Communications – electrical: acoustic wave systems and devices – Echo systems – Side scanning or contour mapping sonar systems
Patent
1994-05-18
1995-06-27
Pihulic, Daniel T.
Communications, electrical: acoustic wave systems and devices
Echo systems
Side scanning or contour mapping sonar systems
367904, G01S 1550
Patent
active
054285810
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to methods which make it possible to compensate, in sonars, for the movements of the antenna which risk disturbing the reception of the acoustic signals.
It is known, in order to detect a moving target with a sonar, to use the Doppler effect by sending out an acoustic pulse whose frequency band is smaller than the Doppler shift likely to affect the echoes originating from moving targets. On reception, the received signals are correlated with several copies of the signals sent out, each copy corresponding to a different Doppler shift. The type of correlation obtained with the copy which corresponds to a Doppler shift identical or substantially identical to that of the target makes it possible to locate this target in distance, as well as to determine its radial speed with respect to the sonar. This method is entirely compatible with the other signal processing systems, in particular those which consist in forming directional receiving channels. This method makes it possible to improve the contrast between the useful echo which is affected by a Doppler shift and the other echoes originating from the reverberation as much from the bottom and from the surface of the sea, as from the volume itself of the underwater medium whose non-uniformities give rise to a diffuse ambient echo which is very troublesome for a simple sonar. In fact, the echoes due to reverberation are not affected by a Doppler shift since the elements at the origin of this reverberation do not move, which makes it possible to eliminate them. This assumes that the target is moving, which is generally the case.
Very often the sonar is carried, or towed, by a boat which is moving at a speed which is not negligible with respect to that of the target. In this case the echoes due to reverberation are affected by a Doppler shift originating from the speed of the sonar itself. As transmission in general covers a wide angular sector, there are always reverberation sources whose radial speed with respect to the sonar is substantially equal to that of the moving target. In these conditions the echo
oise contrast is appreciably limited by these reverberation sources, even if these sources are outside the main lobe of the receiving channel used since, as is known, there are always relatively significant secondary lobes in the diagram of such a channel.
In order not to be too much disturbed by such an effect, it would be necessary for the target to move at a speed at least higher than twice the speed of the vessel which is carrying the sonar. This would lead to very seriously limiting the speed of the vessel and thus its operational capabilities. If, moreover, the target is a torpedo which is heading towards the boat, it is easy to imagine the catastrophic consequences of such a speed reduction on the capability of escaping this very torpedo.
In order to overcome the problems related to movements of the sonar, the invention proposes a method of compensating for the movement of the antenna for a sonar, in which the antenna comprises a set of transducers and is driven in a translational movement, characterized principally in that the signals from these transducers are switched in order to obtain a synthetic movement of the antenna making it possible to compensate for the Doppler shift applied to the fixed echoes by the said translational movement.
Other features and advantages of the invention will appear clearly in the following description, given by way of non-limiting example with regard to the attached figures which represent:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1, a diagram of reception from a target and from a reverberation cell;
FIG. 2, a boat towing a sonar antenna;
FIG. 3, the constructional diagram of the virtual antennae of a sonar according to the invention;
FIG. 4, the variation of phase over time of the reception signal;
FIG. 5, the spectrum of this signal; and
FIG. 6, the block diagram of a device for implementing the invention.
In FIG. 1, a sonar 101, carried by a boat moving with a speed V.sub.A receives t
REFERENCES:
patent: 4987563 (1991-01-01), Gilmour
patent: 5101383 (1992-03-01), Doisy et al.
Baudoux Jean-Pierre
Doisy Yves
Le Parquier Guy
Pihulic Daniel T.
Thomson - CSF
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