Method for multistatic sonar location

Abstract

Disclosed is a method for determining a position of a sonar transmitter for multistatic sonar location by means of a plurality of sonar receivers (22a, 22b, 22c, 22d) that are disposed in a distributed arrangement, the method involving the following steps: a) having the sonar receivers (22a, 22b, 22c, 22d) receive waterborne sound; b) detecting a sonar signal (26) in the waterborne sound, wherein the sonar signal (26) can be detected multiple times in the waterborne sound by each sonar receiver (22a, 22b, 22c, 22d) as a result of reflections; c) determining a plurality of possible positions of the sonar transmitter (24) on the basis of differences in the propagation time of the first received sonar signal to different sonar receivers (22a, 22b, 22c, 22d); d) validating the plurality of possible positions in order to determine the most probable actual position of the sonar transmitter (24).

Description

[0001] Methods for multistatic sonar location

[0002] Description

[0003] The invention relates to multistatic sonar location, in particular the estimation of the position of the sonar transmitter.

[0004] Multistatic sonar detection refers to a sonar detection method using a sonar transmitter that emits a sonar signal and a plurality of distributed sonar receivers, each equipped with an underwater transducer or multiple underwater transducers. The sonar signal can also be referred to as a transmitted pulse. The position of the reflecting object can be determined from the difference in the signal travel time of a reflection relative to the direct sound, the direction of incidence of the reflection at the receiver, and the position of the transmitter, provided the transmitter position is known.

[0005] In multistatic sonar positioning, a distinction is made between cooperative and non-cooperative multistatic positioning. In cooperative multistatic positioning, the transmitted pulse contains information about the transmission time and the transmitter's position during transmission. In non-cooperative multistatic positioning, this information must be estimated. Various degrees of accuracy are possible; for example, if the receiver's position is known, the travel time of the transmitted pulse can be determined from the transmitter's distance to the receiver, and vice versa.

[0006] Within the context of this revelation, uncooperative multistatic localization will be considered when reference is made to “multistatic localization”.

[0007] There are two problems with determining the transmitter's position. First, it's possible that not all sonar receivers can pick up the direct sound. For example, a receiver might be obstructed, meaning there's an obstacle between the transmitter and receiver that reflects the direct sound, preventing it from reaching the receiver. It's also possible, especially with inexpensive receivers like those used in sonar buoys, that the direct sound wasn't received at all or only partially due to dropouts or other defects (e.g., premature saturation). With incomplete reception, matching the known transmitted pulse to the received partial signal, for example using a matched filter, might not result in a sufficient match between the signals.

[0008] The object of the present invention is therefore to create an improved concept for multistatic sonar location.

[0009] The problem is solved by the subject matter of the independent patent claims. Further advantageous embodiments are the subject matter of the dependent patent claims.

[0010] Exemplary embodiments show a method for multistatic sonar detection using a plurality of distributed underwater sound receiving arrangements. An underwater sound receiving arrangement is also referred to as a sonar receiver. The method comprises the following steps a) to e). In step a), underwater sound is received by the underwater sound receiving arrangements from a (particularly single) transmitting underwater sound transducer ((sonar) transmitter). The underwater sound receiving arrangements can have one or more underwater sound transducers for receiving the underwater sound. The underwater sound receiving arrangements and / or the transmitting underwater sound transducer can be freely movable, i.e., not anchored, in a body of water, i.e., floating in or on the water.

[0011] In step b), a sonar signal is detected in the underwater sound current. The sonar signal can be detected multiple times per underwater sound current receiver due to reflections. The sonar signal is preferably known, allowing it to be detected, for example, using a matched filter. Further signal processing is preferably delayed until all underwater sound current receivers, or at least a predominant number of them, have received the sonar signal at least once, regardless of whether it is the direct sonar signal or a reflection. Typically, the first sonar signal received is the direct signal, as it travels the shortest possible distance to the receiver.However, it is possible that the receiver cannot receive the direct sound, for example, because an interfering object between the transmitter and receiver is obscuring the receiver and thus preventing reception of the direct sound, or because the receiving electronics cannot process the direct sound correctly at the time it arrives. In any case, it is ensured that the direct sound can no longer arrive. If further signal processing is started with the following step c), steps a) and b) can still be carried out simultaneously.

[0012] In step c), a multitude of possible positions of the sonar transmitter are determined based on the time-of-flight differences of the first received sonar signal to various underwater sound receivers. A sonar transmitter is defined as an underwater sound transducer or a plurality of underwater sound transducers that (collectively) transmit the sonar signal. As explained in step b), the first received sonar signal can be the direct sound or, if the receiver could not receive the direct sound, a reflection of the sonar signal. From the known positions of the underwater sound receivers and the different reception times of the sonar signal, possible positions of the sonar transmitter can be determined. One method for determining the position of the sonar sensor is the use of Time Difference of Arrival (TDoA) methods. From the aforementioned data from three different underwater sound receivers, a possible position of the sonar transmitter can be determined, i.e.,The position of the sonar signal transmitter is determined. Different combinations of three receivers provide different positions of the transmitter.

[0013] In step d), the multitude of possible positions are then validated to determine the most probable actual position of the sonar transmitter. The multitude of possible positions should result in a cloud of positions that are approximately in the same location. There may be some outliers, for example, if a receiver did not receive the direct sound. In this case, the transmitter positions determined based on the receiver will deviate more from the transmitter's actual position than the positions determined based on receivers that did receive the direct sound. For validation, a position that most likely corresponds to the actual position of the sonar transmitter is then determined. Statistical methods can be used for this validation. For example, the center of gravity of the position cloud can be determined.Preferably, outliers are not taken into account when determining the center of gravity.

[0014] Validating the multitude of possible positions can further involve comparing these positions with known positions of the sonar transmitter to determine the most probable position. For example, if a sonar transmitter's position lies within the cloud of possible positions, the statistically determined position can be selected as the most probable position instead of the one obtained statistically. If no known position of a sonar transmitter is close to the multitude of possible positions, particularly after outliers have been eliminated, the statistically determined position can be used.

[0015] In an optional step e), a reflection point from a sonar signal is determined based on the time difference between the reflected sonar signal and the direct sonar signal, the validated position of the sonar transmitter, and the direction in which the reflected sonar signal is received. The direction in which the reflected sonar signal is received can be determined, for example, by beamforming or sound pressure gradient generation. Thus, classic multistatic sonar positioning can be performed using the validated position of the sonar transmitter.

[0016] The idea is therefore to first filter out the potential direct sound signals for each underwater sound receiver from the multitude of received sonar signals. By determining possible transmission positions of the sonar signal, received sound signals from the potential direct sound signals that do not prove to be direct sound can be actively or passively eliminated when determining the most probable actual transmission position. In active elimination, the underwater sound receivers (or at least the transmission positions determined based on these underwater sound receivers) are identified and excluded from determining the most probable actual transmission position. In passive elimination, the most probable actual transmission position is determined based on all possible transmission positions.Then the statistical method for determining the most probable actual transmission position is preferably chosen to be robust against outliers. For example, a median can be used instead of a mean, to name just one example.

[0017] In exemplary embodiments, the method according to step d) comprises the following steps m) and n), and optionally an adapted step e), which is then referred to as step e'). In step m), a virtual transmission time of the sonar signal is determined based on the various reception times and the validated position of the sonar transmitter. That is, a transmission time corresponding to the validated position is determined. This can also be done using statistical methods. For example, starting from the reception times of the direct sound for the receivers that received the direct sound, the transmission time for the validated transmitter position is determined. This is possible using the speed of sound underwater and the distance between transmitter and receiver. For example, the mean of all transmission times can now be used as the virtual transmission time.

[0018] Optionally, in step n), a virtual reception time of the direct sound of the sonar signal can be determined for at least one underwater sound receiving array. This virtual reception time can be determined at least for those underwater sound receiving arrays that did not receive the direct sound. That is, the virtual reception time is determined at least for the underwater sound receiving array(s) that did not receive the direct sound or received it only with insufficient quality. Thus, it is possible for these underwater sound receiving arrays to contribute to the detection of objects even if they did not receive the direct sound.

[0019] In the optional step e'), the reflection location of the sonar signal for the underwater sound receiving arrangement is determined based on a time difference of the reflection of the sonar signal to the direct sound of the sonar signal based on the virtual transmission time, the position of the sonar transmitter and the virtual reception time and a reception direction of the reflection of the sonar signal.

[0020] In further embodiments, the transmission of the known sonar signal with the sonar transmitter is part of the method in step z). In particular, step z) is performed before step a), or before the sonar signal is to be detected in step b).

[0021] Furthermore, a computer program is disclosed comprising instructions which, when the program is executed by a computer, cause it to perform the steps of the procedure based on received underwater sound signals.

[0022] Likewise, a receiving system comprising a plurality of underwater sound receiving arrangements and a data processing unit is disclosed, wherein the data processing unit is configured to execute the described method using the underwater sound received by the underwater sound receiving arrangements.

[0023] Preferred embodiments of the present invention are explained below with reference to the accompanying drawings. They show:

[0024] Fig. 1 : a schematic block representation of a receiving system for carrying out the multistatic sonar detection method.

[0025] Before exemplary embodiments of the present invention are explained in detail below with reference to the drawings, it should be noted that identical, functionally equivalent, or equivalently acting elements, objects, and / or structures in the different figures are provided with the same reference numerals, so that the description of these elements shown in different exemplary embodiments is interchangeable and can be applied to one another. Fig. 1 shows a schematic block diagram of a receiving system 20. The receiving system 20 comprises a plurality of underwater sound receiver assemblies 22, of which four underwater sound receiver assemblies 22a, 22b, 22c, and 22d are shown here by way of example. Optionally, the receiving system 20 comprises a sonar transmitter 24 with one or more underwater sound transducers for emitting a (known) sonar signal 26. A plurality of propagation paths of the sonar signal 26 originating from the sonar transmitter 24 are shown.The sonar signal 26 reaches the corresponding underwater sound receivers 22a, 22b, and 22c as direct sound along propagation paths d1, d2, and d3. However, the direct sound d4 to underwater sound receiver 22d is interrupted by an obstruction 28, so that underwater sound receiver 22d does not receive any direct sound. Alternatively, underwater sound receiver 22d may also be malfunctioning, preventing it from receiving the direct sound.

[0026] On propagation paths r1, r2, r3 and r4, the sonar signal 26 is reflected from an object 30 to the underwater sound receivers as a reflection 26'. For simplification, only one propagation path r1 / 2 / 3 / 4 between the sonar transmitter 24 and the underwater sound receivers 22 is shown for all four propagation paths between object 30 and underwater sound receivers 22.

[0027] The received underwater sound, containing the sonar signal 26 multiple times due to reflections, is converted into underwater sound signals by one or more underwater sound transducers in the underwater sound receiving arrangements 22. These signals are then transmitted to a data processing unit 32. An external data processing unit 32 is shown as an example. This unit receives the underwater sound signals from the corresponding underwater sound receiving arrangements 22 via radio links f1, f2, f3, and f4. However, it is also possible for the data processing unit 32 to be housed in the same enclosure as one of the underwater sound receiving arrangements. Instead of radio links, other methods of data transmission, such as underwater communication, can also be used. In this case, the data processing unit 32 can also be located on a manned or unmanned underwater vehicle.Typically, the data processing unit 32 will be located on a manned or unmanned surface vessel, particularly a ship. The underwater sound receivers 22 can be integrated into sonar buoys. It is also possible to integrate the sonar transmitter 24 into a sonar buoy. Furthermore, it is possible to integrate the sonar transmitter 24 and an underwater sound receiver 22 into a single sonar buoy, or to use the sonar buoys as sonar transmitters 24. This allows the sonar signal to be transmitted alternately from the sonar buoys, and the reflections can be received by the transmitting buoy and the other sonar buoys. This means, for example, that active or passive DIFAR (Directional Frequency Analysis and Recording) buoys can be used.

[0028] The disclosed method ensures that all "reflection paths" are recognized as reflections and not erroneously as direct sound. This means that if there is an echo only in r4, the method ensures that object 30 is found, even if there is no echo in r1, r2, and r3. This also applies if the original direct sound d4 could not be received by the underwater sound receiver 22d. With known multistatic methods, object 30 would remain undetected.

[0029] The disclosed (underwater) sound transducers are designed for use underwater, particularly in the sea. The transducers can convert underwater sound into an electrical signal (e.g., voltage or current) corresponding to the sound pressure, the (received) underwater sound signal. Furthermore, it is possible for the transducers to convert an applied electrical voltage into underwater sound. The electrical voltage can follow a predefined pattern and then be referred to as the (transmitted) sonar signal, while the underwater sound resulting from the sonar signal to be transmitted is referred to as the (transmitted) sonar signal. Examples of sonar signals are a chirp (frequency-modulated signal) or, as a special case of the chirp, a sweep (linearly frequency-modulated signal). The transducers can therefore be used as underwater sound receivers and / or underwater sound transmitters.The transducers can be made of a piezoelectric material, such as a piezoceramic, to serve as the sensor material. A plurality of underwater transducers, or one or more underwater transducers in conjunction with a signal processing unit (i.e., a data processing unit), can be referred to as a sonar system. The transducers can be used for (active and / or passive) sonar (sound navigation and ranging). The transducers are preferably not suitable for, or are not used for, medical applications. Likewise, the transducers are preferably not used for, or are not suitable for, the ultrasonic testing of materials.

[0030] Although some aspects have been described in connection with a device, it is understood that these aspects also constitute a description of the corresponding process, so that a block or component of a device is also to be understood as a corresponding process step or as a feature of a process step. Similarly, aspects described in connection with or as a process step also constitute a description of a corresponding block, detail, or feature of a corresponding device.

[0031] Depending on the implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be carried out using a digital storage medium, for example, a magnetic or optical storage medium, on which electronically readable control signals, e.g., a computer program, are stored. These signals can interact with a programmable computer system (CPU and / or GPU) in such a way that the respective method is carried out. For this reason, the digital storage medium should be computer-readable. Embodiments can therefore include a data carrier that has electronically readable control signals capable of interacting with a programmable computer system in such a way that one of the methods described herein is carried out.

[0032] In some embodiments, a programmable logic device (for example, a field-programmable gate array, an FPGA) can be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field-programmable gate array can interact with a microprocessor to perform one of the methods described herein. Generally, in some embodiments, the methods are performed by any hardware device. This can be general-purpose hardware such as a computer processor (CPU) or a graphics processing unit (GPU), or hardware specific to the method, such as an ASIC. A distributed implementation on both CPU and GPU is also possible. The embodiments described above merely illustrate the principles of the present invention.It is understood that modifications and variations of the arrangements and details described herein will be obvious to other experts. Therefore, it is intended that the invention be limited only by the scope of protection of the following patent claims and not by the specific details presented herein by way of description and explanation of the exemplary embodiments.

[0033] Reference symbol list:

[0034] 20 receiving system

[0035] 22a, 22b, 22c, 22d Underwater sound receiver arrangement 24 Sonar transmitters

[0036] 26 Sonar signal

[0037] 28 Disturbance object

[0038] 30 (Reflection) Object

[0039] 32 Data processing unit d1, d2, d3, d4 direct propagation path of the sonar signal r1, r2, r3, r4 propagation path of the reflection of the sonar signal at the reflection object

Claims

Patent claims 1. Method for determining the position of a sonar transmitter for multistatic sonar positioning using a plurality of distributed underwater sound receivers (22a, 22b, 22c, 22d) comprising the following steps: a) receiving underwater sound with the underwater sound receivers (22a, 22b, 22c, 22d); b) detecting a sonar signal (26) from the sonar transmitter in the underwater sound, wherein the sonar signal (26) can be detected multiple times in the underwater sound by reflections for each underwater sound receiver (22a, 22b, 22c, 22d); c) determining a plurality of possible positions of the sonar transmitter (24) based on time-of-flight differences of the first received sonar signal to different underwater sound receivers (22a, 22b, 22c, 22d); d) Validating the multitude of possible positions to determine the most probable actual position of the sonar transmitter (24).

2. Method according to claim 1, wherein the determination of the plurality of possible positions of the sonar transmitter (24) is carried out using Time Difference of Arrival (TDoA) methods.

3. Method according to one of the preceding claims, comprising step e) determining a reflection location of a reflection (r1 , r2 , r3, r4) of the sonar signal based on a time-of-flight difference of the reflection (r1 , r2, r3, r4) of the sonar signal to the direct sound (d1 , d2, d3, d4) of the sonar signal (26), the validated position of the sonar transmitter (24) and a reception direction of the reflection of the sonar signal (26).

4. Method according to any of the preceding claims, wherein the method according to step d) comprises the following steps m) and n): m) Determining a virtual transmission time of the sonar signal (26) based on the various reception times and the validated position of the sonar transmitter (24); n) Calculating a virtual reception time of the direct sound (d1 , d2, d3, d4) of the sonar signal (26) for at least one underwater sound receiving arrangement (22a, 22b, 22c, 22d).

5. Method according to claim 4, comprising a step e') determining a reflection location of the reflection (r1 , r2 , r3, r4) of the sonar signal (26) for the underwater sound receiving arrangement (22a, 22b, 22c, 22d) based on a time-of-flight difference of the reflection of the sonar signal to the direct sound (D1 , d2, d3, d4) of the sonar signal (26) based on the virtual transmission time, the position of the sonar transmitter (24) and the virtual reception time and a reception direction of the reflection (r1 , r2, r3, r4) of the sonar signal.

6. Method according to claim 4 or 5, wherein the virtual transmission time and the virtual reception time for the underwater sound receiving arrangement(s) (22a, 22b, 22c, 22d) are determined when the underwater sound receiving arrangement has not received the direct sound (d1 , d2, d3, d4) or has received it only with insufficient quality.

7. Method according to one of the preceding claims, comprising step z) sending a known sonar signal (26) with the sonar transmitter (24).

8. Method according to one of the preceding claims, wherein validating the plurality of possible positions comprises comparing the positions with known positions of the sonar transmitter (24) to determine the most probable position of the sonar transmitter (24).

9. Method according to one of the preceding claims, wherein the underwater sound receiving arrangements (22a, 22b, 22c, 22d) of the plurality of underwater sound receiving arrangements are movably arranged in a body of water.

10. Method according to one of the preceding claims, wherein the sonar transmitter is movably arranged in a body of water.

11. Computer program comprising instructions which, when the program is executed by a computer, cause it to perform the steps of the method according to one of the preceding claims based on underwater sound signals from received underwater sound.

12. Receiving system (20) comprising a plurality of underwater sound receiving arrangements (22a, 22b, 22c, 22d) and a data processing unit (32), wherein the data processing unit (32) is configured to perform the method according to any one of claims 1 to 10.

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