MINIMIZING THE LATENCY OF A TARGET'S RESPONSE TO SONA RELEASE

Description

DOMAINE TECHNIQUE

[0001] This description relates to a target for anti-submarine warfare training.

[0002] It applies in particular to minimizing the latency of the response of such a target to a sonar emission. CONTEXTE

[0003] A crucial step in anti-submarine warfare is to detect the presence and, if possible, the trajectory of underwater vessels using sonar.

[0004] One difficulty is that submarines have varying sonar signatures, which designers seek to reduce to make them as undetectable as possible. Consequently, the signal-to-noise ratio of a sonar echo can be quite low due to ambient noise, including that generated by the ship carrying the sonar and responsible for detecting submarines.

[0005] To improve the effectiveness of this anti-submarine warfare, personnel must be highly trained in the operation of detection equipment and acquire substantial experience. Furthermore, the detection devices themselves (sonar and digital processing software) must be continuously adapted and improved to ensure the required performance levels.

[0006] These current underwater targets are generally autonomous underwater vehicles that can be programmed or remotely piloted to perform a mission and include devices to simulate a response equivalent to that of a conventional submarine upon receiving a sonar signal. Indeed, being substantially smaller than a conventional submarine (which can carry personnel), they cannot provide a sufficient passive response for detection, or, in any case, their response is very weak compared to such a submarine.

[0007] The targets therefore have devices allowing them to simulate the response using an acoustic emitter.

[0008] Some targets are equipped with an acoustic transmitter and receiver positioned on the hull.

[0009] If the transmitter activates while the target is recording the acoustic signal, the signal is disrupted by retransmission. Therefore, according to current best practices, the transmitter and receiver do not operate simultaneously: when a sonar transmission reaches the target, the receiver records the entire transmission. When it detects the end of the transmission, it retransmits the recorded signal to simulate a sonar echo as it would have been obtained from a conventionally sized submarine.

[0010] However, such targets pose problems because they exhibit excessive latency between the emission of a signal by the detection device and its reception. Indeed, since the target must wait for the entire signal to be received before it can re-emit it, the reception of the re-emitted signal by the detector is delayed accordingly. Furthermore, the trend in modern sonars is to use increasingly longer signals. Yet, detectors determine the distance to a detected target based on the time it takes for the emitted signal to return. Therefore, the estimation of the target's distance will be inaccurate. Such a target thus does not allow for effective training of personnel responsible for detecting underwater vehicles. MURPHY STEFAN M ET AL: "Experimental Implementation of an Echo Repeater for Continuous Active Sonar", IEEE JOURNAL OF OCEANIC ENGINEERING, IEEE SERVICE CENTER, PISCATAWAY, NJ, US, vol. 42, no.Document 2, April 1, 2017 (2017-04-01), pages 289-297, XP011645600, proposes a solution to improve latency. This document describes a solution in which the vessel (i.e., the detector) attempting to detect the target is equipped with a sonar transmitting on multiple bands: a primary band on which it listens for the echo for detection, and secondary bands which, in the case of ping-pong mode, are used to transmit a synchronization signal. The synchronization signal is transmitted entirely before the primary band signal. It is an out-of-band signal used to activate the recording and subsequent transmission of the primary band acoustic signals by the target.

[0011] Other types of targets have a transmitter on the hull and a towed antenna. This arrangement allows for simultaneous analysis of the received signal and transmission of a response.

[0012] When an acoustic signal reaches the target, it records, analyzes, and then responds continuously throughout the reception of the incoming signal. As soon as the response is calculated for a portion of the signal, it is emitted without waiting for the end of the entire acoustic signal.

[0013] But such a solution only partially solves the latency problem: the latency time no longer depends on the duration of the signal, but on the time it takes to calculate the response.

[0014] In particular, minimizing latency time, which distorts the estimation of the distance between the target and the detection device, implies a strong constraint on the computing power to be carried on the target and on the precision of the filtering operations (size of FIR filters...).

[0015] Furthermore, the addition of a towed antenna is also a very important constraint: Deployment and recovery of the target is more complex; the drag generated by the antenna prevents high speeds from being reached and greatly reduces the target's autonomy; adding an antenna represents a significant cost.

[0016] State-of-the-art training targets therefore have various drawbacks that hinder their performance and effective use. RESUME

[0017] One objective of this description is to provide a device for underwater targets that at least partially overcomes the aforementioned drawbacks. In particular, the aim is to minimize the latency of a training underwater target's response to an acoustic emission, especially sonar.

[0018] More specifically, according to embodiments, it aims to provide a target that reduces this latency time compared to existing targets, while maintaining or improving the target's speed, lightness and manufacturing cost characteristics.

[0019] To this end, according to a first aspect, the present description can be implemented by an underwater target device comprising an acoustic receiver, an acoustic transmitter, and processing means adapted to perform a first cycle of operations comprising the detection within an acoustic stream captured by said acoustic receiver, of the beginning of a first acoustic signal corresponding to characteristics of at least one acoustic emitter distant from at least one detector; following the detection of the beginning of said first signal, the triggering of the recording of said first signal; the detection within said acoustic stream, of the end of said first signal; following the detection of the end of said first signal, the triggering of the stopping of said recording; then a second cycle of operations comprising: the detection within said acoustic stream, of the beginning of a second acoustic signal of the same nature as the first signal and corresponding to said characteristics; and following the detection of the beginning of said second signal, the triggering of the emission, via said acoustic emitter, of the first signal recorded during the first cycle of operations.

[0020] Depending on the embodiment, the device or method includes one or more of the following features, which may be used separately, in partial combination, or in total combination: said second cycle is iterated several times, preferably 5 times; said processing means are adapted to detect first and second signals corresponding to characteristics of a plurality of acoustic receivers distant from distinct detectors; said processing means comprise an analog-to-digital converter to convert said acoustic stream into a series of digital samples, digital circuits to process said series, a digital-to-analog converter to convert said series into an analog signal and an amplifier to amplify said analog signal before its emission via said acoustic emitter; said digital circuits are adapted to transform said digital samples in the frequency domain and to detect a signal start based on a comparison of a power for each frequency to at least one threshold;said digital circuits are adapted to perform a first decimation of said series, by a first constant factor, then, to perform a second decimation of said series, by a factor depending on a bandwidth corresponding to said characteristics of said at least one remote acoustic emitter;

[0021] According to a second aspect, an underwater target is described which includes a device as previously defined.

[0022] According to another aspect, a system is described which includes at least one underwater target as previously defined and at least one detector.

[0023] According to another aspect, a method is described intended to be implemented by an underwater target comprising an acoustic receiver and an acoustic transmitter, said method comprising a first cycle of operations including: the detection within an acoustic stream captured by said acoustic receiver, of the beginning of a first acoustic signal corresponding to characteristics of at least one acoustic emitter distant from at least one detector; following the detection of the beginning of said first signal, the triggering of the recording of said first signal; the detection within said acoustic stream, of the end of said first signal; following the detection of the end of said first signal, the triggering of the stopping of said recording; then a second cycle of operations comprising: the detection within said acoustic stream, of the beginning of a second acoustic signal of the same nature as the first signal corresponding to said characteristics; and following the detection of the beginning of said second signal, the triggering of the emission, via said acoustic emitter, of the first signal recorded during the first cycle of operations.

[0024] According to another aspect, a computer program is also described which includes instructions which, once loaded into a memory of a processor embedded in an underwater target, allow the implementation of a process as previously defined.

[0025] The device and method described in this document require no specific detector programming, as they do not use any specific signal or signal processing technology. Therefore, the solution is compatible with the use of standard detectors—that is, the same detectors used in operation. The solution described here offers great flexibility in its use.

[0026] Other features and advantages will become apparent upon reading the following description of an embodiment, given by way of non-limiting example and with reference to the attached drawings. BREVE DESCRIPTION DES FIGURES

[0027] The attached drawings illustrate a device and method as described in this document: There figure 1 schematically represents an example of a context in which embodiments can be implemented. figure 2 illustrates a schematic state machine according to one embodiment. figure 3 illustrates an example of how processing means function in one embodiment. figure 4 illustrates a possible functional architecture for the processing means in one embodiment. figure 5 illustrates a possible functional architecture for digital circuits in one embodiment. DESCRIPTION DETAILLEE DE MODES DE REALISATION

[0028] A training target can be an autonomous underwater vehicle. These targets can be programmable and / or remotely piloted by an operator. The target is designed to be able to capture acoustic signals of the type used by active sonar detectors, simulate a response corresponding to a conventional underwater vehicle, and transmit it for detection by the detectors (sonars).

[0029] It should be noted that a training target is typically substantially smaller. This is because it does not need to carry personnel and must be less expensive in terms of manufacturing and operating costs. Furthermore, the response of such a target is inherently very different from that of a conventional underwater vehicle and does not allow for personnel training.

[0030] The acoustic detector is typically a sonar (acronym derived from English) « sound navigation and ranging » This device uses the specific properties of sound propagation in water to detect and locate underwater objects, indicating their direction and distance. Specifically, for the purposes of this description, an active sonar may be used; that is, one that emits an acoustic signal and then analyzes the response, or "echo," of any potential targets in the aquatic environment.

[0031] Analyzing these responses requires both appropriate technical means and human resources trained in the use of these technical means and in the analysis of the responses as presented by the technical means.

[0032] The training targets allow personnel to be trained both in the use of detection tools in near-real-life conditions and in the interpretation of the results provided by these tools. The goal is to both detect all underwater targets (minimizing false negatives) and avoid false detections (minimizing false positives).

[0033] An example of a training target is the SEMA MK-II target from RTsys. It simulates the acoustic signature of a submarine, responds to active sonars, including the latest generation TBF1 sonars and torpedo seekers. It can descend to a depth of 300 meters and navigate for approximately 1.5 hours at 15 knots. This allows it to immerse a training crew in a realistic operational scenario and surprise them with kinematic maneuvers.

[0034] It has a length of approximately 2 meters and a weight of approximately 33 kilograms, which allows it to be handled quite easily, deployed at sea and recovered from any type of vessel (from a Zodiac to a frigate).

[0035] There figure 1 presents a general context in which a device or method such as those described in this document may be used.

[0036] An underwater target 1 is deployed and operational. Operators (military or civilian personnel) use a detector 2, such as an active sonar, to attempt to detect the underwater target 1.

[0037] Detector 2 includes a remote acoustic emitter 21 adapted to emit an acoustic signal with certain characteristics. These characteristics include, in particular, the characterization of a frequency band. The emission can also vary in power, duration, etc.

[0038] Generally, a detector emits a signal repeatedly to track the behavior (particularly the movement) of underwater vehicles and maximize detection rates. Sometimes, certain signals do not provide sufficient echoes for detection, making redundancy crucial. This involves transmitting multiple signals over time to maximize the probability of receiving an echo. Analyzing the different responses obtained over time also provides additional information such as the direction and speed of movement.

[0039] The underwater target includes an acoustic receiver 11 adapted to capture an acoustic stream 31.

[0040] This acoustic stream 31 can include the acoustic signal 30 emitted by the remote transmitter 21 with attenuation and distortions related to the distance and relative velocities of the target 1 and the detector 2. If these are too far apart, the acoustic stream will no longer contain detectable signals from the remote transmitter 21.

[0041] In some embodiments, several detectors may be provided, including several active detectors (sonars). In which case, the acoustic stream 31 captured by the acoustic receiver 11 may comprise different signals emitted by these detectors.

[0042] Target 1 also contains processing means 10 adapted to analyze the acoustic stream captured by the acoustic receiver 11, and, if necessary, trigger the emission of an acoustic response 32 via an acoustic emitter 16.

[0043] This acoustic response 32 is determined so as to form an echo similar to that which a submersible vehicle would have provided passively (i.e., by simple reverberation of the acoustic signal 31 on the hull). This emitted response 32 is therefore of similar form (in frequency and duration) to the captured signal 31, but the levels cannot be compared because they have different units (dBre1µPa 2< for the received level and dBre1µPa 2< m 2< for the re-emitted level).

[0044] The level of amplification at the output can depend in particular on the type of submarine that one wishes to simulate.

[0045] In particular, the processing means 10 are adapted to detect within the acoustic stream 31 received by the acoustic receiver 11 the beginning of a first signal which corresponds to characteristics of a distant acoustic emitter 21.

[0046] Here, "corresponding to the characteristics of a distant acoustic emitter" means that the detector seeks a specific signal that matches the one emitted by an acoustic emitter, such as a sonar. The goal is to search for such a signal while excluding other types of signals that could be received by the receiver, which may be of natural or artificial origin (noise from the target's engine, noise related to the target's movement in the aquatic environment, etc.).

[0047] In addition, the characteristics of the remote acoustic emitter 21 may include the frequency band used by it, so that the processing means 10 can be adapted to seek to detect the onset of signal in this frequency band.

[0048] In the case where several distant acoustic emitters are planned, the processing means 10 can be adapted to seek to detect the beginnings of signals in the respective frequency bands.

[0049] There figure 2 illustrates a schematic state machine which, according to one embodiment, can represent the operation of the processing means 10.

[0050] State S1 corresponds to this state of detection of a signal beginning in a captured acoustic stream.

[0051] When a signal start is detected, the T12 transition is triggered and the processing means 10 switch to an S2 state.

[0052] In this S2 state, the recording of the detected signal is activated. The signal is recorded in a memory embedded in the underwater target 1.

[0053] In this same state S2, the processing means 10 seek to detect the end of this signal within the acoustic flow 31.

[0054] The detection of the end of this signal triggers the T23 transition, causing the detection means to switch to an S3 state.

[0055] States S1 and S2 form the first cycle of operations aimed at capturing and recording the signal from a detector. During this cycle, the target does not emit a response.

[0056] States S3 and S4 constitute a second cycle of operations aimed at sending a response to the remote detector. During this cycle, the target no longer analyzes the acoustic stream reaching receiver 11.

[0057] In state S3, the processing means 10 seek to detect, within the acoustic flow 31, the beginning of a second signal corresponding to the characteristics of a distant acoustic emitter.

[0058] This stage is similar to that of state S1, and the detection methods essentially seek to detect a signal of the same nature. The terms "first" and "second" are used solely to distinguish them in the following explanation and relate only to the use made of their detection by the processing methods.

[0059] The T34 transition corresponds to the detection of the beginning of a (second) signal matching the characteristics of a distant acoustic emitter. These characteristics correspond to those of the first signal and are as previously described.

[0060] This detected signal beginning (leading to the validation of this T34 transition), the processing means switch to an S4 state.

[0061] In one embodiment, the processing means can detect a signal different from the signal already recorded. In which case, the processing means return to state S2 (validation of the T32 transition) and record the new signal in place of the old one, then wait, in state S3, for the repetition of this signal.

[0062] In state S4, the processing means may no longer analyze the received acoustic stream 31 and trigger the emission of the first signal memorized during the first cycle of operation, and, more precisely in state S2 of the processing means.

[0063] To achieve this, the processing means 10 transmit the stored signal to the acoustic emitter 16, which transmits an acoustic stream 32 into the aquatic environment. This signal corresponds to a simulated echo of the signal emitted by the acoustic emitter 21 located away from the detector 2. The detector also includes a receiver 22 adapted to receive the acoustic streams and analyze them to detect such a signal 32. Thus, the detector 2 can detect the presence of a simulated underwater vehicle represented by the underwater target.

[0064] This second cycle of operation can be repeated a predefined number of times.

[0065] In a limiting case, it is iterated only once; that is, following the re-emission of a stored signal, the processing means validate the T41 transition and enter state S1, corresponding to the first operational cycle. In such a case, the target stores one signal out of every two and emits a stored signal every two signal detections.

[0066] In a preferred case, the second cycle is repeated 5 times, before a new first cycle of operations is triggered again.

[0067] According to one embodiment, in state S4, the detection means 10 increment a counter. When the stored signal is transmitted, this counter is tested. If it is less than a predefined number (for example, 5), the transition T43 is enabled and the processing means return to state S3 to wait for the start of a new second signal. If the predefined number is reached, then the transition T41 is enabled and the processing means return to state S1 to wait for the start of a first signal (for its storage).

[0068] The periodic triggering of the first cycle of operations allows the received signal from detector 2 to be refreshed, because as it can evolve over time, it is important to maintain the adequacy of the simulated echo to it. A contrario, Too many refreshes of this signal would mechanically generate a smaller number of simulated echo transmissions and would impair the detection of this echo by detector 2. The number of 5 re-emissions of the stored signal before refresh by new storage represents an optimal compromise determined by experimentation.

[0069] There figure 3 This illustrates an example of how processing means function, focusing on a reception activity (AR), a memorization activity (AM), and an emission activity (AE). figure 3 illustrates three chronograms corresponding to these activities taking place in parallel, the horizontal axes representing time t.

[0070] The AR reception activity represents the incoming acoustic stream, that is, captured by the receiver 11 and provided to the processing means 10 for analysis. The AM storage activity represents the storage of a (first) signal detected by the acquisition means (in step S2 in the example of the figure 2 ). The emission activity AE represents the transmission of a stored signal towards the acoustic emitter 16 (in step S4 in the example of the figure 2 ).

[0071] It is assumed that initially the processing means have not yet detected any acoustic signals and are therefore in a state corresponding to state S1 of the example of the figure 2 .

[0072] At time t1, a signal begins to be received. The processing means 10 analyze the incoming acoustic stream and detect the beginning of this signal, triggering its storage at time t1 + δ. The duration δ corresponds to the time required for the processing means to analyze the incoming acoustic stream and detect the beginning of the signal.

[0073] The processing means analyze the incoming stream to detect the end of the signal and interrupt the memorization.

[0074] At that point, the first cycle of operations ends and the second cycle of operations begins.

[0075] At time t 2, a new signal is received. At time t 2 +δ (we assume that the latency time δ is constant, for clarity of explanation), the processing means 10 begin to emit the stored signal via the acoustic emitter 16.

[0076] In this example, the second cycle of operations is iterated 5 times.

[0077] Similarly, at times t3, t4, t5, t6, new signals are received. At times t3 +δ, t4 +δ, t5 +δ, t6 +δ, the processing means 10 begin to emit the stored signal via the acoustic emitter 16.

[0078] At time t 7, a new signal is received. But the number of iterations planned for the second cycle of operations being reached, a new first cycle of operations begins.

[0079] The processing means 10 analyze the incoming acoustic flow and detect the beginning of this signal and trigger its memorization at time t 7 +δ.

[0080] The process can continue with a new second cycle of operations which can be iterated 5 times again and so on.

[0081] We therefore alternate between first cycles during which the target captures the acoustic streams and does not itself emit any acoustic signal, and second cycles during which, on the contrary, it emits an acoustic response (simulating an echo of a signal previously detected within the acoustic streams) but does not analyze the acoustic streams, nor does it memorize the received signals.

[0082] Thus, in the example illustrated by the figure 3 , we note that out of 6 signals emitted by detector 2 and received by target 1, five responses are emitted by the latter and likely to be captured by detector 2.

[0083] This principle of not seeking to analyze all received acoustic streams, and of transmitting a simulated echo of a previously received signal when receiving a new signal makes it possible to significantly improve performance compared to state-of-the-art solutions.

[0084] Through experimentation, the inventors determined that not responding to all received signals (those corresponding to the first cycle) does not cause any major drawbacks. Indeed, under real-world conditions, propagation conditions in the aquatic environment are such that, in any case, a significant portion of sonar echoes are not received or detected by sonars.

[0085] There figure 4 illustrates a possible functional architecture for processing means 10.

[0086] According to this embodiment, the processing means 10 include an analog-to-digital converter 12 for converting the acoustic stream from the acoustic receiver 11 into a series of digital samples, digital circuits 13 for processing this series, and a digital-to-analog converter 14 for converting this series into an analog signal for transmission via the acoustic transmitter 16. The processing means 10 may further include an amplifier 15 for amplifying the analog signal before its transmission via the transmitter 16.

[0087] According to one embodiment, these processing means 10 can be implemented by an electronic board adapted to be carried on board the underwater training target. This therefore includes a watertight housing in which this board can be carried.

[0088] This electronic card can perform the following functions: to ensure the analog-to-digital conversion of the signal captured by the acoustic receiver; to provide the computing and memory resources necessary for the processing algorithm, such as, for example, the one described above with references to figures 2 And 3 ; perform the digital-to-analog conversion of the response.

[0089] The signal can then be transmitted to the voltage amplifier 15 for the acoustic emitter 16.

[0090] According to one embodiment, the electronic board may include an analog-to-digital converter operating at 10 MHz. The data is then processed by the digital circuits 13.

[0091] According to one embodiment, the digital circuits may include a processor associated with a memory storing software instructions to implement the signal processing algorithm that will be described.

[0092] According to one embodiment, as illustrated by the figure 5 , the digital circuits 13 may include a programmable logic circuit 131, or more specifically an in-situ programmable gate network or FPGA (for « Field-programmable Gate Area » in English), and a digital signal processing unit 132, or DSP (for " Digital Signal Processor » (in English). The DSP processor can be in charge of implementing the algorithm described above, which calculates the target's response.

[0093] The programmable logic circuit can, in one embodiment, perform a preliminary decimation step combined with anti-aliasing filtering. This decimation can be performed by a constant and predetermined factor. For example, a factor of 128 can be used.

[0094] According to one embodiment, the data is transmitted from the programmable logic circuit 131 to the digital signal processing unit 132 in blocks of constant size. This size can, for example, be 128 samples.

[0095] Receiving a block triggers a new analysis cycle by the digital signal processing unit 132. At each analysis cycle, the processor 132 determines the presence or absence of a sonar signal. A key constraint is that this analysis must be performed before a new block is received to ensure real-time analysis of the acoustic stream 31 and minimize the latency δ.

[0096] The response calculated by the digital circuits 13 can then be transformed by the digital-to-analog converter.

[0097] According to a preferred embodiment, this electronic card is dedicated to the processing algorithm, which is therefore not interrupted by concurrent processes.

[0098] Furthermore, the DSP 132 processor features optimized signal processing functions and can therefore ensure that a data block is processed before the next block is recorded (real-time). In one embodiment, a data block consists of 128 samples and thus corresponds to 1.7 ms.

[0099] The algorithm primarily fulfills two functions: detect the beginning and end of a signal within an acoustic flow, and thus enable, in particular, its recording; and, manage the response logic, that is to say, order the cycles of recording a detected signal and of emitting a recorded signal.

[0100] In particular, according to one embodiment, a first step consists of isolating the frequency band in which the signal to be detected is located.

[0101] Indeed, the target (or more precisely, the target processing methods) can be configured according to the detection device used for its detection. Thus, the target can adapt to the equipment and conditions set up for personnel training. Each detection device can operate at a specific frequency for emitting acoustic signals, and in order to best simulate an appropriate response, the target may need to know this specific frequency, as well as any other parameters characterizing the signal.

[0102] According to one embodiment, the target can simultaneously process several signals from detectors 2 with non-overlapping frequency bands.

[0103] Indeed, to optimize the detection of underwater vehicles, it is advantageous to deploy multiple detectors. For example, in the case of a fleet of ships, each vessel can be equipped with a sonar detector.

[0104] In such situations, the target can provide simulated echoes for each of the operating sonar detectors.

[0105] The user of the training target can configure the frequency bands corresponding to these different sonar detectors. In particular, a number of detectors, N channels, can be defined, as well as for each frequency band, or channel, corresponding to a detector, a center frequency fc and a bandwidth bw.

[0106] The processing means (and in particular the processor 132 according to one embodiment) are adapted to detect first and second signals corresponding to characteristics of a plurality of acoustic receivers 21 distant from distinct detectors 2. They are adapted to carry out these analyses for each channel in the time imposed by the continuous reception of the data blocks transmitted by the programmable logic circuit 131.

[0107] The received data stream is analyzed to isolate data that may correspond to signals matching the characteristics of the remote acoustic transmitter(s). Specifically, in one embodiment, this involves detecting the beginning (and then the end) of a sonar-type signal within the received data stream.

[0108] According to one embodiment, a frequency analysis can be performed since the center frequency and bandwidth are known for each remote acoustic emitter.

[0109] Thus, for example, as soon as a block of samples is received, for each channel (i.e., remote acoustic emitter), the samples are modulated by a complex sinusoidal signal of frequency -fc in order to center the useful frequency band on zero. This yields N channels of modulated signals that can then be processed in parallel.

[0110] For example, an acoustic stream can contain signals from 3 distinct acoustic emitters. A first acoustic emitter emits sonar signals in a central frequency band fc1 and width bw1, a second acoustic emitter emits sonar signals in a central frequency band fc2 and width bw2 and the third acoustic emitter emits sonar signals in a central frequency band fc3 and width bw3.

[0111] The modulated signals corresponding to each frequency band are then decimated and filtered by a polyphase filter.

[0112] Decimation involves subsampling the incoming samples to retain only a portion of them. A polyphase filter is a filter implemented after decimation. Its purpose is to prevent spectral aliasing associated with decimation, and it is applied afterward to reduce computation and improve efficiency.

[0113] Polyphase filters are well known to those skilled in the art and explained in the literature. For example, one can refer to the following Wikipedia page: https: / / en.wikipedia.org / wiki / Polyphase_quadrature_filter

[0114] This is typically a finite impulse response (FIR) low-pass filter. Finite impulse response (FIR) filters, or FIR filters (for " Finite Impulse Response » In English, these are filters whose impulse response has a finite duration. A digital FIR filter is characterized by a response based solely on a finite number of input signal values. Therefore, regardless of the filter, its impulse response will be stable and of finite duration, dependent on the number of filter coefficients. The terms "non-recursive filter" or "moving average filter" are sometimes used to refer to the same class of filters, although the term "moving average filter" primarily refers to low-pass filters.

[0115] The decimation factor depends on the bandwidth. It follows that the wider the bandwidth, the less decimation occurs due to the anti-aliasing low-pass filter. Thus, the wider the bandwidth, the lower the decimation factor.

[0116] One of the advantages of this decimation is the ability to reduce the volume of data to be processed and stored in subsequent steps. Indeed, as with any embedded system, memory capacity is a significant constraint for the underwater target. Therefore, in order to store the signal to be echoed by the target, it is crucial that this signal occupies as little space as possible. This is all the more important when multiple channels must be managed, corresponding to as many remote detectors. The decimation factor is thus a compromise between the need to conserve memory and the requirement to avoid losing the frequency information of the processed signals, especially for wide frequency bands.

[0117] The acoustic streams received by the acoustic receiver 11 include noise components and (possibly) signals emitted by the remote transmitters 21.

[0118] With each new block of data received, for each frequency band, the modulated and filtered acoustic streams are analyzed over a specific period. The analysis focuses on the most recently recorded samples. The algorithm calculates the average signal power and compares this power to a detection threshold.

[0119] If, within a frequency band, the average signal power over the analysis period exceeds the detection threshold, the algorithm treats the signal from that frequency band as coming from a distant acoustic emitter: either the processing means are in a first cycle of operations, and the signal has been stored in a memory associated with the processing means 10; or the processing means are in a second cycle of operations and the previously stored signal must be emitted.

[0120] Due to the decimation explained previously, a stored signal occupies a reduced data volume corresponding to the constraints of embedded systems.

[0121] The detection threshold is chosen to be high enough so that noise components do not generate false detections so that only signals emitted by remote transmitters 21 generate detections.

[0122] The transmission may include processing symmetric to that which was applied between the reception of the acoustic stream and its storage in the on-board memory.

[0123] In particular, according to some embodiments, the signal can be interpolated to restore the initial sampling before decimation.

[0124] Each of the channels can be modulated by a complex sinusoidal signal of the respective center frequency fc, with a polyphase filter, in order to reconstruct the original signal.

[0125] The signal can then be transmitted to the digital-to-analog converter 14, and then, possibly, to an amplifier 15 in order to give it the amplitude corresponding to a realistic echo and which can be perceived by the distant acoustic receiver 22.

[0126] The signal can then be emitted by an acoustic transmitter 16. This acoustic signal propagates, in a conventional way, in the aquatic environment and can be received by a distant acoustic receiver 22, belonging to detector 2. The detector can thus detect the presence of the training target and interpret it as an underwater vehicle.

[0127] The solution described here allows for a low latency, such that the time between the emission of a signal by the remote acoustic transmitter 21 and the reception of its simulated echo by the remote acoustic receiver 22 provides a very good approximation of the time it would have taken for the transmission of the signal and its echo from a real underwater vehicle. Consequently, a good estimate of the distance between the underwater target 1 and the detector 2 can be deduced from this time. This good estimate allows the users of detector 2 to accurately track the position and movements of the training target simulating an underwater vehicle.

[0128] Of course, the invention is not limited to the examples and embodiments described and illustrated, but is defined by the claims. In particular, it is susceptible to numerous variations accessible to those skilled in the art.

Claims

1. Device for a submarine target (1)comprising an acoustic receiver (11), an acoustic transmitter (16) and processing means (10) suitable for carrying out a first cycle of operations including - detecting, within an acoustic stream (31) captured by said acoustic receiver, the onset of a first acoustic signal corresponding to characteristics of at least one remote acoustic transmitter (21) of at least one detector (2); - following the detection of the onset of said first signal, triggering recording of said first signal; - detecting, within said acoustic stream, the end of said first signal; - following the detection of the end of said first signal, triggering stopping of said recording; followed by a second cycle of operations including: - detecting, within said acoustic stream, the onset of a second acoustic signal of the same kind as the first signal and corresponding to said characteristics; and following the detection of the onset of said second signal, triggering transmission, by means of said acoustic transmitter (16), of the first signal recorded during the first cycle of operations.

2. Device according to the preceding claim, wherein said second cycle is iterated several times, preferably 5 times.

3. Device according to either of the preceding claims, wherein said processing means are suitable for detecting first and second signals corresponding to characteristics of a plurality of remote acoustic receivers (21) of separate detectors.

4. Device according to any of the preceding claims, wherein said processing means comprise an analog-to-digital converter (12) for converting said acoustic stream into a series of digital samples, digital circuits (13) for processing said series, a digital-to-analog converter (14) for converting said series into an analog signal, and an amplifier (15) for amplifying said analog signal prior to its transmission by means of said acoustic transmitter (16).

5. Device according to the preceding claim, wherein said digital circuits are suitable for transforming said digital samples into the frequency domain and for detecting a signal onset according to a comparison between a power for each frequency and at least one threshold.

6. Device according to any of claims 3 to 5, wherein said digital circuits are suitable for: - carrying out a first decimation of said series, by a first constant factor, - then, carrying out a second decimation of said series, by a factor depending on a bandwidth corresponding to said characteristics of said at least one remote acoustic transmitter (21).

7. Submarine target, comprising a device according to any of the preceding claims.

8. System comprising at least one submarine target according to the preceding claim and at least one detector (2).

9. Method, implemented by a submarine target (1) comprising an acoustic receiver (11) and an acoustic transmitter (16), said method comprising a first cycle of operations including - detecting, within an acoustic stream (31) captured by said acoustic receiver, the onset of a first acoustic signal corresponding to characteristics of at least one remote acoustic transmitter (21) of at least one detector (2); - following the detection of the onset of said first signal, triggering recording of said first signal; - detecting, within said acoustic stream, the end of said first signal; - following the detection of the end of said first signal, triggering stopping of said recording; followed by a second cycle of operations including: - detecting, within said acoustic stream, the onset of a second acoustic signal of the same kind as the first signal and corresponding to said characteristics; and following the detection of the onset of said second signal, triggering transmission, by means of said acoustic transmitter (16), of the first signal recorded during the first cycle of operations.

10. Computer program comprising instructions which, when loaded onto a memory of a processor on board a submarine target comprising a device according to claim 1, enable a method according to the preceding claim to be implemented.