous pinger method, the AUV's position is only measured by receiving, which also solves problem (1) that was an issue with conventional positioning methods.

[0030] 8 is a block diagram of a portion of the circuitry included in the signal processing unit 4 of one embodiment of the present invention. Power, for example +5 V, is supplied to the GPS receiver 2 via a cable 7. Power from a battery unit 5 is supplied to each unit and also to the GPS receiver 2 via a slip ring 11. A winding motor 12 is provided to wind up the cable 7. The winding motor 12 is operated periodically by a timer 13. The GPS receiver 2 rises to the surface at regular intervals to receive GPS signals. It has the ability to wind up the surfaced portion at its own discretion once it has completed receiving the GPS signal.

[0031] A GPS signal 14 is supplied from the GPS receiver 2 via a cable 7 to a GPS signal receiving section and a synchronization signal generating section 15. A high-precision oscillator 16 is connected to the GPS signal receiving section and a synchronization signal generating section 15. The high-precision oscillator 16 generates a synchronization signal that is synchronized in phase with and has the same frequency as the 1 pps signal of the GPS signal 14. The synchronization signal is supplied to a timer 13 and a synchronization transmitting circuit 17.

[0032] A Gold code is supplied to the synchronous transmission circuit 17 from a Gold code signal generation circuit 18. The Gold code signal generation circuit 18 generates a Gold code in synchronization with the transmission trigger pulse. The Gold code is a different code for each underwater lighthouse in order to identify the underwater lighthouse. A PN (Pseudorandom Noise) sequence other than the Gold code, such as an M sequence, may also be used.

[0033] In the synchronous transmission circuit 17, the Gold code is digitally modulated, for example, by BPSK (Binary Phase Shift Keying), to form a transmission signal. The carrier frequency is a single frequency in the range of several tens to several hundred kHz. The transmission signal is one or more pulse trains. If there are, for example, 32 types of Gold codes, and two pulses are sent from one underwater lighthouse, 32 x 32 = 1024 different underwater lighthouses can be identified by combining the two codes. In this way, a transmission signal synchronized with the synchronization signal is transmitted.

[0034] The output signal of the synchronous transmission circuit 17 is supplied to a transmission amplifier 19, where it is amplified and processed. The output signal of the transmission amplifier 19 is supplied to an ultrasonic wave transmitter 20. The transmitter 20 transmits underwater ultrasonic waves (transmission signals).

[0035] When the GPS receiver 2 is above the water surface, it can receive GPS signals and generate a synchronization signal synchronized with the GPS signal. When the GPS receiver 2 is submerged, it continues to generate a synchronization signal with the precision of the high-precision oscillator 16. In recent years, inexpensive high-precision oscillators 16 with a frequency stability of, for example, about + / - 0.005 ppm have become available.

[0036] If the accuracy is about 0.01 ppm, the deviation over 24 hours is only 0.864 ms (= 0.01 ppm x 3600 s x 24 hours). This equates to an error of about 1.296 m, assuming the speed of sound in water is 1500 m / s. Therefore, by correcting the time by receiving GPS signals about once a day, it is possible to adopt the highly accurate synchronous pinger method as the positioning method. If the synchronous pinger method is used, there is no limit to the number of AUVs that can be used.

[0037] Figure 9 is a timing chart for explaining the synchronization signal generation and synchronization pinger method. The GPS receiver 2 surfaces periodically, for example, once a day, to receive radio waves from satellites and generate a 1 pps signal. The high-precision oscillator 16 generates a synchronization signal synchronized with the 1 pps signal, for example, with a 10-second period. All underwater lighthouses can generate synchronization signals with a negligible deviation. Then, an ultrasonic transmission signal synchronized with the synchronization signal is sent out.

[0038] Figure 9 shows a case where an AUV receives ultrasonic transmission signals from three underwater lighthouses. It shows how, after receiving a GPS signal, the internal high-precision oscillator generates a synchronization signal after the GPS signal is lost, and then the signal from the underwater lighthouse is received. The time accuracy of a GPS 1 pps signal is approximately 10^-12, so the accuracy of a 10-second synchronization pulse is expressed as 10.00000000000 (12 zeros). ^ indicates exponentiation. Meanwhile, the accuracy of the internal clock after the GPS signal is lost is, for example, 0.01 ppm (10^-8), so it is expressed as 10.0000000 (8 zeros).

[0039] Like the underwater lighthouses, the AUVs are equipped with GPS receivers that periodically surface to receive radio waves and generate synchronization signals synchronized with the 1 pps signal. Therefore, the AUV knows the time each underwater lighthouse transmitted ultrasound and can accurately measure the times t1, t2, and t3 from that time to reception, allowing it to calculate the distance from each underwater lighthouse.

[0040] Figures 10A and 10B show a plan view and a side view of an example of underwater lighthouse installation. When installing a GPS-synchronized underwater lighthouse, positioning is performed using GPS, and the underwater installation is performed with the highest possible accuracy. The installation distance between underwater lighthouses is set to approximately 10 km. This is because, when 10 kHz is selected as the frequency of the ultrasonic synchronization signal, this distance can be ensured as a transmission distance. Assuming there are short and long sides, the short side can be set at a distance that allows reception by the AUV, and the long side can be set to, for example, approximately 20 km. In this case, another underwater lighthouse can be placed near the center.

[0041] 11A and 11B are diagrams assuming a case where multiple AUVs are in the same sea area. In this case, with the synchronized pinger method, each AUV simply receives ultrasonic waves, so even if many AUVs are present at the same time, there is no problem with position measurement.

[0042] 12 is a block diagram of a receiver mounted on an AUV for performing synchronous pinger positioning. The AUV surfaces about once a day to receive GPS signals and corrects its internal clock. The GPS receiver 8 receives radio waves from GNSS satellites and generates a GPS signal (1 pps signal) 31, which is supplied to a pinger signal reception time calculation and positioning calculation circuit 32.

[0043] A high-precision oscillator 33 is connected to the pinger signal reception time calculation and positioning calculation circuit 32. Using the output signal of the high-precision oscillator 33, a synchronization signal having a phase synchronized with and the same frequency as the 1 pps signal of the GPS signal 31 is generated.

[0044] Ultrasonic waves are received by the ultrasonic receiver 9, and the received signal is supplied to a correlator 35 via a receiving amplifier 34. The correlator 35 performs correlation detection using a Gold code to identify which underwater lighthouse transmitted the ultrasonic wave that corresponds to the received signal. The output signal from the correlator 35 is supplied to a pinger signal reception time calculation and positioning calculation circuit 32, and is used to measure the time from the transmission time to the reception time of each underwater lighthouse. The position of the AUV is detected based on this time.

[0045] The next important thing is to calculate the distance between the underwater lighthouse and oneself from the time received by the AUV, but as shown in the above equations (4), (5), and (6), when calculating the distance from the time difference, it is multiplied by the speed of sound in water, c, so it is necessary to accurately calculate this speed of sound in water, c. For example, Mackenzie's equation (7) can be used to calculate the speed of sound in water.

[0046]

[0047] where D is depth (m), S is salinity concentration (‰), and T is water temperature (°C). That is, to accurately calculate the speed of sound, D, S, and T must be known accurately. The AUV is equipped with a pressure sensor 36, a water temperature sensor 37, and a salinity sensor 38 to measure these parameters. The detection signals of these sensors are supplied to a sound speed calculation circuit 39, which calculates the underwater sound speed at that time using equation (7). The calculated underwater sound speed data is supplied to the pinger signal reception time calculation and positioning calculation circuit 32.

[0048] Figure 13 is a graph showing how changes in water temperature affect the speed of sound in water. It shows that for a sound speed of 1500 m / s, a change in water temperature of 5°C results in a change of -1.95%, and a change in water temperature of 25°C results in a change of +2.3%. Note that D = 10 m and S = 35‰ are fixed. We can see that a change of 20°C from 5°C to 25°C results in a change of (1.95 + 2.30) = 4.25%. Therefore, the change per 1°C is 4.25 / 20 = 0.2125% / °C. If water temperature is measured with an accuracy of 0.1°C, the error in the speed of sound can be kept to 0.02125%.

[0049] The water temperature near the surface varies by nearly 15°C between summer and winter, as shown in Figure 14. Although the water temperature change is thought to be smaller in the ocean where the AUV is cruising, sound speed correction is performed using the water temperature sensor 37 to improve the accuracy of position measurement.

[0050] Next, another embodiment of the present invention will be described. In one embodiment, a floating GPS receiver is used as a method for realizing a synchronous pinger-type underwater lighthouse. In another embodiment, a synchronous pinger-type underwater lighthouse is realized without using a floating GPS receiver. This is intended for use in situations where a floating GPS receiver cannot be installed deep offshore, or where an underwater lighthouse is installed near enemy territory, where there is a risk of it being detected and retrieved by an enemy country when it surfaces, even if only once a day.

[0051] 15A and 15B are schematic diagrams showing a plan view and a side view of a system according to another embodiment of the present invention. One lighthouse is installed relatively close to land. This lighthouse is referred to as the reference underwater lighthouse 51. Furthermore, multiple underwater lighthouses are installed offshore. These underwater lighthouses are referred to as offshore underwater lighthouses 52a, 52b, 52c, ... (hereinafter, when there is no need to distinguish between individual offshore underwater lighthouses, they will simply be referred to as offshore underwater lighthouses 52). The reference underwater lighthouse 51 refers to the underwater lighthouse that is the initial starting point when a synchronization signal is transmitted sequentially to the offshore underwater lighthouses. The reference underwater lighthouse 51 and the offshore underwater lighthouse 52 are used for position measurement using the synchronous pinger method. That is, an underwater robot receives ultrasonic transmission signals from these underwater lighthouses to measure its position.

[0052] The distance R1 between the reference underwater lighthouse 51 and the offshore underwater lighthouse 52a, the distance R2 between the offshore underwater lighthouse 52a and the offshore underwater lighthouse 52b, the distance R3 between the offshore underwater lighthouse 52b and the offshore underwater lighthouse 52c, and the distance R4 between the offshore underwater lighthouse 52c and the offshore underwater lighthouse 52d are determined by accurate measurements when the reference underwater lighthouse 51 and the offshore underwater lighthouse 52 are installed. The reference underwater lighthouse 51 is connected to a GPS receiver 50 on land via a cable 53.

[0053] The GPS receiver 50 receives radio waves from GNSS satellites and generates a GPS signal (1 pps signal). The reference underwater lighthouse 51 receives the GPS signal continuously or periodically via a cable 53. However, in other embodiments of the present invention, the reference underwater lighthouse 51 may have the same configuration as in the first embodiment and may have a floating buoy-type GPS receiving function. In this case, there is no need to connect it to the land-based GPS receiver 50 via a cable. On the other hand, the offshore underwater lighthouse 52 does not need to be equipped with a floating GPS receiver as in the first embodiment. However, although it may be equipped with a GPS receiver, it does not need to be operated.

[0054] The reference underwater lighthouse 51 generates a synchronization signal, for example, at a 10-second cycle, from the GPS signal. The GPS receiver 50 may have the function of generating a synchronization signal and transmit the generated synchronization signal to the reference underwater lighthouse. The reference underwater lighthouse 51 transmits a synchronous ultrasonic signal synchronized with the GPS signal, i.e., a transmission signal U1, at a 10-second cycle, for example.

[0055] This transmission signal U1 is, for example, a 10 kHz ultrasonic wave modulated by the Gold code, and is synchronized with the 1 pps signal of the constantly connected GPS receiver 50. The reason for using a Gold code modulated signal is that, as in the first embodiment, multiple types of transmission signals can be generated, and by assigning a signal with a different code to each underwater lighthouse, it is possible to identify which lighthouse the signal received by the AUV is from.

[0056] The offshore underwater lighthouse 52a that receives the transmission signal U1 from the reference underwater lighthouse 51 calculates the arrival time t1 = R1 / c using the distance R1 to the reference underwater lighthouse that was measured in advance and the underwater sound speed c. For example, if R1 = 10 km and c = 1500 m / s, then t = 6.6666 sec, and the timing of transmission of the transmission signal U2 that the offshore underwater lighthouse 52a will emit is calculated.

[0057] 16 is a timing chart showing the above-mentioned processing of the offshore underwater lighthouse 52a. The reference underwater lighthouse 51 generates a synchronization signal with a 10-second period synchronized with the GPS signal (1 pps signal), and transmits a transmission signal U1 synchronized with this synchronization signal.

[0058] The offshore underwater lighthouse 52a receives the transmitted signal U1 after a time equal to the distance R1 divided by the underwater sound speed c. In order to synchronize the timing with the reference underwater lighthouse 51 with a transmission period of 10 seconds, the time t is calculated by subtracting 6.6666 seconds from 10.0 seconds. delay = 3.3333 seconds later, the offshore underwater lighthouse 52a emits its own transmission signal U2. In this way, the offshore underwater lighthouse 52a can send out a transmission signal U2 synchronized with the transmission signal U1 of the reference underwater lighthouse 51.

[0059] Do this calculation: delay A high-precision oscillator is not required to count the time. This is because this operation is repeated every 10 seconds, which is an extremely short calculation time compared to correction using a surface-mounted GPS signal, which is performed approximately once a day, so there is no significant error. Furthermore, the offshore underwater lighthouse 52b receives the transmission signal U2 from the offshore underwater lighthouse 52a and performs the same operation, so that the offshore underwater lighthouse 52b can also transmit a transmission signal U3 synchronized with the reference underwater lighthouse 51. In this way, transmission signals synchronized with the reference underwater lighthouse 51 can be transmitted from the offshore underwater lighthouse 52b onwards.

[0060] The reference underwater lighthouse 51 has the same configuration as the underwater lighthouse of the above-mentioned embodiment except that it does not have a GPS receiver. Figure 17 is a block diagram showing the configuration of the offshore underwater lighthouse 52. A signal received by an ultrasonic receiver 61 is amplified by a receiving amplifier 62 and then supplied to a Gold code signal correlator 63. Only the desired transmission signal (e.g., transmission signal U1 from the reference underwater lighthouse 51) is extracted through the correlator 63.

[0061] The transmission signal extracted by the correlator 63 is supplied to a calculation circuit 64. The calculation circuit 64 calculates the arrival time t=R1 / c and calculates t to determine how many seconds after which it should transmit. delay A signal from a high-precision oscillator 65 is supplied to the calculation circuit 64. Calibrated underwater sound speed data is supplied to the calculation circuit 64 from a sound speed calculation circuit 66. Detection signals are supplied to the sound speed calculation circuit 66 from a pressure sensor 67, a water temperature sensor 68, and a salinity concentration sensor 69 to accurately calculate the underwater sound speed.

[0062] The time t calculated by the calculation circuit 64 delayis supplied to a Gold code signal generating circuit 70 for forming a transmission signal. The Gold code signal generating circuit 70 generates a Gold code signal assigned to itself. The Gold code signal is supplied to a transmission signal generating circuit 71, which generates a transmission signal, for example, by phase modulation. In the case of the offshore underwater lighthouse 52a, a transmission signal U2 is generated. The transmission signal is amplified by a transmission amplifier 72 and supplied to a wave transmitter 73. The wave transmitter 73 emits the transmission signal into the water as ultrasonic waves.

[0063] In this other embodiment of the present invention, the AUV position measurement method is a synchronous pinger method, and therefore has the same configuration as the first embodiment (see FIG. 12).

[0064] As mentioned above, an important factor in this system is the speed of sound in water. The speed of sound in water can be calculated using the Mackenzie formula or the like by measuring the water pressure, water temperature, and salinity. In a further embodiment of the underwater lighthouse system, the speed of sound in water can be calculated in the following way.

[0065] 18A and 18B show a plan view and a side view illustrating the concept of measuring the underwater sound speed between underwater lighthouses 81 and 82. The underwater lighthouses 81 and 82 may be either reference underwater lighthouses or offshore underwater lighthouses. If the water temperatures are different at the locations of the underwater lighthouses 81 and 82, the underwater sound speed estimated based on the water temperature near the underwater lighthouse 81 will differ from the underwater sound speed estimated based on the water temperature near the underwater lighthouse 82. Meanwhile, the distance R12 between the underwater lighthouses is accurately measured in advance using GPS, optical measurement methods, or the like when the underwater lighthouses are installed.

[0066] The propagation times t1 and t2 of the transmitted signals of each sound wave emitted from each underwater lighthouse should be the same because the paths are exactly the same. The underwater sound velocities c1 and c2 measured from the propagation times are compared with the underwater sound velocities c1 (D, T, S) and c2 (D, T, S) calculated from the measurements of the sound velocity calculation sensors of each underwater lighthouse. If there is a difference between these values ​​that exceeds a certain level, it is best to adopt the sound velocity data measured from the propagation time as the true value. This is because the sound velocity calculation by the sensor does not reflect the average water temperature or depth of the propagation path, but the value calculated from the propagation time reflects the average water temperature and depth information of the propagation path.

[0067] Figure 19 is a timing chart illustrating a method for measuring the underwater sound speed from the propagation time shown in Figure 18. The underwater lighthouses 81 and 82 transmit transmission signals U11 and U12 at 10-second intervals at synchronized timing. The underwater lighthouses 81 and 82 receive the transmission signals from the other side. The times t1 and t2 until reception are calculated by dividing the distance R12 by the underwater sound speed.

[0068] Once the most accurate calculated value of the speed of sound between the underwater lighthouses has been measured, the method for transmitting this data to the AUV is as follows. The underwater lighthouse transmits two pulses as a synchronization signal, which is also used for self-identification. Therefore, if the interval between the two transmitted pulses is set to an interval proportional to the underwater speed of sound obtained through mutual communication between the two underwater lighthouses, the AUV can obtain the most likely accurate sound speed data between the underwater lighthouses by decoding the interval between the two pulses in the received signal. As a result, the accuracy of its own position measurement is improved.

[0069] Next, referring to Figure 20, we will explain how to communicate the underwater sound speed to the AUV using the interval between two pulses. The underwater sound speed obtained and calculated using the method described above is converted into the interval between two pulses and transmitted from the underwater lighthouse. The AUV receives this sound wave and calculates the distance between the underwater lighthouse and itself using the underwater sound speed that is most likely to be correct based on the positional relationship between the underwater lighthouse and itself. This makes it possible to improve the accuracy of its own position measurement. If the interval between the two pulses is T velocity If the relationship between the signal and the underwater sound speed c (m / s) is, for example, as shown in equation (8), the underwater sound speed can be converted into a pulse interval for transmission.

[0070] T velocity = c / 10 (msec) (8)

[0071] On the receiving side, the interval between the two pulses is measured, and by multiplying the result by 10, the most accurate underwater sound speed can be obtained every 10 seconds. Using this underwater sound speed to calculate the position of the AUV, a position with minimal error can be determined. Figures 21A and 21B are plan and side views showing the concept of position measurement by an AUV using sound speed data c1, c2, c3, and c4 sent from underwater lighthouses 81a, 81b, 81c, and 81d.

[0072] 22 is a flowchart showing the flow of processing until an offshore underwater lighthouse, for example, 52a, receives a transmission signal U1 from a reference underwater lighthouse 51 and emits its own transmission signal U2. By repeating the same processing in sequence from the offshore underwater lighthouse 52a onwards, even offshore underwater lighthouses several hundred kilometers offshore can generate synchronization signals synchronized with the reference underwater lighthouse.

[0073] Step ST1: Calculate the underwater sound speed c from the values ​​of the pressure, water temperature, and salinity sensors. This is done approximately once a day. Step ST2: Calculate the propagation time from the distance R1 between the reference underwater lighthouse and the offshore underwater lighthouse and the underwater sound speed c. T R1 =R1 / c Step ST3: Receive the transmission signal U1 from the reference underwater lighthouse and store the time.

[0074] Step ST4: Time T from the reception time to the sending of the own transmission signal delay Calculate T delay =10.0-T R1 Step ST5: Generate a transmission signal and calculate T delay Then, the transmission signal U2 is transmitted, and the process returns to step ST3. Steps ST3, ST4, and ST5 are performed every 10 seconds.

[0075] Step ST6: Receive the transmitted signal from the reference underwater lighthouse to obtain sound speed data at the reference underwater lighthouse. Step ST7: Average sound speed c of the underwater sound speed of the underwater lighthouse and the sound speed of the reference underwater lighthouse ave The propagation time is calculated using T R1 =R1 / c ave The process proceeds to step ST4.

[0076] Next, the processing flow for the AUV to determine its position is shown in Figure 23. Step ST11: The AUV surfaces approximately once a day to receive GPS signals and generate its own transmission signal. Step ST12: The underwater sound speed c is calculated from the values ​​of the pressure, water temperature, and salinity sensors. This process is performed every 10 seconds.

[0077] Step ST13: Receive sound waves from the underwater lighthouse and calculate distance r from the time t from the reference time. r=t*c Step ST14: Calculate distances r1, r2, and r3 from at least three underwater lighthouses. Step ST15: Calculate position from the three distances. Step ST16: Control the AUV. Processing returns to step ST12.

[0078] Step ST17: Underwater sound speed data c sent from the underwater lighthouse is acquired. The acquired underwater sound speed data c is used to calculate the distance in step ST13. Whether to use the underwater sound speed data calculated in step ST12 or the underwater sound speed data acquired in step ST17 depends on the case.

[0079] One method for externally correcting the underwater lighthouse's transmission signal is to use an ROV suspended from an AUV or support vessel to transmit a transmission signal synchronized with the 1 pps signal in the vicinity of the underwater lighthouse, and then use a circuit in which the underwater lighthouse that receives the transmission signal transmits its own synchronization signal at the same timing as the transmission signal.

[0080] The application of the present invention to an actual device will now be described. One embodiment and other embodiments are currently feasible. It is certain that a large number of unmanned and manned AUVs will be deployed into the ocean in future marine applications. Underwater lighthouses are essential infrastructure for the underwater operation of these AUVs. The early practical application of the underwater lighthouse according to the present invention will contribute to the safe operation of AUVs in oceans around the world.

[0081] Figures 28A and 28B are plan and side views of an image of an AUV test site as an embodiment of the present invention. When an AUV is manufactured, a test site is required to verify that the AUV is designed and operates correctly. This image shows a site where an operational test is conducted using an AUV equipped with an underwater lighthouse and an AUV receiving system designed based on the present invention to verify whether the AUV meets predetermined standards. The example shows a scenario in which the AUV departs from underwater lighthouse 91a and is guided by underwater lighthouses 91b, 91c, and 91d to reach underwater lighthouse 91e. The AUV navigates while avoiding obstacles 92 and fishing nets 93 along the way.

[0082] Although the embodiments of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments, and various modifications based on the technical concept of the present invention are possible. The configurations, methods, processes, shapes, materials, and numerical values ​​given in the above-described embodiments are merely examples, and different configurations, methods, processes, shapes, materials, and numerical values ​​may be used as necessary.

[0083] 1... Underwater lighthouse, 2... GPS receiver, 7... Cable, 8... GPS receiver of AUV, 15... GPS signal receiving unit and synchronization signal generating unit, 16... High-precision oscillator, 18... Gold code signal generating circuit, 39... Sound speed calculation circuit, 51... Reference underwater lighthouse, 52... Offshore underwater lighthouse, 63... Correlator, 64... Calculation circuit, 73... Transmitter

Claims

1. A synchronous pinger type underwater lighthouse system having at least three underwater lighthouses installed on the bottom of the water at predetermined locations, wherein each of the underwater lighthouses comprises: a first GPS receiver that surfaces at set intervals, receives radio waves from GNSS satellites, and outputs GPS signals; a first synchronization signal generation unit having a first high-precision oscillator and generating a first synchronization signal of a predetermined period in phase synchronized with the received GPS signal; and a transmission unit that synchronizes with the first synchronization signal generated by the first synchronization signal generation unit and emits a transmission signal of a predetermined period consisting of a PN series code as ultrasonic waves into the water; and an underwater robot having a reception unit that receives the transmission signal from the underwater lighthouse comprises: a second GPS receiver that surfaces at set intervals, receives radio waves from GNSS satellites, and outputs GPS signals; a second synchronization signal generation unit having a second high-precision oscillator and generating a second synchronization signal of the same period as the first synchronization signal in phase synchronized with the received GPS signal; and a correlator for identifying the transmission signal transmitted from the underwater lighthouse. The underwater robot receives a transmission signal from the underwater lighthouse, calculates the arrival time from the second synchronization signal generated by the second synchronization signal generation unit and the reception time of the transmission signal, calculates the distance between the underwater robot and the identified underwater lighthouse from the arrival time and the underwater sound speed, and measures its position based on the distance.

2. An underwater lighthouse system in which an underwater robot determines its position by receiving first, second, and third ultrasonic waves from at least three synchronized underwater lighthouses, namely, first, second, and third underwater lighthouses, wherein the first, second, and third underwater lighthouses are installed at predetermined positions underwater and their distances to each other are measured in advance, and the first underwater lighthouse is equipped with a first synchronization signal generation unit that generates a first synchronization signal synchronized with a GPS signal, and a first transmission unit that transmits a first transmission signal synchronized with the first synchronization signal as the first ultrasonic wave at a predetermined period into the water, the second underwater lighthouse comprises a first receiving unit that receives the first transmission signal and forms a first reception signal, a first calculation circuit that calculates a first time by dividing the distance between the first underwater lighthouse and the second underwater lighthouse by the speed of sound in water and calculates a first delay time by subtracting the first time from the predetermined period, and a second transmitting unit that transmits a second transmission signal into the water as the second ultrasonic wave the first delay time after the reception time of the first transmission signal; and the third underwater lighthouse comprises a second receiving unit that receives the second transmission signal and forms a second reception signal, a second calculation circuit that calculates a second time by dividing the distance between the second underwater lighthouse and the third underwater lighthouse by the speed of sound in water and calculates a second delay time by subtracting the second time from the predetermined period, and a third transmitting unit that transmits a third transmission signal into the water as the third ultrasonic wave the second delay time after the reception time of the second transmission signal.

3. An underwater lighthouse system according to claim 1 or 2, wherein the transmission signal is a single-frequency ultrasonic signal phase-modulated with a Gold code and transmitted in a pulse train of one or more pulses.

4. An underwater lighthouse system according to claim 3, wherein the time between pulses in the pulse train is related to the underwater sound speed, and said underwater sound speed is transmitted to the underwater robot.

5. An underwater lighthouse system according to claim 1 or 2, wherein each of said underwater lighthouses calculates the underwater sound speed using output signals from a pressure sensor, a water temperature sensor and a salinity sensor.

6. A position detection method using a synchronous pinger type underwater lighthouse system having at least three underwater lighthouses installed on the bottom of the water at predetermined locations and used, and one or more underwater robots having receiving units for receiving transmission signals from the underwater lighthouses, wherein the multiple underwater lighthouses surface at set intervals and have a first GPS receiver that receives radio waves from GNSS satellites and outputs GPS signals, the multiple underwater lighthouses generate first synchronization signals with a predetermined cycle in phase synchronized with the GPS signals, the multiple underwater lighthouses synchronize with the first synchronization signals and emit transmission signals with a predetermined cycle consisting of PN series codes as ultrasonic waves into the water, the underwater robot surfaces at set intervals and has a second GPS receiver that receives radio waves from GNSS satellites and outputs GPS signals, the underwater robot generates a second synchronization signal with a phase synchronized with the GPS signals and has the same cycle as the first synchronization signal, A position detection method using an underwater lighthouse system in which the underwater robot receives a transmission signal from the underwater lighthouse, calculates the arrival time from the second synchronization signal and the reception time of the transmission signal, calculates the distance to the identified underwater lighthouse from the arrival time and the underwater sound speed, and measures its position based on the distance.

7. A method for measuring position using underwater lighthouses, in which an underwater robot determines its position by receiving first, second, and third ultrasonic waves from at least three synchronized underwater lighthouses, namely, first, second, and third underwater lighthouses, wherein the first, second, and third underwater lighthouses are installed at predetermined positions underwater and their distances to each other are measured in advance, the first underwater lighthouse generates a first synchronization signal synchronized with a GPS signal by a first synchronization signal generating unit, and transmits a first transmission signal synchronized with the first synchronization signal into the water as the first ultrasonic wave at a predetermined cycle by a first transmitting unit, a first calculation circuit for calculating a first delay time by subtracting the first time from the predetermined period; and a second transmitting unit for transmitting a second transmission signal into the water as the second ultrasonic wave after the first delay time from the reception time of the first transmission signal; and a second receiving unit for receiving the second transmission signal into the water as the second ultrasonic wave after the first delay time from the reception time of the first transmission signal; and a third underwater lighthouse for receiving the second transmission signal into the water as the second ultrasonic wave after the second delay time from the reception time of the second transmission signal. A position measurement method using an underwater lighthouse system.

8. A position measurement method using an underwater lighthouse system according to claim 6 or 7, wherein the transmission signal is a single-frequency ultrasonic signal phase-modulated with a Gold code and transmitted as a pulse train of one or more pulses.

9. A position measurement method using an underwater lighthouse system according to claim 8, wherein the time interval between pulses in a pulse train is correlated with the underwater sound speed, and said underwater sound speed is transmitted to the underwater robot.

10. A position measurement method using an underwater lighthouse system according to claim 6 or 7, wherein each of said underwater lighthouses calculates the underwater sound speed using output signals from a pressure sensor, a water temperature sensor and a salinity sensor.

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