Communications – electrical: acoustic wave systems and devices – Acoustic image conversion
Patent
1991-08-09
1992-11-10
Pihulic, Daniel T.
Communications, electrical: acoustic wave systems and devices
Acoustic image conversion
367101, 367104, G01S 1500
Patent
active
051630265
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a sonar system for the detection of objects and the imaging of sea bottoms. Such a sonar is intended to be installed in an underwater vehicle or in a ship's hull, at low cost; it must therefore be simple and cheap, and have a good image rate. In fact, because of the low speed of propagation of sound in water, the rate of renewal of information must be high so as to carry out correct sampling of the terrain or to carry out automatic extraction of targets, which extraction calls for the greatest possible independent returns from the target.
Classically, there exist three major types of operation for sonar systems, and each leads to specific hardware.
In a pulse-action single-beam sonar with rotatable mechanical scanning, the antenna is a single-element one and a single electronic reception system is required. This sonar is therefore simple and cheap. On the other hand, the transmission and reception transducers are generally directional and the reception transducer must remain aimed in the direction of transmission so long as the signal likely to return from the maximum distance has not arrived at the antenna. The speed of rotation of the antenna then remains limited to low values, and the image rate, in particular for systems having good angular resolution, is very low.
In a sonar with preformed channels, a wide sector is filled with sound on transmission, and at reception, with each pulse, channels are formed electronically throughout the sound-filled sector. The image of a complete sector is then obtained at a high image rate. This principle is very powerful but the complexity of the hardware is great, especially for installation in a small underwater vehicle.
Finally, in a continuous transmission frequency modulation (CTFM) sonar, the image rate is high but the resolution in distance is often low since it is inversely proportional to the number of spectral analysis filters present in the reception system. Moreover, the low reception band, after spectral analysis, renders the targets fluctuating, this being prejudicial to a good probability of detection.
In order to overcome these disadvantages and obtain a sonar which is simple, cheap, has wide-sector transmission, and offers 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 movable transmission antenna covering a current sector of angular width .theta..sub.E and a movable reception antenna covering a current sector of angular width.theta..sub.R centred in relation to an axis turning like the reception antenna, called the reception antenna axis, characterized in that it consists: to Cn) in a current sector of angular width .theta..sub.E exactly equal to n.theta..sub.R, the transmission and reception antennae continuing to turn during this time in such a way that the rate of transmission of the codes corresponds to the time taken by the reception antenna to pass from the current sector of angular width .theta..sub.R to a following sector of angular width .theta..sub.R, in the direction of turning of the reception antenna; echoes of these n codes, coming from n propagation regions which are adjacent in relation to the reception antenna axis, and lying between 0 and dmax in space, each of them having a depth equal to dmax
.
The subject of the invention is also a sonar for the implementation of this method.
Other features and advantages of the invention will emerge clearly in the following description given by way of non-limiting example and made with reference to the attached figures which represent:
FIG. 1: The region of terrain observed on transmission and on reception, for a single-beam sonar with rotatable mechanical scanning according to the prior art,
FIG. 2: The chart of the times of the transmission pulses and the position of the antennae as a function of time for a pulse-action single-beam sonar with rotatable mechanical scanning according to the invention,
FIG. 3: The region of terrain observed by a pulse-action s
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Proceedings: ICASSP 87--1987 International Conference on Acoustics, Speech, and Signal Processing, 6-9 Apr. 1987, Dallas, Texas, vol. 3 de 4, IEEE, P.M. Cassereau et al.: "Frequency hopping patterns for simultaneous multiple-beam sonar imaging", pp. 1704-1707.
"Thomson-CSF"
Pihulic Daniel T.
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