WO2011117304A1

WO2011117304A1 - Discreet active proximity sonar for a submarine

Info

Publication number: WO2011117304A1
Application number: PCT/EP2011/054461
Authority: WO
Inventors: Laurent Kopp; Michel Eyries
Original assignee: Ixwaves Sarl
Priority date: 2010-03-23
Filing date: 2011-03-23
Publication date: 2011-09-29
Also published as: FR2958047A1

Abstract

The present invention relates to a discreet active proximity sonar for a submarine, the code of which is suited to the target to be avoided. The general principle is to use the natural signal transmission from the target, passively detected using a high-gain receiving antenna, so as to send a code constructed from said signal. The discreetness comes from the fact that the transmitted signal is similar to the noise present on board the targeted submarine and in that the latter will therefore have much trouble intercepting an external signal transmission that resembles the natural or radiated noise of said target submarine itself.

Description

SONAR FURTIVE ACTIVE PROXIMITY FOR SUBMARINE

The present invention relates to a stealth sonar for submarine more particularly adapted to the detection of near and mobile obstacles. Also part of the invention is a method of implementing such a sonar.

The collisions of a submarine with a "threat" such as another mobile, surface vessel upon return to the periscopic immersion of the submarine in question, or other submarine immersion are not rare, the term "threat" is used in the rest of the text to refer to all these kinds of motives. These collisions can be of particular gravity on the human level, and also on the political level (case of the ballistic submarines). The use of its passive sonar system to avoid a collision obviously assumes that the object to be avoided by the submarine is an acoustic source of a level sufficiently high to be detectable in the surrounding noise conditions encountered. However, the progress of naval architecture is such that the noise radiated by submarines or modern surface buildings can be very low.

In a proximity situation, the avoidance of a collision also supposes that this sound source is located in distance in time. However, with an entirely passive system, it is impossible to measure a distance in the front zone by curvature of the wavefront.

Some submarines have active sonar capable of covering all fields. However, the transmission modes so far used are recognizable and therefore favor interception by an opponent. This results in a strong reluctance, even doctrinaire, to the use of an active sonar, a reluctance that leads to accepting situations of doubt and ignoring the existence of evidence of the presence of a threat. risk of a collision. The use of an active near sonar by a submarine would be the only solution to avoid collisions. It would be acceptable only if the risk of interception by an adversary was extremely low, more precisely if this use brought an operational advantage in comparison to the only use of the passive sonar system. Now, with the current means, said risk of interception is real.

The signals conventionally used by an active sonar are generally designed to facilitate a Doppler insensitive detection (FMH code (Hyperbolically Modulated Frequency)) to improve the detection range in the ambient noise (long FM code) or in the reverberation (FP code ( Pure frequency) or PRN (Pseudo Random Noise)). These conventional signals are different from natural signals and are therefore relatively easy to intercept.

The present invention relates to an active stealth sonar proximity (for distances of the order of about 2km) for submarine to locate a potential threat or suspected to be dangerous, and to do it as discreetly as possible, sonar whose transmitting and receiving characteristics pose the least possible risk of interception by the threat.

The acoustic detection device active and stealth proximity for submarine according to the invention is a detection device comprising a transmitting device connected to a transmitting antenna and a receiver connected to a passive antenna, this device further comprising detection and localization means, and is characterized in that the receiver is connected to means for analyzing signals received from a potential threat and its immediate environment, and in that this device comprises means for generating the stealth signal connected to the receiver, these generation means being connected to memory and acoustic signal production circuits connected to a passive receiving antenna to produce a stealth signal from that perceived by this passive antenna. The method according to the invention is a method of acoustic detection active and stealthy proximity for submarine to locate a potential threat, and it is characterized in that it consists in collecting the ambient noise with the help of a passive gain receiving antenna, then emitting a stealth sonar signal constructed from this collected signal and to locate the threat and assess its relative radial velocity.

According to another characteristic of the invention, the ambient noise collected and from which the furtive sonar signal is formed mainly contains the noise radiated by the threat.

According to yet another characteristic of the invention, the ambient noise from which the stealth sonar signal is formed is a prerecorded noise.

According to yet another characteristic of the invention, the stealth sonar signal contains the noise radiated by a surface vessel or another selected submarine in a contextual database.

According to yet another characteristic of the invention, the stealth sonar signal contains a biomimetic signal selected from a contextual database. "Signal selected in a contextual database" means a signal chosen to be most plausible in the situation considered, for example if the zone in question is a fishing zone, a noise radiated by a trawler is chosen as the signal. .

The method of the invention, implemented on board an underwater vessel, consists of collecting on a high-gain passive reception antenna, the signal coming from the direction of the potential threat or suspected to be dangerous, then to issue a stealth sonar signal constructed from this received signal and to locate the threat.

The stealth comes from the fact that the signal emitted contains the noise radiated by the threat and ambient noise, and is therefore similar to the noise perceived constantly on board the threat. The threat will therefore have a lot it is hard to distinguish this external emission from the sound background that its sonars constantly perceive.

The present invention will be better understood on reading the detailed description of an embodiment, taken by way of nonlimiting example and illustrated by the appended drawing, in which:

FIG. 1 is a simplified diagram illustrating the provision, on a submarine, of an acoustic projector and a panoramic reception antenna, according to an implementation characteristic of the invention,

FIG. 2 is a simplified diagram illustrating the organization of the transmission chain, comprising a part related to the development of the emission code and a part intended for the effective transformation of this code into an acoustic signal transmitted in the water by the electroacoustic device, in accordance with an implementation characteristic of the invention, and

FIG. 3 is a simplified diagram illustrating the organization of the reception chain, comprising the processing intended to elaborate the transmission code from the signals received by the passive antenna, the elaboration of the reception parameters and the panoramic detection. echoes according to the invention.

The present invention is described below with reference to a sonar system equipping a submarine and intended to detect and locate, at least roughly, potential threats close (about 2km) may come from surface or submersible buildings.

The active sonar of the invention consists of a transmitter (shown in FIG. 2) connected to an omnidirectional or sector transmission antenna capable of transmitting a broadband signal (at least 3 kHz, for example). A Realistic example for this transmitter is the underwater "phone" (active sonar) installed aboard all submarines.

The receiver (shown schematically with its antenna and reception circuits in Figure 3) comprises a passive broadband antenna (compatible with that of emission), that of bow if one favors the deposits of the front. This receiver has a very large gain (more than 20 dB).

In the diagram of Figure 1, there is shown in a very simplified way the front of a submarine 1 equipped with an acoustic projector of an underwater transmission device 2, which is here a "phone" under and a receiving antenna, for example a standard cylindrical antenna 3, disposed in the bow of the submarine for passive watch purposes. The projector is usually part of an acoustic communication device and the cylindrical antenna is part of the passive sonar of the submarine. These two systems (acoustic communication and passive acoustic monitoring) are generally completely independent systems that the present invention combines in a manner described hereinafter.

The transmitting device 2 schematized in FIG. 2 essentially comprises: a database 4 of stealth signals to be transmitted, connected to a recording device 5 (for recording samples of acoustic signals), to a source 6 of biomimetic signals, to a source 7 of complex signals difficult to interpret by the threat (for example multi-state phase codes, frequency evasion codes or pseudorandom signals), to a source 8 of innocuous signals (for example white noise) and a pre-processing device 3A of signals from the receiver 3 (selection of a time slice, time reversal, spectral shift, ... signals from the receiver). In general, the devices 5 to 8 and 3A, produce signals likely to make the submarine 1 as stealthy as possible by avoiding attracting the attention of the threat by passing these signals as false alarms. The database 4 is followed by a signal selection circuit 9 and a transmission level adjustment acoustic transmitter 10 whose output is connected to an acoustic projector 11. An output of the circuit 9 and an input of triggering of the device 10 are connected to the receiver 3.

The receiver schematized in FIG. 3 essentially comprises: a high-gain passive reception base 12, connected to a dedicated channel formation device 3 (respecting the temporal integrity of the signal designated by the operator) and to a generic device 14 These two devices 13 and 14 are connected to a passive monitoring device 15. This receiver also comprises a Doppler copy base 16 connected to the transmitter device 2 and to an active standby device 17 receiving the triggering signals of the transmitter 10.

The method of the invention preferably consists in using as a furtive sonar emission signal the signal produced from that perceived by a passive antenna of the submarine and coming from the zone where a threat is suspected to be. This preferred implementation is a particular case of the more general method of the invention in which the chosen stealth signal is "adapted to the situation" in the broad sense of the term. This adaptation to the situation is achieved by exploiting the knowledge of the context, that is to say the knowledge of all the acoustic signals that can be perceived or likely to occur in the marine area surrounding the threat at the moment at which it is desired transmit said stealth signal. Non-limiting examples are described below.

The signal received by the passive antenna (FIG. 3) and estimated and subsequently processed is a broadband signal whose temporal structure has nothing to do with that of the conventional signals used by an active sonar, and which are easy to detect, as specified. upper. The transmitted signal used by the invention is a "natural" signal (or, more accurately, a brief excerpt from such a signal) much less likely to awaken the attention of the submarine operator representing the potential threat. which will easily confuse him with a whiff of clean noise or ambient noise.

It should be noted that this stealth sonar signal is developed in order to remove doubt about the presence and allow the localization of a nearby threat (in a radius of 2000m for example), especially following an index, possibly brief (transient), provided by a passive sonar of the submarine, without revealing its own presence, so as to create a crucial operational advantage in proximity.

The important point to consider is that the stealth sonar signal must be made "on the fly" (dynamically) in a context-appropriate way, that is, by imitating in a better way the signals normally perceived by the threat to the environment. moment of manufacture considered. The example mentioned here is that of a stealth sonar signal made from the signal received from the threat zone. This signal, containing the noise radiated by the threat, will be all the more difficult to interpret by the threat.

One can just as easily (for example if no signal is collected in passive but it is necessary to control the anticollision process) use the signal of another object recorded elsewhere (boat located in the area, signal biomimetic, etc.).

The implementation of the method assumes an almost direct connection

(to emit with minimal delay from the first passive perception of a suspicious signal) between the dotted tracks of the passive antenna (bow, for example) and the transmitter of furtive sonar signals. In parallel with the acquisition of the signal and the choice of its characteristics (chosen duration for example) it is necessary to elaborate all the Doppler copies to allow the localization. In FIG. 3, the Doppler copies of the signal are elaborated so conventional digital signal processing (interpolation-decimation) and stored to then constitute the coefficients of a battery of suitable filters with which the signals of the reception channels will be correlated. It will be noted that this Doppler processing is particularly necessary because of the unambiguous emission code. In addition, knowledge of the relative radial velocity of the threat is a valuable risk indicator to avoid a collision. The number of Doppler copies is a function of the duration of the signal (Doppler resolution) and the excursion of the relative speeds of the obstacles. To improve the probability of detection, the stealth signal transmitted is quite long (for example 400 ms) for a band of the order of that of the underwater telephone (of the order of 3000 Hz). The Doppler resolution in this example is of the order of 2.5 Hz in spectral terms, which corresponds to about 1 Node in terms of speed. It should be about 64 to 128 Doppler filters to explore the possible speed ranges.

Then, the localization is performed according to a usual strategy in active sonar. After transmission by an appropriate antenna (that of the underwater phone for example), the passive antenna operates in echo detection mode, not only in the suspect direction but also in all other directions.

Another example concerns the use of a biomimetic signal either recorded on site a few moments before, or extracted from a contextual database, signal in which the biomimetic code corresponds to a possible animal behavior for the spatial position and the period of time considered.

The discretion of the furtive emission sonar signals is, of course, relative. On the one hand, the threat will not benefit from the temporal processing gain (10 to 15dB). On the other hand, submarine 1 will suffer the loss of transmission on the return path {30 to 60 dB depending on whether the divergence is cylindrical or spherical) and benefits from the reflection index of the threat (5 to 25 dB); these elements generally lead to an unfavorable balance sheet to the submarine 1. To reduce the mistrust of the adversary remains necessary by playing on the stealth of the signal emitted by the submarine 1 as explained above.

Claims

1. An active and furtive proximity acoustic detection device for a submarine comprising a transmitting device (2) connected to a transmitting antenna (1 1) and a receiver (3) connected to a passive antenna (12), this device comprising in in addition to detection and localization means, characterized in that the receiver is connected to means for analyzing signals received from a potential threat and its immediate environment, and in that this device comprises means for generating the signal stealth (5 to 8, 3A) connected to the receiver, these generation means being connected to circuits (4 to 7 and 3A) for storing and producing acoustic signals connected to a passive reception antenna (12) to produce a signal stealthy from that perceived by this passive antenna.

2. A method of acoustic detection active and stealth proximity to submarine to locate a potential threat, characterized in that it consists in collecting the ambient noise using a high gain passive receiving antenna, then to emit a stealth sonar signal built from this collected signal and to locate the threat and assess its relative radial velocity.

3. Method according to claim 2, characterized in that the ambient noise collected and from which the furtive sonar signal is formed contains mainly the noise radiated by the threat.

4. Method according to claim 2 or 3, characterized in that the ambient noise from which the stealth sonar signal is formed is a prerecorded noise.

5. Method according to claim 2 or 3, characterized in that the furtive sonar signal contains the noise radiated by a surface vessel or another submarine selected in a contextual database.

6. Method according to claim 2, characterized in that the furtive sonar signal contains a biomimetic signal selected from a contextual database.