Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
  • G01S15/88—Sonar systems specially adapted for specific applications
  • G01S15/89—Sonar systems specially adapted for specific applications for mapping or imaging
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
  • G01S15/02—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems using reflection of acoustic waves
  • G01S15/06—Systems determining the position data of a target
  • G01S15/42—Simultaneous measurement of distance and other co-ordinates

Definitions

• the present invention relates to a sonar system for object detection and seabed imagery.
• 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
• the antenna 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
• 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.
• the weak reception band after spectral analysis, makes the targets fluctuating which is detrimental to a good probability of detection.
• 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
• the invention also relates to a sonar for the implementation of this method.
• 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
• FIG. 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 ⁇ -, and a reception antenna, 30, covering an angular sector ⁇ -, such that ⁇ p is greater than ⁇ _.
• 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 ⁇ * p and ⁇ -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.
• 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.
• 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 ⁇ -p current to a sector of angular width
• 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.
• 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.
• v n ⁇ * p C / 2dmax, C being the speed of sound in water
• each code is received in a sector of angular width ⁇ - p for a duration equal to 2 dmax / n C.
• the echoes of the n previously transmitted codes are received in the angular sector ⁇ - p centered on the axis ⁇ ,.
• the echoes of these n codes come from different regions of space, contiguous along the reception axis ⁇ , 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 ⁇ - p centered on the antenna axis position of reception.
• the receiving antenna receives the code signal C. coming from the hatched area between 0 and dmax / n.
• ⁇ In the axis position ⁇ resort, it receives the code signal C Cosmetic 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 ⁇ 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.
• a legend illustrates the codes of the signals coming from the different zones.
• 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.
• 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.
• 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.
• 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.
• n 4
• the antenna rotation speed is 60 ° / s.
• 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.
• 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.
• a quarter of this possible band is used, ie 75 kHz, the reception bands being distant from 25 kHz.
• 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;
• each pulse is time-weighted to generate the lowest possible frequency side lobes.
• the weighting used is Gauss or Hamming, or other known weights;
• the reception chain includes:
• This preamplifier 8 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 (4x250 ⁇ s) 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.
• the first filter will output the signal between 0 and 0
• 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.
• a processing circuit 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.
• 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.

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• Engineering & Computer Science (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Physics & Mathematics (AREA)
• Acoustics & Sound (AREA)
• Computer Networks & Wireless Communication (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Velocity Or Position Using Acoustic Or Ultrasonic Waves (AREA)

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• 1989
  • 1989-02-17 FR FR8902080A patent/FR2643464B1/fr not_active Expired - Lifetime
• 1990
  • 1990-02-16 WO PCT/FR1990/000111 patent/WO1990009600A1/fr not_active Application Discontinuation
  • 1990-02-16 US US07/741,516 patent/US5163026A/en not_active Expired - Fee Related
  • 1990-02-16 CA CA002046273A patent/CA2046273A1/fr not_active Abandoned
  • 1990-02-16 EP EP90903849A patent/EP0458885A1/de not_active Ceased

Non-Patent Citations (1)

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