Underwater lighthouse system and position measurement method using underwater lighthouse

Abstract

Underwater position measurement is possible even when multiple underwater robots are present. [Solution] The underwater lighthouse system has multiple underwater lighthouses installed underwater, with the distances between them measured in advance, and one underwater lighthouse starts as the base and emits a transmission signal synchronized with a synchronization signal.Other underwater lighthouses that receive the transmission signal control the transmission timing so that they synchronize with the synchronization signal and transmit the transmission signal to still other underwater lighthouses, and receives the synchronization signals emitted by the multiple underwater lighthouses to measure their underwater position.

Description

[Technical Field]

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

[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 vehicles (hereafter referred to as AUVs (Autonomous Underwater Vehicles)) to perform autonomous navigation. Underwater position measurement is an important technology not only for AUVs, but also for remotely operated robots (hereafter referred to as ROVs (Remotely Operated Vehicles)) and manned submersibles (hereafter referred to as HOVs (Human Occupied Vehicles)). When simply referring to an underwater robot, it means either an AUV or an ROV. An underwater lighthouse is defined as follows: (1) An underwater lighthouse system that allows multiple underwater robots to simultaneously determine their underwater position. (2) An underwater lighthouse is fixed to the seabed. (3) Synchronized sound waves are emitted from the underwater lighthouse, and each sound wave has a unique ID. (4) The sound waves emitted from the underwater lighthouse are in multiple unique codes, such as M-sequence codes.

[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. This 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]

number

[0007] where x ROV ,y ROV ,z ROV are the xyz coordinates of the underwater robot's position to be determined, 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. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-138589 Summary of the Invention [Problem to be solved by the invention]

[0009] Another example of a conventional underwater position detection method, as shown in Figure 3, involves using marine buoys B1, B2, and B3 floating on the water surface to receive radio waves from a Global Positioning System (GPS) and link the underwater position to the latitude and longitude on land. In this case, the positions of the marine buoys B1, B2, and B3 on the water surface change depending on meteorological and hydrographic conditions. Therefore, to accurately measure the underwater position of the AUV, the position information of the marine 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 explanation, 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, they are often installed suspended almost directly below the waterline using GPS. Furthermore, since each transponder responds to a call signal from an underwater robot during operation, if there are a large number of underwater robots, there are cases where a transponder cannot return a response signal due to interference or overlap, and because so many response signals are returned, it becomes difficult for the underwater robot to identify which response signal is the one that is responding to its own call, resulting in the problem that many robots are unable to detect their positions at the same time.

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

[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, providing 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 marine 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 marine 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 ocean, the AUV will receive the GPS 1 pps signal and synchronize itself with the time transmitted by the buoys. This will allow the time at which the signals from the three buoys are received, that is, 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]

number

[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 ship or theft of the buoy.

[0018] The problems to be solved in conventional underwater position measurement methods are as follows: (1) It is difficult to simultaneously measure the positions of multiple underwater robots using the transponder method. (2) The marine buoy method can receive GPS signals, but the position of the buoy itself moves, so the underwater robot needs to be notified of its position. (3) Marine buoys are at risk of collision with ships and theft. (4) There is concern that underwater ultrasonic wave propagation may be adversely affected by bubble breakdown when the ocean buoy is used in severe sea conditions.

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

[0020] The present invention has a plurality of underwater lighthouses whose distances to each other are measured in advance and which are installed underwater, This underwater lighthouse system starts from one underwater lighthouse and emits a transmission signal synchronized with a synchronization signal, and other underwater lighthouses that receive the transmission signal control the transmission timing so that they synchronize with the synchronization signal and transmit the transmission signal to still other underwater lighthouses, receiving the synchronization signals emitted by multiple underwater lighthouses to measure their underwater position. [Effects of the Invention]

[0021] According to the present invention, it is possible to synchronize multiple underwater lighthouses that are not equipped with GPS receivers. Note that the effects described here are not necessarily limited to those described herein, and any of the effects described in this specification may be used. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a schematic diagram for explaining the concept of an underwater lighthouse. [Figure 2]FIG. 2 is a schematic diagram for explaining the LBL positioning method, which is a conventional method for measuring positions using an underwater lighthouse. [Figure 3] FIG. 3 is a schematic diagram for explaining another example of a conventional underwater position detection method. [Figure 4] Figure 4 is a timing chart to explain AUV position measurement using the synchronous pinger method. [Figure 5] 5A and 5B are schematic diagrams of an example of an underwater lighthouse. [Figure 6] FIG. 6 is a schematic diagram showing the change over time in the operation of the GPS receiver 2 as it rises. [Figure 7] FIG. 7 is a schematic diagram illustrating an example of an AUV. [Figure 8] FIG. 8 is a block diagram showing a configuration of a part of the circuit included in the signal processing unit. [Figure 9] FIG. 9 is a timing chart for explaining the synchronization signal generation and synchronization pinger method. [Figure 10] 10A and 10B show plan and side views of an example of an underwater lighthouse installation. [Figure 11] Figures 11A and 11B are diagrams that assume multiple AUVs are in the same sea area. [Figure 12] FIG. 12 is a block diagram of a receiving device mounted on an AUV for performing synchronous pinger positioning. [Figure 13] FIG. 13 is a graph showing how changes in water temperature affect the speed of sound in water. [Figure 14] Figure 14 is a graph showing seasonal changes in water temperature near the surface. [Figure 15] 15A and 15B are plan and side views of a system according to one embodiment of the present invention. [Figure 16] FIG. 16 is a timing chart showing the processing of the offshore underwater lighthouse. [Figure 17] Figure 17 is a circuit block diagram inside the underwater lighthouse. [Figure 18]18A and 18B are plan and side views illustrating the concept of a further embodiment of the present invention relating to underwater sound speed measurements between underwater lighthouses. [Figure 19] Figure 19 is a timing chart used to explain the method for measuring the underwater sound speed between underwater lighthouses. [Figure 20] FIG. 20 is a timing chart used to explain a method of transmitting underwater sound speed between underwater lighthouses. [Figure 21] 21A and 21B are a plan view and a side view for explaining the process of determining an accurate distance using the underwater sound speed transmitted from the underwater lighthouse. [Figure 22] FIG. 22 is a flowchart showing the flow of processing performed by the offshore underwater lighthouse. [Figure 23] FIG. 23 is a flowchart showing the flow of processing performed by an AUV. [Figure 24] 24A and 24B are a plan view and a side view for explaining the process of generating a synchronization signal by sequentially transmitting a synchronization signal from a reference lighthouse. [Figure 25] FIG. 25 is a timing chart for explaining the process of generating a synchronization signal by sequentially transmitting the synchronization signal from the reference lighthouse. [Figure 26] FIG. 26 is a timing chart for explaining a method for receiving AUV synchronization signals and adjusting them to synchronize timing. [Figure 27] Figure 27 is a circuit block diagram inside an underwater lighthouse that does not have a GPS receiver. [Figure 28] 28A and 28B are top and side views of the AUV test site image. DETAILED DESCRIPTION OF THE INVENTION

[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 example of an underwater lighthouse 1. This underwater lighthouse 1 is installed on the seabed and has a buoyant GPS receiver 2 mounted on the top part, and is normally underwater and therefore unable to 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 to the surface. 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 a GPS signal, acquires the necessary data, and transmits the GPS signal (1 pps signal) 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 in the water.

[0028] As shown in Figure 7, the AUV has a configuration similar to that of a conventional one, but with the addition of a GPS receiver 8 and an acoustic receiver 9. Like an underwater lighthouse, the AUV periodically surfaces to receive radio waves, generates a reference signal, and 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. Note that the AUV may monitor the sea surface conditions from underwater when surfacing, and may suspend or postpone surfacing if the sea conditions are high.

[0029] Such an underwater lighthouse 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 that the object to be measured (underwater robot, such as an AUV) only has 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 is a synchronous pinger method, the AUV's position is determined only by reception, which also solves problem (1) that was present with conventional positioning methods.

[0030] Figure 8 is a block diagram of part of the circuitry included in the signal processing unit 4. Power, for example +5V, is supplied to the GPS receiver 2 via a cable 7. Power from the 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 unit and a synchronization signal generating unit 15. A high-precision oscillator 16 is connected to the GPS signal receiving unit and a synchronization signal generating unit 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 wave frequency is a single frequency in the range of several tens of kHz to several hundreds of kHz. The transmission signal is one or more pulse trains. If the number of types of Gold codes N is, for example, 32, and two pulses are sent from one underwater lighthouse, then 32 x 32 = 1024 different underwater lighthouses can be identified by combining the two codes. In this way, the 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 subjected to processing such as amplification. 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 receiving GPS signals about once a day and correcting the time, 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 once a day to receive radio waves from a satellite 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, ultrasonic waves are transmitted in synchronization with the synchronization signal.

[0038] Figure 9 shows, for example, what happens when an AUV receives ultrasonic transmission signals from three underwater lighthouses. After receiving a GPS signal, the AUV generates a synchronization signal in its internal high-precision oscillator after the GPS signal is lost, and then receives the signal from the underwater lighthouse. The time accuracy of a GPS 1 pps signal is about 10^-12, so the accuracy of a 10-second synchronization pulse is expressed as 10.00000000000 (12 zeros). ^ represents 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 and periodically surface to receive radio waves and generate a synchronization signal synchronized with the 1 pps signal. Therefore, the AUV knows the time when each underwater lighthouse transmitted ultrasound and can accurately measure the time 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 an underwater lighthouse installation. When installing a GPS-synchronized underwater lighthouse, positioning is performed using GPS and installation on the seabed is performed with as much precision as possible. The installation distance between underwater lighthouses was set to approximately 10 km. This is because if 10 kHz is selected as the frequency of the ultrasonic synchronization signal, this distance can be ensured as a transmission distance. As an idea, if there are short and long sides, the short side can be set at a distance that the AUV can receive, and the long side can be set to, for example, approximately 20 km. In that case, another underwater lighthouse can be placed near the center.

[0041] Figures 11A and 11B show the case where multiple AUVs are in the same sea area. In this case, with the synchronous 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] Figure 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 is generated that is synchronized in phase with and has the same frequency as the 1 pps signal of the GPS signal 31.

[0044] Ultrasonic waves are received by the ultrasonic receiver 9, and the received signal is supplied to the correlator 35 via the 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 the 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 itself 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]

number

[0047] Here, D is depth (m), S is salinity (‰), 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 a pinger signal reception time calculation and positioning calculation circuit 32.

[0048] Figure 13 is a graph showing how much a change in water temperature affects 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%. However, 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, per 1°C, the change 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. It is thought that the water temperature changes less in the ocean where the AUV is cruising, but in order to improve the accuracy of position measurement, sound speed correction is performed using a water temperature sensor 37.

[0050] Next, an embodiment of the present invention will be described. In the above-described example, a surface-mounted GPS receiver is used as a method for implementing a synchronous pinger-based underwater lighthouse. In one embodiment, a synchronous pinger-based underwater lighthouse is implemented without using a surface-mounted GPS receiver. This is intended for use in situations where a surface-mounted 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 the plan and side views of a system according to one 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 the 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 first starting point when transmitting a synchronization signal to the offshore underwater lighthouses in sequence.

[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 constantly or periodically via a cable 53. However, in one embodiment of the present invention, the reference underwater lighthouse 51 may have the same configuration as the example and may have a floating buoy-type GPS receiving function. In this case, it does not need to be connected 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. 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 with 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 example, multiple types of transmission signals can be generated, and by assigning a signal with a different code to each underwater lighthouse, the AUV can identify which lighthouse the signal received 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 calculated by dividing the distance R1 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 number of times the GPS signal is received. This is because this operation is repeated every 10 seconds, which is an extremely short calculation time compared to corrections using a surface-mounted GPS signal that are 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 a configuration similar to the example of the underwater lighthouse described above, except that it does not have a GPS receiver. Figure 17 is a block diagram showing the configuration of the offshore underwater lighthouse 52. The signal received by the 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 (for example, the transmission signal U1 of 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 delay is 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 one embodiment of the present invention, the position measurement system for an AUV is a synchronous pinger system, and therefore has the same configuration as the example (see FIG. 12).

[0064] As mentioned above, the key factor in this system is the speed of sound in water. The speed of sound in water can be calculated using the Mackenzie formula by measuring the water pressure, temperature, and salinity. In addition to this method, the underwater lighthouse system can also calculate the speed of sound in water using the following method.

[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 explaining a method for measuring the underwater sound speed from the propagation time shown in Figure 18. The underwater lighthouses 81 and 82 send out 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 be 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 show 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 that receives this sound wave uses the underwater sound speed that is most likely to be correct based on the positional relationship between the underwater lighthouse and itself to determine the distance between the underwater lighthouse and itself. This can improve the accuracy of its own position measurement. The interval between the two pulses is called 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 the result is multiplied by 10 to obtain the most accurate underwater sound speed every 10 seconds. Using this underwater sound speed to calculate the AUV's position allows for a position with minimal error. Figures 21A and 21B are plan and side views showing the concept of an AUV measuring its position 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 this same processing in sequence after the offshore underwater lighthouse 52a, 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 pressure, water temperature, and salinity sensor values. 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. 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 transmission of the own transmission signal delay Calculate T delay =10.0- TR1 Step ST5: Generate a transmission signal and calculate T delay Then, the transmission signal U2 is transmitted. Then, the process returns to step ST3. Steps ST3, ST4, and ST5 are performed every 10 seconds.

[0075] Step ST6: A signal transmitted from the reference underwater lighthouse is received to obtain sound speed data at the reference underwater lighthouse. Step ST7: Average speed of sound c of the underwater lighthouse and the standard 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 AUV positioning is shown in Figure 23. Step ST11: Surface once a day to receive GPS signals and generate its own transmission signal Step ST12: Calculate the underwater sound speed c from the values ​​of the pressure, water temperature, and salinity sensors. This process is performed every 10 seconds.

[0077] Step ST13: Receive the sound wave from the underwater lighthouse and calculate the 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 the position from the three distances Step ST16: Control the AUV, and then return 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] The following describes the errors that may occur in the above-described embodiment of the present invention. (a) Position measurement error δBouyposition when installing an underwater lighthouse on the seabed (b) Error due to time lag of the transmitted signal using GPS_1pps δGPStime (c) Estimation error of the speed of sound in water δsoundvelocity (d) In the case of a method that does not use GPS_1pps, the cumulative error δnΔt occurs when the transmitted signal is transmitted to the offshore lighthouse by ultrasound.

[0080] If these four error factors (a), (b), (c), and (d) are not correlated with each other, the total error δsum can be expressed by equation (9).

[0081]

number

[0082] The measurement error when installing an underwater lighthouse on the seabed cannot be determined in general terms, as it depends on the installation depth and installation method, but in relatively shallow waters (for example, less than 50m), it is thought possible to install an underwater lighthouse on the seabed with an accuracy of less than 1m by using the Michibiki satellite or similar for construction work.

[0083] If time synchronization is performed by receiving a 1 pps GPS signal, synchronization with an accuracy of 10^-12 or better is possible if GPS reception is always possible. However, in this system, GPS reception is limited to about once a day, and the transmission signal is then generated using an internal clock (high-precision oscillator). Therefore, assuming that the time synchronization is off by the accuracy of the internal clock (for example, 0.01 ppm), the amount of deviation over 24 hours can be calculated using the following formula.

[0084] 24Hx3600Sx0.01ppm=0.000864sec=0.864msec

[0085] If the speed of sound in water is 1500 m / sec, the error in distance is 0.864x1500=1.296m Increasing the number of GPS receptions reduces the error, but receiving GPS signals at intervals longer than one day increases the error.

[0086] The next important factor is the estimated error in the underwater speed of sound, which is used to calculate the distance between the underwater lighthouse and the AUV. The speed of sound in water is significantly affected by water temperature, so the estimated value for the speed of sound in water is determined by how accurately water temperature can be measured. If the measurement accuracy of water temperature is 0.1°C, the estimated error in the speed of sound in water is 0.02125%.

[0087] If the distance between the underwater lighthouse and the AUV is 10 km, that error is included in the calculation of the propagation distance, so we assumed an error of 2.125 m. In other embodiments that do not receive GPS signals, the sound waves reach the next lighthouse, including the error δΔt that occurred between the reference underwater lighthouse and the first lighthouse farther away, so an error also occurs there and the error accumulates to become δnΔt. It is thought that at the 10th lighthouse from a lighthouse 10 km away, 10 errors will accumulate. Therefore, the values ​​of these four errors and the total error are shown in Table 1.

[0088] [Table 1]

[0089] Table 2 shows the estimated total error when the estimated error in the speed of sound in water is set to 1 m.

[0090] [Table 2]

[0091] The overall error of 3.7m is the basis for the belief that this system has ample potential for practical use. The reason is that if an AUV is operated with an optical camera mounted on it, 3.7m is considered to be a sufficient visibility distance for the optical camera, except in particularly murky waters, so once the AUV reaches the vicinity of the underwater lighthouse using acoustics, the next mission can be carried out using an optical camera, etc.

[0092] Next, we will explain how to reduce the error. The three error reduction methods mentioned above can be considered as follows.

[0093] (a) Installation error of underwater lighthouse Although it depends on the sea conditions when the underwater lighthouse is installed, it is not impossible to install it with an accuracy of less than 1m if the installation is done under conditions with small tidal currents and waves and a large installation vessel is used. The error of the Michibiki satellite is said to be less than 10cm, so it is thought that an accuracy of 0.1m will be easily achievable in the near future.

[0094] (b) Reducing time synchronization discrepancies This depends on the accuracy of the internal clock (high-precision oscillator), and even more accurate ones (around 10^-10) are commercially available, but these are called OCXOs (Oven Controlled Crystal Oscillators), and have the disadvantage of consuming a lot of power because they use a heater to keep the crystal oscillator at a constant temperature.Underwater lighthouses and AUVs are battery-powered, so it is essential to keep circuit power consumption as low as possible.

[0095] The oscillator used in this invention is also an OCXO, but it consumes a relatively small amount of current: 5V 200mA, and we selected one with a frequency stability of ±10ppb (0.01ppm). If we allow for a current consumption of about 2.5 times higher (5V 500mA), there are also models with a frequency stability of about ±1.0ppb (0.001ppm). Using this can reduce the time synchronization error to 1 / 10. Conversely, if we can tolerate a time error that is one order of magnitude worse, we can use a TCXO (Temperature Compensated Crystal Oscillator), which consumes less current (1 / 20 or less). In this case, we can reduce the error by increasing the frequency of GPS reception to about once every two hours and performing time synchronization via GPS more frequently.

[0096] (c) Underwater sound velocity estimation error Insofar as the speed of sound is calculated using water temperature, depth, and salinity, the estimation error for the speed of sound in water depends on how accurately these parameters are measured; however, the error was estimated assuming a water temperature measurement accuracy of 0.1°C. The water temperature measurement only measured the water temperature around the AUV, so the water temperature sent from the underwater lighthouse was not corrected for along the entire propagation path. In the ocean, the physical structure of the water temperature and salinity can vary even at distances of 10 km, so if it were possible to measure the water temperature distribution along the propagation path, the estimation error for the speed of sound in water could be reduced.

[0097] Therefore, since the distance between the underwater lighthouses is known, the accurate underwater sound speed can be obtained by dividing that distance by the propagation time. The obtained underwater sound speed is an answer that reflects the water temperature, depth, and salinity concentration within the propagation path, so it is more accurate than sound speed values ​​corrected by water temperature sensors, etc. By using this method, it is possible to determine the AUV's position more accurately.

[0098] (d) Accumulated error due to ultrasonic propagation in an embodiment that does not receive GPS transmitted signals The further the distance from the reference underwater lighthouse, and when transmitting signals using a relay system, the more errors in measuring the arrival time accumulate. In the example shown in Table 2, a lighthouse 100 km offshore is thought to have an accumulated error of 3.7 m. Since no greater error can be tolerated underwater where the visibility distance is short, we will consider how to reduce the time difference in the transmitted signal from an underwater lighthouse located quite far offshore.

[0099] One of them is to reduce the estimation error of the speed of sound in water. If the measurement accuracy of water temperature in the estimation formula (7) for the speed of sound in water is made higher than 0.1°C, the estimation error of the speed of sound in water will be reduced accordingly. In reality, it seems possible to measure with an accuracy of about 0.05°C, so the error can be reduced by about half.

[0100] The second method does not measure the water temperature or the like to determine the underwater sound speed, but instead calibrates the timing close to the true value of the transmitted signal using a different method. Figures 24A and 24B are plan and side views of a system that generates transmitted signals by sequentially transmitting signals from a reference underwater lighthouse. Offshore underwater lighthouses 521, 522, ..., 52n-1, 52n sequentially receive the transmitted signals, calculate the synchronization timing, and transmit their own transmitted signals.

[0101] Figure 25 is a timing chart to explain the accumulation of errors due to the sequential transmission of transmission signals between underwater lighthouses. After receiving the transmission signal from the reference underwater lighthouse 51, errors accumulate in the process of transmitting the transmission signal to the nth underwater lighthouse in sequence, and although the error does not simply increase by n times, it does increase. The cause of this error is an error in estimating the speed of sound in water.

[0102] In this method of transmitting signals sequentially, timing errors accumulate, and the accumulated errors become larger as the number of underwater lighthouses offshore increases. Therefore, we present a method to reduce the transmission timing errors of the transmission signals using AUVs.

[0103] If this AUV surfaces and transmits a signal synchronized with the GPS_1pps signal, it can approach an underwater lighthouse and synchronize its own transmission signal to be received by the target underwater lighthouse. Once the underwater lighthouse receives the AUV's transmission signal, it can correct the timing discrepancy with its own transmission signal, allowing it to transmit a signal with extremely little discrepancy from the reference underwater lighthouse's transmission signal. If this method can be used, there is no need to use the underwater sonic speed, making it possible to continuously transmit a transmission signal without accumulating errors.

[0104] Figure 26 is a timing chart to explain the method for receiving an AUV's transmission signal and adjusting it to have the same timing. If the nth underwater lighthouse adjusts its own transmission signal to receive the AUV's transmission signal at the same timing, the transmission signal will have very little time lag from the reference underwater lighthouse's transmission signal.

[0105] Figure 27 is a circuit block diagram of an underwater lighthouse synchronized with the AUV's transmission signal. Since it has the same configuration as the underwater lighthouse shown in Figure 17, the same reference numerals will be used for corresponding components.

[0106] However, since this method does not require correction for the underwater speed of sound, sensors for calculating the speed of sound, such as water temperature sensors, and sound speed calculation circuits are not required, and a high-precision oscillator is also not required.The reason for this is that the transmission signal sent from the reference underwater lighthouse has a repetition period of 10.00000000000 seconds, and the accuracy of this period is maintained at a high precision of 10^-12 because it is directly linked to GPS.Once synchronization is corrected on the AUV, it can generate its own transmission signal synchronized with the transmission signal from the reference underwater lighthouse simply by correcting the Δt shown in Figure 26.Therefore, since there is no need to generate a transmission signal using an internal clock for a long period of time, a high-precision oscillator is not required.

[0107] Furthermore, in a system consisting of one reference underwater lighthouse equipped with GPS receiving capabilities and multiple offshore underwater lighthouses installed on the seafloor that transmit signals by receiving transmission signals from the reference underwater lighthouse, the offshore underwater lighthouse's transmission signal may be corrected at least once per predetermined period with an external transmission signal. One method for correcting the offshore underwater lighthouse's transmission signal at least once per predetermined period, for example, once per week, is for the underwater robot that most recently surfaced, received a GNSS signal, and generated a synchronization signal to approach or come into contact with the offshore underwater lighthouse and transmit an ultrasonic synchronization signal modulated with a Gold code every 10 seconds, which is then received by the underwater lighthouse. The underwater lighthouse is equipped with a circuit for synchronizing its own synchronization signal with the synchronization signal from the underwater robot, but does not have a GNSS receiver.

[0108] 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 signal synchronized with the 1 pps signal in the vicinity of the underwater lighthouse, and then use a circuit that enables the underwater lighthouse to receive the transmitted signal and transmit its own synchronization signal at the same time as the transmitted signal.

[0109] This section explains the application of the present invention to actual equipment. The embodiments of the present invention are currently feasible. There is no doubt that a large number of unmanned and manned AUVs will be deployed in the ocean in future marine applications. Underwater lighthouses are essential infrastructure for the underwater navigation of these AUVs. We are confident that the underwater lighthouse of the present invention will be put into practical use soon, contributing to the safe navigation of AUVs in oceans around the world. We hope to work toward practical application as soon as possible.

[0110] 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 operational tests are 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 case where 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. Further application examples of the present invention include the following. (1) A support system for autonomous navigation of AUVs used to inspect undersea cables for offshore wind power generation (2) Position measurement support system for underwater robots that explore and mine mineral resources (3) Position measurement system for unmanned and manned submersibles

[0111] 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. [Explanation of symbols]

[0112] 1...Underwater lighthouse, 2...GPS receiver, 7...Cable, 8...AUV GPS receiver, 15...GPS signal receiver and synchronization signal generator, 16...High-precision oscillator, 18...Gold code signal generator circuit, 39...Sound speed calculation circuit, 51...Reference underwater lighthouse, 52...Offshore underwater lighthouse, 63...Correlator, 64...Calculation circuit, 73...Transmitter

Claims

1. A synchronized pinger type underwater lighthouse system having at least three underwater lighthouses installed on the bottom of a water body at a predetermined location, Each of the underwater lighthouses includes a first GPS receiver that surfaces at set times, receives radio waves from GNSS satellites, and outputs GPS signals; a first synchronization signal generating 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; a transmitter that synchronizes with the first synchronization signal generated by the first synchronization signal generator and transmits a transmission signal having a predetermined period and consisting of a PN series code as an ultrasonic wave into water, The underwater robot has a receiving unit that receives the transmission signal from the underwater lighthouse, and a second GPS receiver that surfaces at set time intervals, receives radio waves from GNSS satellites, and outputs GPS signals. a second synchronization signal generating unit having a second high-precision oscillator and generating a second synchronization signal having a phase synchronized with the received GPS signal and the same period as the first synchronization signal; a correlator for identifying a 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 underwater lighthouses, first, second and third, that are in a synchronous relationship, The first, second and third underwater lighthouses are installed at predetermined positions underwater, and the distances between them are measured in advance; The first underwater lighthouse includes a first synchronization signal generating unit that generates a first synchronization signal synchronized with a GPS signal, and a first transmitting unit that transmits a first transmission signal synchronized with the first synchronization signal as the first ultrasonic wave into the water at a predetermined period, 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 water as the second ultrasonic wave after the first delay time from the reception time of the first transmission signal, The third underwater lighthouse includes 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 water as the third ultrasonic wave after the second delay time from the reception time of the second transmission signal. Underwater lighthouse system.

3. 3. The underwater lighthouse system according to claim 1, 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. 4. The underwater lighthouse system according to claim 3, wherein the time interval between pulses in the pulse train is correlated with the underwater sound speed, and the underwater sound speed is transmitted to the underwater robot.

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

6. A method for detecting a position using a synchronous pinger type underwater lighthouse system, comprising at least three underwater lighthouses installed on the bottom of the water at predetermined locations and one or more underwater robots each having a receiving unit for receiving a signal transmitted from the underwater lighthouses, The plurality of underwater lighthouses each have a first GPS receiver that surfaces at a set time, receives radio waves from a GNSS satellite, and outputs a GPS signal; The plurality of underwater lighthouses generate a first synchronization signal having a predetermined period in phase synchronized with a GPS signal; The plurality of underwater lighthouses synchronize with the first synchronization signal and transmit a transmission signal having a predetermined period and consisting of a PN series code as an ultrasonic wave into the water, the underwater robot has a second GPS receiver that surfaces at set time intervals, receives radio waves from GNSS satellites, and outputs GPS signals; the underwater robot generates a second synchronization signal having the same period as the first synchronization signal in a phase synchronized with a GPS 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 a position using an underwater lighthouse, in which an underwater robot determines its position by receiving first, second, and third ultrasonic waves from at least three underwater lighthouses, first, second, and third, that are in a synchronous relationship, comprising: The first, second and third underwater lighthouses are installed at predetermined positions underwater, and the distances between them are measured in advance; The first underwater lighthouse generates a first synchronization signal synchronized with a GPS signal by a first synchronization signal generation unit, and transmits a first transmission signal synchronized with the first synchronization signal as the first ultrasonic wave into the water at a predetermined period by a first transmission unit, The second underwater lighthouse receives the first transmission signal using a first receiving unit to form a first reception signal, 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 using a first calculation circuit, calculates a first delay time by subtracting the first time from the predetermined period, and transmits a second transmission signal as the second ultrasonic wave into the water after the first delay time from the reception time of the first transmission signal using a second transmitting unit. The third underwater lighthouse receives the second transmission signal by a second receiving unit to form a second reception signal, calculates a second time by dividing the distance between the second underwater lighthouse and the third underwater lighthouse by the underwater sound speed by a second calculation circuit, calculates a second delay time by subtracting the second time from the predetermined period, and transmits a third transmission signal into the water as the third ultrasonic wave after the second delay time from the reception time of the second transmission signal by a third transmitting unit. A method for measuring position using an underwater lighthouse system.

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

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

Images