JP2024081960A - Acoustic positioning method and system - Google Patents

Abstract

To provide an acoustic positioning method and a system that enable simultaneously position locations of a plurality of objects while eliminating cumbersome pre-processing as much as possible on the basis of an acoustic pulse a pinger transmits.SOLUTION: An acoustic positioning system 1 includes: a plurality of pingers 10 that transmits an acoustic pulse; a receiver 20 that positions a location of each pinger 10 on the basis of the acoustic pulse of the pinger. A cycle of a time slot transmitting the acoustic pulse is registered in a cycle setting part 101 of the pinger 10. The time slot cycle to be set to each pinger 10 is different from each other. Each pinger 10 is configured to transmit the acoustic pulse every time slot cycle. The receiver 20 mainly consists of a pulse reception unit 202, and a positioning unit 203, which are provided with at least three hydrophones 201. The positioning unit 203 is configured to calculate a direction and depth of each pinger 10 on the basis of the acoustic pulse each hydrophone 201 receives; and position the location of each pinger 10 on the basis of a calculation result.SELECTED DRAWING: Figure 1

Description

The present invention relates to an acoustic positioning method and system that determines the positions of multiple pingers or the position of a moving object equipped with the pingers based on acoustic pulses transmitted by the pingers, and in particular to an acoustic positioning method and system that is suitable for simultaneously determining the positions of multiple underwater robots, drones, or divers equipped with pingers.

Non-Patent Document 1 discloses a "combined water-air drone" in which an aerial drone flies holding an underwater drone, lands in the target water area, separates the underwater drone and dives, and retrieves it and takes off after the operation is completed, as shown in Figure 5.

One of the features of the combined water-air drone is that it is equipped with "acoustic positioning" technology, which transmits acoustic pulses from a transmitter (pinger) attached to the underwater drone, receives them with an underwater microphone (hydrophone) on the aerial drone, and automatically analyzes them to calculate the position of the underwater drone. Acoustic positioning technology is disclosed, for example, on pages 236-245 of Non-Patent Document 2.

The combined water-air drone uses the SSBL (Super Short Base Line) method for acoustic positioning, which first calculates the time difference between acoustic pulses received by three or more hydrophones, and then calculates the position of the transmitter from that. There is also another acoustic positioning method called SBL (Short Base Line) (Non-Patent Document 2), which has higher positioning accuracy than SSBL. As disclosed in Patent Document 1, the combined water-air drone can also use the SBL method for acoustic positioning.

Non-patent document 3 discloses an example of an SSBL acoustic positioning system installed on a combined water-air drone. In the SSBL acoustic positioning system, a set of acoustic pulses is emitted within a certain period of time (e.g., one second). A set of pulses consists of a certain number of pulses (e.g., two pulses), and positioning is performed by receiving these pulses with a hydrophone and processing the signals.

As a method for simultaneously locating the positions of two pingers, Non-Patent Document 2, pages 125 and 177, discloses a technique in which one pinger transmits an acoustic pulse as an up-chirp and the other transmits a down-chirp, thereby separating the acoustic pulses when calculating the cross-correlation even if the acoustic pulses collide.

Patent Application No. 2022-145518

KDDI/KDDI Research/PRODRONE, News Release "World's first water-air combined drone succeeds in remote underwater photography ~ Carrying out safe and efficient inspection of offshore wind power generation equipment without launching a ship ~", December 14, 2021https://news.kddi.com/kddi/corporate/newsrelease/2021/12/14/5593.html Marine Acoustics Society, "Fundamentals and Applications of Marine Acoustics", Seizando Shoten, pp. 236-245 Kawada, Nishitani, and Kojima: "Acoustic Positioning System for a Combined Water-Air Drone", Proceedings of the Marine Acoustics Society of Japan, No. 22-2, pp. 3-4 (2022)

Consider the case where there is not just one positioning target, but multiple targets. A chirp signal pulse is a signal whose frequency changes continuously within the duration of the pulse (for example, from 40 kHz to 60 kHz, with 50 kHz as the center). There are uplink and downlink change forms (uplink: start 40 kHz, end 60 kHz, downlink: start 60 kHz, end 40 kHz), so if there are two positioning targets, if an uplink and downlink chirp signal is generated for each, they can be separated during correlation calculations even if the two signals are received simultaneously.

However, when there are three or more positioning targets, for example when two of the same upstream chirp signals are received simultaneously, it becomes difficult to separate them, making positioning impossible, which is an issue. One solution to avoid this is to appropriately shift the emission timing of each pinger so that the reception timing at the hydrophone does not overlap. However, shifting the emission timing of the pingers requires synchronization processing to set the clocks of each pinger to a common clock, which makes pre-processing complicated.

Furthermore, even when chirp signals are used to separate colliding acoustic pulses, there is a technical challenge in that accurate separation is not possible when calculating cross-correlation if the levels of the colliding acoustic pulses differ significantly.

The object of the present invention is to solve the above technical problems and provide an acoustic positioning method and system that can simultaneously determine the positions of multiple objects based on acoustic pulses transmitted by their pingers while minimizing cumbersome pre-processing.

To achieve the above object, the present invention is an acoustic positioning method for determining the position of each pinger based on acoustic pulses transmitted by multiple pingers at a predetermined period, and is characterized by having the following configuration.

(1) Each pinger is assigned a time slot with a different period, and each pinger transmits an acoustic pulse during each time slot.

(2) The time slot period of each pinger is set to a different prime number multiple of the time slot length for each pinger.

(3) Multiple pingers are classified based on two types of encoding methods for their acoustic pulses that correspond to the code division multiplexing method, and the time slots of each pinger are time-division multiplexed for each group, with the time slot period set for each pinger being different for each group.

(4) Multiple pingers are classified based on two types of encoding methods for their acoustic pulses that correspond to the code division multiplexing method, and one pinger is selected from each group and combined into pairs, in which the time slots of each pinger are time-division multiplexed, and the time slot period set for each pinger is made different for each pair.

The present invention can be realized not only as an acoustic positioning method that includes these characteristic processing steps, but also as an acoustic positioning system that configures these steps with hardware.

(1) Each pinger is assigned a time slot with a different period, and each pinger transmits an acoustic pulse for each time slot. This not only reduces the chance of collisions between acoustic pulses transmitted by each pinger, but also prevents the collision target from becoming fixed. This reduces the probability of a pinger causing positioning failure.

(2) The time slot period of each pinger is set to a different prime multiple of the time slot length for each pinger, which further reduces the frequency of collisions between acoustic pulses transmitted by each pinger.

(3) The acoustic pulses of multiple pingers are classified based on two types of encoding methods corresponding to the code division multiplexing method, and the time slots of each pinger are time division multiplexed for each group. The time slot period set for each pinger is different for each group. This reduces the chance of collision between acoustic pulses transmitted by each pinger, and even if a collision does occur, the collision partner is not fixed. Therefore, efficient simultaneous positioning can be achieved by combining the time division multiplexing method and the code division multiplexing method.

(4) Multiple pingers are classified based on two types of encoding methods for their acoustic pulses corresponding to the code division multiplexing method, and one pinger is selected from each group and combined into a pair, and the time slots of each pinger are time division multiplexed for each pair. The time slot period set for each pinger is made different for each pair, thereby reducing the chance of collision between acoustic pulses transmitted by each pinger, and making it possible to separate them even if a collision does occur. Therefore, it becomes possible to realize efficient simultaneous positioning by combining the time division multiplexing method and the code division multiplexing method.

1 is a functional block diagram showing a configuration of an acoustic positioning system to which the present invention is applied. 1A to 1C are diagrams showing a method of transmitting an acoustic pulse according to a first embodiment of the present invention. 6A to 6C are diagrams showing a method of transmitting an acoustic pulse according to a second embodiment of the present invention. 13A to 13C are diagrams showing a method of transmitting an acoustic pulse according to a third embodiment of the present invention. This is a diagram showing an example of a combined water and air drone.

Below, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a functional block diagram showing the configuration of an acoustic positioning system 1 to which the present invention is applied, and its main components are multiple pingers 10 that transmit acoustic pulses and a receiver 20 that determines the position of each pinger 10 based on the acoustic pulses. When the present invention is applied to a combined water-air drone, the receiver 20 is mounted on the air drone, and at least one of the pingers 10 is mounted on the underwater drone.

Each pinger 10 mainly comprises a period setting unit 101, a pressure sensor 102, a depth measurement unit 103, a pulse generation unit 104, and a pulse transmission unit 105, and the period setting unit 101 registers the time frame (time slot) for transmitting acoustic pulses and its period. The time slot periods preset in each pinger 10 are different from each other, and each pinger 10 transmits an acoustic pulse for each unique time slot period set in its own period setting unit 101.

The receiver 20 mainly comprises a pulse receiving section 202 equipped with at least three hydrophones 201, and a positioning section 203. The positioning section 203 includes a direction calculation section 203a and a depth calculation section 203b, and calculates the direction and depth of each pinger 10 based on the acoustic pulses received by each hydrophone 201, and locates the position of each pinger 10 based on the calculation results.

Such a receiver 20 can be configured by implementing applications (programs) that realize the functions detailed below on a general-purpose computer or server equipped with a CPU, ROM, RAM, bus, interface, etc. Alternatively, it can be configured as a dedicated machine or a single-function machine in which part of the application is implemented as hardware or software.

Figure 2 shows a method for transmitting acoustic pulses according to a first embodiment of the present invention, characterized in that multiple pingers 10 repeatedly transmit acoustic signals (pulses) AP at different time slot periods.

In this embodiment, each pinger 10 transmits a positioning pulse AP1 and a depth pulse AP2 as an acoustic pulse AP in that order for each time slot. The positioning pulse AP1 is transmitted for the purpose of allowing the receiver 20 to calculate the direction of the pinger 10 based on the difference in reception time at the three hydrophones 201. The depth pulse AP2 is transmitted for the purpose of allowing the receiver 20 to calculate the depth of the pinger 10 based on the time difference with the positioning pulse AP1.

The duration of the positioning pulse AP1 and the depth pulse AP2 are both set to 1.6 ms. In this embodiment, the transmission interval and time slot length of each pulse AP1 and AP2 are set based on the maximum arrival time of the acoustic pulse, 50/1500=0.033 [s] = 33 ms, assuming that the maximum distance from the pinger 10 to the receiver 20 is 50 m and the speed of sound is 1500 [m/s].

For each pinger 10, a time that is a prime multiple of the time slot length (150 ms) is preregistered in the period setting unit 101 as the time slot period. In this embodiment, a time that is the time slot length multiplied by a different prime number for each pinger 10 is preregistered as the time slot period so that the time slot period for each pinger 10 is set to a different prime multiple of the time slot length for each pinger.

The pulse generating unit 104 generates a positioning pulse AP1 with a duration of 1.6 ms for each time slot. The pulse generating unit 104 further generates a depth pulse AP2 at a time that is a time interval proportional to the depth measured by the depth measuring unit 103 based on the output of the pressure sensor 102, with the time 50 ms after the transmission time of the positioning pulse AP1 being set as a depth of 0 m.

For example, if the measurement depth is 10 m, the depth pulse AP2 is generated when 50 + 10 = 60 ms have elapsed since the transmission time of the positioning pulse AP1. If the measurement depth is the maximum depth of 50 m, the depth pulse AP2 is generated when 50 + 50 = 100 ms have elapsed. The pulse transmitting unit 105 transmits the positioning pulse AP1 and depth pulse AP2 at their generation times.

In this embodiment, the time slot length is set to 150 ms, so even if the pinger 10 is located at the maximum distance and maximum depth from the receiver 20, the transmission time from transmission to reception of each pulse signal AP1, AP2 can be kept within the time slot.

In the receiver 20, the three hydrophones 201 receive the acoustic pulses AP transmitted by each pinger 10. In this embodiment, before positioning begins, the timing of transmitting the acoustic pulse is learned (recorded) for each pinger 10, for example, when the pinger 10 and the receiver 20 are in contact and the distance between them is zero. This allows the receiver 20 to predict the time slots in which it is likely to receive the acoustic pulses AP from each pinger 10, making it possible to separate and process the acoustic pulses from each pinger 10. This advance processing is simpler than the process of synchronizing the clocks of all pingers 10 and the receiver 20, and can be completed in a short time.

In the positioning unit 203, the direction calculation unit 203a calculates the direction of the pinger 10 based on the difference in the time when each hydrophone 201 receives the positioning pulse AP1. The depth calculation unit 203b calculates the depth of the pinger 10 based on the difference in the reception time between the positioning pulse AP1 and the depth pulse Ap2. The positioning unit 203 locates the position of the pinger 10 based on the calculation results of the direction and depth.

In this embodiment, the time slot period of each pinger 10 is set to a length obtained by multiplying the time slot length by a different prime number. In the example of FIG. 2, the time slot period of one pinger 10a is set to the time slot length (150 ms) multiplied by prime number 5 (750 ms), while the time slot period of the other pinger 10b is set to the time slot length multiplied by prime number 7 (1050 ms).

If the time slot period of each pinger 10 is set as above, for example, if the time slots of pingers 10a and 10b overlap initially, the next time they overlap will be after a time equal to 15, the least common multiple of the prime numbers 3 and 5, multiplied by the time slot length of 150 ms (15 x 150 ms = 5250 ms). In other words, the acoustic pulse transmitted by pinger 10a will collide with another acoustic pulse only once in seven times, and the acoustic pulse transmitted by pinger 10b will collide with another acoustic pulse only once in five times. Therefore, if only the periods of collision are considered missing, the position of each pinger 10 can be accurately and simultaneously determined without any problems.

If there are three pingers 10 (10a, 10b, and 10c), the time slot period of pinger 10c can be set to the time obtained by multiplying the time slot length by a different prime number 11 (150 x 11 = 1650 ms).

In this case, the collision frequency of acoustic pulses transmitted by pingers 10b and 10c is every time 55, the least common multiple of prime numbers 5 and 11, multiplied by the time slot length of 150 ms (55 x 150 ms = 8250 ms), and the collision frequency of acoustic pulses transmitted by pingers 10a and 10c is every time 33, the least common multiple of prime numbers 3 and 11, multiplied by the time slot length of 150 ms (33 x 150 ms = 4950 ms), both of which can be essentially ignored.

In the above example, the time slot length is set to 150 ms in order to transmit two acoustic pulses (positioning pulse AP1 and depth pulse AP2) in each time slot, but it is also possible to omit the depth pulse AP2 and transmit only the positioning pulse AP1. In that case, the time slot length can be shortened to about 50 ms (assuming a maximum distance of about 50 meters), so if the time slot period is set to about 1 second, the time slot period can be set as follows even when simultaneously positioning the positions of five pingers 10.

1st: 50 ms x 13 = 650 ms
2nd: 50 ms x 17 = 850 ms
3rd: 50 ms x 19 = 950 ms
4th: 50 ms x 23 = 1150 ms
5th: 50 ms x 29 = 1450 ms

In this way, according to this embodiment, time slots with different periods are set for each pinger 10, and each pinger 10 transmits an acoustic pulse for each time slot. This not only reduces the chance of collisions between acoustic pulses transmitted by each pinger 10, but also prevents the collision partner from becoming fixed. This makes it possible to keep the probability of a pinger that is unable to be positioned low.

Furthermore, by setting the time slot period of each pinger 10 to a different prime number multiple of the time slot length for each pinger, the frequency of collisions between acoustic pulses transmitted by each pinger 10 can be further reduced.

Figure 3 shows an acoustic positioning method according to a second embodiment of the invention, focusing on the timing at which ten pingers 10 transmit acoustic pulses. Simultaneous positioning of multiple pingers is achieved by combining time division multiplexing and code division multiplexing.

In this embodiment, a chirp signal is used as the code division multiplexing method, and a large number of pingers 10 are classified into a group that modulates and transmits the acoustic pulse using an upward chirp method in which the pulse frequency gradually increases, and a group that modulates and transmits using a downward chirp method in which the pulse frequency gradually decreases. This embodiment is characterized in that the time slots of each pinger are time division multiplexed for each group, and the time slot period set for each pinger is different for each group.

Figure (a) shows an example of time slots set for channels in group A that transmit acoustic pulses as upstream chirps. The transmission frame (1000 ms) is divided equally into five time slots, and each time slot is assigned to one of five pingers (0, 1, 2, 3, 4). The period of each time slot in the channel is set to a prime multiple of the time slot length (200 ms) (here, 5 times 1000 ms).

Figure (b) shows an example of time slots set for group B channels that transmit acoustic pulses in downstream chirps. The transmission frame (1000 ms) is divided equally into five time slots, and each time slot is assigned to five pingers (5, 6, 7, 8, 9). The period of each time slot in the channel is set to a prime multiple of the time slot length (200 ms) different from that of group A (here, 7 times, or 1400 ms).

In this way, according to this embodiment, the period of each time slot in group A and the period of each time slot in group B are set to a time obtained by multiplying the time slot length by a different prime number, so that the combination of colliding time slots can be made different for each period.

For example, the time slots of pinger 5 and pinger 6 collide with the time slots of pinger 0 and pinger 1, respectively, in the first cycle, but collide with the time slots of pinger 2 and pinger 3, respectively, in the next cycle.

Therefore, even if the level of the acoustic pulses transmitted by, for example, pingers 0 and 1 is particularly low and not the same as the level of the acoustic pulses transmitted by pingers 5 and 6, making separation by correlation signal processing difficult, positioning is possible in the next period because neither of the time slots of pingers 0 and 1 collide with other time slots.

Furthermore, since the time slots of pingers 5 and 6 collide with the time slots of pingers 2 and 3, if the level of the acoustic pulses transmitted by pingers 2 and 3 is equivalent to the level of the acoustic pulses transmitted by pingers 5 and 6, positioning is possible by separation using correlation signal processing.

As described above, according to this embodiment, the acoustic pulses of multiple pingers 10 are classified based on two types of encoding methods corresponding to the code division multiplexing method, the time slots of each pinger 10 are time division multiplexed for each group, and the time slot period set for each pinger is made different for each group. This reduces the chance of collision between acoustic pulses transmitted by each pinger, and even if a collision does occur, the collision partner is not fixed. Therefore, efficient simultaneous positioning can be achieved by combining the time division multiplexing method and the code division multiplexing method.

Figure 4 shows an acoustic positioning method according to a third embodiment of the present invention, focusing on the timing at which ten pingers 10 transmit acoustic pulses, and realizes simultaneous positioning of multiple pingers by combining time division multiplexing and code division multiplexing.

In this embodiment, a large number of pingers are classified into Group A, which modulates and transmits acoustic pulses using the upstream chirp method, and Group B, which modulates and transmits using the downstream chirp method. This embodiment is characterized in that the time slots of each pinger are time-division multiplexed for each pair, combining the time slot of one pinger selected from Group A with the time slot of one pinger selected from Group B, and the time slot period set for each pinger is made different for each pair.

In the first channel in the figure (a), the transmission frame (100 ms) is divided equally into two time slots, with the leading time slot being assigned to the time slot of pinger 0 selected from group A, and the trailing time slot being assigned to the time slot of pinger 5 selected from group B.

Similarly, in the second channel in the same figure (b), the leading time slot is assigned to the time slot of pinger 1 selected from group A, and the trailing time slot is assigned to the time slot of pinger 6 selected from group B.

Similarly, in the third channel, the time slot of pinger 2 selected from group A is assigned to the leading time slot, and the time slot of pinger 7 selected from group B is assigned to the trailing time slot. In the fourth channel, the time slot of pinger 3 selected from group A is assigned to the leading time slot, and the time slot of pinger 8 selected from group B is assigned to the trailing time slot.

In the fifth channel in the figure (c), the leading time slot is assigned to the time slot of pinger 4 selected from group A, and the trailing time slot is assigned to the time slot of pinger 9 selected from group B.

Here, the time slot period in the first channel is set to a time length (650mm) obtained by multiplying the time slot length (50mm) by the prime number 13. In the second channel, it is set to a time length (850ms) obtained by multiplying the time slot length by the prime number 17. Similarly, in the third channel, it is set to a time length (950ms) multiplied by a multiple of the prime number 19, in the fourth channel to a time length (1150ms) multiplied by the prime number 23, and in the fifth channel to a time length (1450ms) multiplied by the prime number 29.

In this embodiment, the time slot period is set to a time obtained by multiplying the time slot length by a prime number that is different for each channel, which makes it possible to reduce the frequency of time slot collisions.

In addition, since the transmission frame of each channel is composed of a pair of an uplink chirp time slot and a downlink chirp time slot, even if the time slots collide, if it is an uplink chirp time slot and a downlink chirp time slot, positioning is possible by separating them using correlation signal processing.

Furthermore, according to each of the above-mentioned embodiments, the positions of a large number of moving objects can be determined simultaneously without cumbersome pre-processing, which makes it possible to contribute to Goal 9 "Build resilient infrastructure and promote inclusive and sustainable industrialization" and Goal 11 "Make cities inclusive, safe, resilient and sustainable" of the Sustainable Development Goals (SDGs) led by the United Nations.

10...pinger, 20...receiver, 101...period setting unit, 102...pressure sensor, 103...depth measurement unit, 104...pulse generation unit, 105...pulse transmission unit, 201...hydrophone, 202...pulse reception unit, 203...positioning unit, 203a...direction calculation unit, 203b...depth calculation unit

Claims (16)

1. An acoustic positioning method in which a computer locates the position of each pinger based on acoustic pulses transmitted from a plurality of pingers at a predetermined period, comprising:
Each pinger is assigned a time slot with a different period.
4. The acoustic positioning method according to claim 1, wherein each pinger transmits an acoustic pulse in each of said time slots. The acoustic positioning method according to claim 1, characterized in that the time slot period of each pinger is set to a different prime number multiple of the time slot length for each pinger. classifying the plurality of pingers into a first group whose acoustic pulses are modulated by a first coding scheme corresponding to a code division multiplexing scheme and a second group whose acoustic pulses are modulated by a second coding scheme;
The clocks of each pinger are synchronized for each group.
time-division multiplexing the time slots of each pinger for each group based on the time of the clock;
2. The acoustic positioning method according to claim 1, wherein the time slot period set for each pinger is different for each group. The acoustic positioning method according to claim 3, characterized in that the time slot period set for each pinger is set to a different prime number multiple of the time slot length for each group. classifying the plurality of pingers into a first group whose acoustic pulses are modulated by a first coding scheme corresponding to a code division multiplexing scheme and a second group whose acoustic pulses are modulated by a second coding scheme;
One pinger is selected from each group and the clocks are set for each pair.
time-division multiplexing the time slots of each pinger for each pair based on the time of said clock;
2. The acoustic positioning method according to claim 1, wherein a time slot period set for each pinger is different for each pair. The acoustic positioning method according to claim 5, characterized in that the time slot period set for each pinger is set to a different prime multiple of the time slot length for each pair. The acoustic positioning method according to any one of claims 3 to 6, characterized in that the first and second encoding methods are upstream chirp and downstream chirp, respectively. each pinger includes a pressure sensor for determining depth;
Transmitting a positioning pulse and a depth pulse in that order for each time slot;
7. The acoustic positioning method according to claim 1, wherein an interval between the positioning pulse and the depth pulse is variable depending on a result of depth positioning. In an acoustic positioning system in which a receiver receives acoustic pulses transmitted from a plurality of pingers at a predetermined period and determines the position of each pinger,
Each pinger
A means for setting time slots having mutually different periods;
means for transmitting an acoustic pulse for each of said time slots. The acoustic positioning system according to claim 9, characterized in that the means for setting the time slot sets the time slot period of each pinger to a different prime number multiple of the time slot length for each pinger. classifying the plurality of pingers into a first group whose acoustic pulses are modulated by a first coding scheme corresponding to a code division multiplexing scheme and a second group whose acoustic pulses are modulated by a second coding scheme;
Each pinger is
a means for synchronizing the clocks of each pinger for each group;
means for time-division multiplexing the time slots of each pinger for each group based on the time of said clock;
10. The acoustic positioning system according to claim 9, wherein a time slot period set for each pinger is different for each group. The acoustic positioning system according to claim 11, characterized in that the time slot period set for each pinger is set to a different prime number multiple of the time slot length for each group. classifying the plurality of pingers into a first group whose acoustic pulses are modulated by a first coding scheme corresponding to a code division multiplexing scheme and a second group whose acoustic pulses are modulated by a second coding scheme;
Each pinger is
a means for selecting one pinger from each group and pairing them to set the clock for each pair;
means for time-division multiplexing the time slots of each pinger for each pair based on the time of said clock;
10. The acoustic positioning system according to claim 9, wherein a time slot period set for each pinger is different for each pair. The acoustic positioning system according to claim 13, characterized in that the time slot period set for each pinger is set to a different prime number multiple of the time slot length for each pair. The acoustic positioning system according to any one of claims 11 to 14, characterized in that the first and second encoding methods are upstream chirp and downstream chirp, respectively. each pinger includes a pressure sensor for determining depth;
The transmitting means transmits a positioning pulse and a depth pulse in that order for each time slot;
15. The acoustic positioning system according to claim 9, wherein an interval between the positioning pulse and the depth pulse is variable depending on a result of depth positioning.

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