EP0458885A1

EP0458885A1 - Method for increasing the image rate of a sonar and sonar for implementing such method

Info

Publication number: EP0458885A1
Application number: EP90903849A
Authority: EP
Inventors: François Peynaud
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1989-02-17
Filing date: 1990-02-16
Publication date: 1991-12-04
Also published as: WO1990009600A1; FR2643464A1; FR2643464B1; US5163026A; CA2046273A1

Abstract

L'invention concerne un système sonar simple, bon marché et à cadence image élevée pour la détection d'objets et l'imagerie des fonds marins. Elle consiste à émettre n codes successifs non corrélés dans un secteur de largeur angulaire exactement égale à n fois la largeur angulaire du secteur de réception R, l'antenne de réception continuant d'évoluer pendant ce temps alors que le premier signal émis n'a pas encore atteint la portée maximale dmax, et à recevoir dans le secteur de largeur angulaire R les échos de ces n codes provenant de n zones de propagation contiguees selon l'axe de réception, et dans l'espace compris entre 0 et dmax, chacune d'entre elles ayant une profondeur égale à dmax/n. Elle s'applique à un sonar destiné à être installé sur un véhicule sous-marin ou dans une coque de navire.A simple, inexpensive, high frame rate sonar system for object detection and seabed imaging is provided. It consists in transmitting n successive uncorrelated codes in a sector of angular width exactly equal to n times the angular width of the reception sector R, the reception antenna continuing to evolve during this time while the first signal transmitted has not not yet reached the maximum range dmax, and to receive in the sector of angular width R the echoes of these n codes coming from n contiguous propagation zones along the reception axis, and in the space between 0 and dmax, each of them having a depth equal to dmax / n. It applies to a sonar intended to be installed on an underwater vehicle or in a ship hull.

Description

PROCESS FOR INCREASING THE IMAGE RATE OF A SONAR AND SONAR FOR IMPLEMENTING THIS PROCESS

The present invention relates to a sonar system for object detection and seabed imagery. Such a sonar is intended to be installed on an underwater vehicle or in a ship's hull and this at low cost; it must therefore be simple, inexpensive and have a good frame rate. Indeed, because of the low speed of sound propagation in water, the rate of renewal of information must be high in order to carry out a correct sampling of the ground or to carry out an automatic extraction of the targets, extraction

10 which requires as much independent feedback as possible from the target. Conventionally, there are three main types of operation for sonar systems and each leads to specific equipment.

In a pulsed mono-beam sonar with rotating mechanical scanning, the antenna is mono-element and only one electronic reception chain is required. This sonar is therefore simple and inexpensive. On the other hand, the emission and reception transducers are generally directive and the reception transducer must remain pointed in the direction

20 transmission until the signal likely to return from the maximum distance has not arrived on the antenna. The antenna rotation speed then remains limited to low values, and the image rate, in particular for systems with good angular resolution, is very low.

In a sonar with preformed channels, a large sector is soundproofed on transmission and in reception, at each pulse, channels are formed electronically throughout the soundproofed sector. The image of a complete sector is then obtained at a high image rate. This principle is very effective but the 0 complexity of the material is important, especially for installation on a small underwater vehicle. Finally, in a continuous emission and frequency modulation sonar (CTFM), the frame rate is high but the distance resolution is often low because it is inversely proportional to the number of analysis filters

- spectral present in the reception chain. In addition, the weak reception band, after spectral analysis, makes the targets fluctuating which is detrimental to a good probability of detection.

To overcome these drawbacks and obtain a sonar which

[g is simple, inexpensive, the emission of which is broad sector and having an image rate increased by a factor n, the invention proposes another method, to be implemented in a sonar having a maximum range dmax and comprising a mobile transmitting antenna covering a wide sector

15 angular θ-, current and a mobile receiving antenna covering a sector of angular width θ- _p current centered along an axis evolving like the receiving antenna, said axis of the receiving antenna, characterized in that it consists :

- to transmit, in the form of pulses, n successive codes Cl to 20 Cn not correlated in a sector of angular width θ _p . current exactly equal to nβ * -,, the transmitting and receiving antennas continuing to evolve during this time, so that the transmission rate of the codes corresponds to the time taken by the receiving antenna to pass from the sector of angular width θ-p current to a sector of angular width θ-p following in the direction of evolution of the receiving antenna;

- and to receive in the angular width sector θ- current the echoes of these n codes coming from n zones of

30 contiguous propagation along the axis of the receiving antenna, and in the space between 0 and dmax, each of them having a depth equal to dmax / n.

The invention also relates to a sonar for the implementation of this method.

35 Other features and advantages of the invention will appear clearly in the following description given by way of nonlimiting example and made with reference to the appended figures which represent: - Figure 1: the area of land observed on transmission and at reception for a single-beam sonar with mechanical scanning rotating according to the prior art,

FIG. 2, the time diagram of the transmission pulses and the position of the antennas as a function of time of a pulsed single-beam sonar with mechanical scanning rotating according to the invention,

FIG. 3: the area of terrain observed by ^* a pulsed mono-beam sonar with mechanical scanning rotating for progressive receiving antenna positions, according to the invention,

FIG. 4: the area of terrain observed by a lateral sonar with parallel preformed tracks according to the invention,

FIG. 5: an example of distribution of the emission frequencies in a single-beam sonar with - pulses and with rotating mechanical scanning according to the invention,

- Figure 6: the general block diagram of a pulsed mono-beam sonar with rotating mechanical scanning, according to the invention.

FIG. 1 represents the area of ground observed on transmission and on reception for a single-beam sonar with rotating mechanical scanning of the prior art comprising a transmitting antenna, 20, covering an angular sector θ-, and a reception antenna, 30, covering an angular sector θ-, such that θ _p is greater than θ_. In the figure, the positions of the transmit, 20 and receive 30 antennas are shown respectively at the time of transmission and at the end of reception. Between transmission and reception, the reception antenna 30 must rotate only at a small angle A in order to allow the two angular sectors θ * p and θ-p covered by the two antennas to overlap. But this condition is not sufficient because the area of the observed terrain evolves with the receiving antenna. So that all the ground is observed, that is to say not to have a loss of more than 6dB (3dB on transmission and 3 dB on reception) on the echo received at the maximum range dmax, it is the angle A must be at most equal to Θ-. . The angular sector θ-p being defined as the opening of the diagram corresponding to 3 dB below the maximum range dmax, the speed of rotation v of the receiving antenna must be less than v = θ _τ , C / 2dmax, C being the speed of sound in water. The transmission rate is set to the maximum range dmax, which means that before re-transmitting, you must wait until you have received the most distant signal.

If, on the other hand, several successive transmissions are carried out during the propagation and a particular code is assigned to each of them to distinguish them, the codes not being correlated, it then becomes possible to rotate the antenna reception between transmission and end of reception, by an angle A greater than θ-p and thus increasing the frame rate. An easy code to carry out consists, for example, in transmitting distinct pure frequencies.

FIG. 2 represents the diagram of the transmission pulses as a function of time as well as the position of the antennas as a function of time. Each transmission is assigned a distinct frequency code C_. to C. During the time that elapses between the transmission of a code and the end of reception of the same code, the transmit and receive antennas, chosen to be collinear, continue to rotate so that the rate of transmission of the codes corresponds to the time taken by the receiving antenna to pass from the sector of angular width θ-p current to a sector of angular width

Θ, - following in the direction of evolution of the reception antenna. The angular positions of the axis of the receiving antenna θ_,, θ „,. . . . θ, θ_. + θ, θ „+ θ etc. . are

^ 1 V 'n' 1 n '2 n represented as a function of time considering that they are respectively in synchronism with the pulses of code C_. , VS" , . . . CC_. , C „, etc. .

FIG. 3 represents the area of terrain observed by a pulsed mono-beam sonar with mechanical scanning rotating for positions of reception antenna axes progressing from θ. To θ as a function of the transmitted code C_. to C. For clarity of the figure, n was chosen n = 4.

The transmission rate is n times greater and the maximum range dmax of the sonar, for each code, is n times lower than the corresponding values obtained when the emissions are not coded. Similarly, the maximum speed of rotation of the antennas of a sonar being inversely proportional to the maximum range, it is then possible to increase the speed of rotation v of the antennas by a factor n compared to a sonar with non-coded transmission. , let v = nθ * _p C / 2dmax, C being the speed of sound in water, and each code is received in a sector of angular width θ- _p for a duration equal to 2 dmax / n C. For a position of reception antenna axis θ,, k being between 1 and n, the echoes of the n previously transmitted codes are received in the angular sector θ- _p centered on the axis θ,. The echoes of these n codes come from different regions of space, contiguous along the reception axis θ, and which have a depth equal to dmax / n. These regions correspond to the time necessary for the coded acoustic waves emitted to propagate to their maximum range dmax / ri, to be reflected by a possible obstacle and to be received in the sector θ- _p centered on the antenna axis position of reception. In the axis position θ .., the receiving antenna receives the code signal C. coming from the hatched area between 0 and dmax / n.

In the axis position θ „, it receives the code signal C„ coming from the zone between 0 and dmax / n and C. code signal from the shaded area between dmax / n and 2DMax / n, and so on until the position where it θ n oit the RECEIVE * sig nal ^& code C n obtained from zone between 0 and dmax / n, the code signal C __, coming from the area between dmax / n and 2dmax / n, etc. . . , the code signal C. from the area between (n-1) dmax / n and dmax.

In FIG. 3, a legend illustrates the codes of the signals coming from the different zones.

Referring to Figure 3, it should be noted that at the beginning and at the end of antenna rotation, the observation sector has "holes", 40, and its edges are in the form of stair treads. These "holes" correspond to the time necessary for the signal of code C. and to the signal corresponding to the last code transmitted to reach the distance dmax. This must be taken into account when planning a greater rotation in relation to the desired observation sector.

This process is applicable to any sonar system regardless of its essential characteristics such as its frequency, range and resolution. The frequency coding can be arbitrary, in practice for reasons of simplicity, a pure frequency code will be chosen so that the sonar retains its impulse characteristic therefore its good resolution in distance (which is not the case for CTFM devices ). The complexity of the sonar is very little increased, the antennas remain identical, only one reception channel is preserved, it is simply necessary to filter the signal according to the antenna position therefore time.

In addition, as for the sonars with preformed channels, it is necessary to insonify a wider sector, ie exactly nθ-,.

As with the other methods, the transmitting and receiving antennas can be collinear or partially common. FIG. 4 represents the area of terrain observed by a lateral sonar with parallel preformed channels when the emissions are coded according to the invention.

In underwater acoustics, it is known to use a lateral sonar with parallel preformed tracks mounted on a towed fish to obtain an image of the seabed. In this case there is no rotation of the antennas over time, but this rotation is replaced by the movement of the carrier. The invention then makes it possible to increase the speed of the wearer and the frame rate or to form only one lane while keeping the same frame rate.

To obtain a complete observation sector in the case of lateral sonar, it is simply necessary to extend the observation time by 3dmax / 4C, which corresponds to the acquisition phase.

A sonar intended to equip a low cost vehicle, although efficient, must also be low cost; the choice is then made on a single-beam sonar with mechanical scanning because the formation of channels is always expensive since it requires a large number of reception chains in parallel.

By way of example, the characteristics of an embodiment of the sonar according to the invention have been chosen such that the range is 75m, the resolution in field is 1 ° 5, the observation sector is 60 ° , the sector observation time is ls, the distance resolution is 0.2m. For the realization, it was chosen, using the rules of calculation of the acoustic systems well known from the prior art, an operating frequency of 750 kHz, a pulse duration of 250 μs, a reception band of 4 H & and a mechanical sweep over 60 °.

Since single-channel use requires 4s to generate an image, the method according to the invention was applied by choosing n = 4. In this case, the antenna rotation speed is 60 ° / s. To achieve such a sonar, a simple solution consists in choosing a code of distinct pure frequencies, that is to say four frequencies, in the reception band of the sonar. With current acoustic technologies, a sonar can operate in a frequency band equal to 40% of the operating frequency, i.e. in a band equal to 300 kHz for the selected operating frequency equal to 750 kHz. In fact, in this exemplary embodiment, only a quarter of this possible band is used, ie 75 kHz, the reception bands being distant from 25 kHz. But it is possible to increase the number of frequencies of this sonar up to n = 16, that is to say to make in single channel the equivalent of a sonar with 16 preformed channels.

FIG. 5 represents an example of distribution of the transmission frequencies over time.

The frequencies chosen are as follows: 700kHz, 725kHz, 750kHz and 775kHz. This choice is made asymmetrically to avoid excessively high frequencies whose absorption is important. A transmission is triggered every 25 milliseconds, the equivalent of 18.75m in the field.

The general block diagram of this sonar, represented in FIG. 6, comprises a transmission chain connected to the transmission antenna, 20, and a reception chain connected to the reception antenna, 30. The two antennas are represented collinear but they can be separated. They consist of acoustic transducers transforming the electrical signal received into an acoustic wave into emission, or transforming the acoustic wave into an electrical signal into reception.

The transmitting antenna, 20, comprises a single acoustic transducer 1, used for both transmission and reception, but only part of the transducer, 1, will be used at transmission to cover an equal angular sector at 6 °, the equivalent of 4 channels. The directivity of the antenna on transmission will in fact only be 12 ° for reasons of symmetry and to be able to explore space in both directions. A motor, 2, fitted with its control electronics, 3, drives the antennas through 60 °.

The broadcast chain includes:

- a frequency generator, 4, which develops the frequencies of 700 kHz, 725 kHz, 750 kHz, and 775 kHz;

- a signal cutter, 5, which forms the pulses to be transmitted. In order to prevent the transmitted pulses from disturbing the reception sectors, each pulse is time-weighted to generate the lowest possible frequency side lobes. The weighting used is Gauss or Hamming, or other known weights;

- a power amplifier, 6, which sends electronic energy to the transmission part of the antenna;

- a sequencer, 7, triggers the sequences in good time and ensures the synchronization of all the sonar.

The reception chain includes:

- reception transducers forming the reception antenna to which is connected a linear preamplifier, ^* 8, which simultaneously receives the 4 frequencies and which has a filter allowing these 4 frequencies to pass, with rejection for all the frequencies which are not between 698kHz and 777kHz. This preamplifier has an inhibition signal which is active when the emission occurs. There will therefore be no reception during the 4 transmissions, i.e. for a very short teijjps (4x250μs) which represents 1% of the time. This inhibition is necessary because there is a large difference in amplitude between the signal sent and the signal received (from 120dB to 150dB) and it is not possible to receive during transmission. This preamplifier 8 has a variable gain over time to compensate for the losses by absorption. If the lack of reception during transmissions is intolerable, it is possible to double the number of codes transmitted n and to change the synchronization of transmission, by moving it by a half-recurrence, ie 12.5 ms, every n emissions and thus reconstruct an image without inhibition every 2n recurrences. In this case the antenna must rotate twice as fast and only one image out of two is presented complete;

- four filters, 9 to 12, allowing the complete image to be reconstructed in real time over the 75m of propagation. So, for example, the first filter will output the signal between 0 and

18.75m the second between 18.75m and 37.5m, the third between 37.5m and 56.25m and the fourth between 56.25m and 75m. Particular attention will be paid to the quality of these filters which must have a high dynamic range, of the order of 80dB, and a faculty of rejection of neighboring frequencies of the order of 60dB. In the current state of the art, quartz filters are very well suited to this problem;

- Finally, the signals from the four filters undergo conventional processing carried out in a processing circuit, 13, such as precise control of the amplitude of the signals by an amplitude regulator, detection, integration to eliminate the rapid signal fluctuations and improve signal-to-noise ratio, scanning to convert analog signal to digital signal. The signals from this processing circuit, 13, are then presented on a display device.

The system described in this example reflects a conventional analog embodiment of the reception part. It is also possible to use an analog to digital conversion system at the output of the preamplifier, 8, with filtering and digital processing.

In addition, a change in frequency or any demodulation can also be envisaged. The treatments equivalent to that precisely described, do not call into question the application of the process.

Finally, the invention is not limited to a code of pure frequencies, it is possible to use other codes, for example a code of disjoint modulated frequencies.

Claims

1. Method for increasing by a factor n the image rate of a sonar having a maximum range dmax and comprising a mobile transmitting antenna covering a sector of angular width Θ--, current and a mobile receiving antenna covering a sector of angular width θ-, current centered along an axis evolving like the receiving antenna, said axis of the receiving antenna, characterized in that it consists:

- to transmit, in the form of pulses, n successive codes (Cl to Cn) not correlated in a sector of angular width Θ _p . current exactly equal to nθ- _p , the transmitting and receiving antennas continuing to evolve during this time so that the transmission rate of the codes corresponds to the time taken by the receiving antenna to pass from the angular width sector θ- _p current to a sector of angular width Θ--, following in the direction of evolution of the receiving antenna;

- and to receive in the angular width sector θ- _p current the echoes of these n codes coming from n contiguous propagation zones along the axis of the receiving antenna, and in the space between 0 and dmax, each of them having a depth equal to dmax / n.

2. Method according to claim 1, characterized in that the successive codes are distinct pure frequencies.

3. Sonar for implementing the method according to any one of the preceding claims, comprising a transmitting antenna (20) and a receiving antenna (30), a transmitting chain connected to the transmitting antenna (20), a reception chain connected to the reception antenna (30), characterized in that the transmission chain comprises, in series: - a frequency generator (4) controlled by a sequencer (7) to develop frequency codes, - a signal cutter (5) controlled by the sequencer (7) for shaping the pulses to be transmitted;

- And an amplifier (6) connected to the transmitting antenna (20).

4. Sonar according to claim 3, characterized in that the reception chain comprises, in series: a linear pre-amplifier (8) connected to the reception antenna, controlled by the sequencer (7), and of sufficient band wide to let pass all the frequencies of the codes,

- filters (9) to (12) connected to the preamplifier (8) and 10 respectively centered on the different frequencies of the code to reconstruct in real time the complete image of the corresponding observation sector, on the propagation depth dmax ,

- a signal processing circuit (13) reconstituted by the 1 different filters (9) to (12) comprising analog signal processing operations such as amplitude control, detection, integration, digitization.

5. Sonar according to claim 4, characterized in that the preamplifier (8) has an inhibition signal which

20 is active when the transmission occurs.

6. Sonar according to claim 4, characterized in that the filters (9) to (12) are quartz filters.

7. Sonar according to any one of claims 3 to

6, characterized in that the transmit and receive antennas 25 are collinear or partially common.

8. Sonar according to any one of claims 3 to

7, characterized in that it is a front monobeam sonar, the mobile transmitting and receiving antennas being rotating.

9. Sonar according to claim 8, characterized in that it is pulsed, mechanically scanned and broad sector and multifrequency emitted.

10. Sonar according to any one of claims 1 to 7, characterized in that it is a lateral sonar with parallel channels, the transmitting and receiving antennas being

35 mobile due to the movement of the carrier.