Communications – electrical: acoustic wave systems and devices – Acoustic image conversion – With memory means
Reexamination Certificate
1999-07-19
2001-05-29
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Acoustic image conversion
With memory means
Reexamination Certificate
active
06240050
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of readjusting the images from a synthetic-antenna sonar, which is formed by the shifting of a linear antenna of transducers with length L. All the transducers are active in reception and some of them are also active in transmission. The transmitters may also not form part of the antenna.
2. Discussion of the Background
Synthetic antennae are known in which a virtual antenna is formed from the different spatial positions of a physical antenna.
When the nominal motion (without movement error) of the carrying vehicle is collinear with the physical antenna, a synthetic linear antenna is obtained. One known embodiment relates to lateral sonars for seabed imaging, for which the synthetic antenna makes it possible considerably to enhance the lateral resolution.
When the nominal motion of the carrying vehicle is perpendicular to the physical antenna, a bidimensional synthetic antenna is obtained, often having gaps. One embodiment relates to frontal minehunting sonars. The synthetic antenna then allows supplementary antenna gain with respect to the physical antenna in order to enhance the performance in terms of detection and classification. This enhancement results from the fact that the coherence, from one recurrence to the next, of the echo from a mine is generally higher than that of a bottom echo, which is itself higher than that of the noise.
The focusing of a synthetic antenna at a given point F, however, requires compensation for the differences in outwards and return path between F and the physical sonar for all the K consecutive recurrences which it is desired to integrate. However, an error in the movement of the carrying vehicle of &lgr;/4 between two consecutive recurrences, i.e. less than a millimeter in the case of the wavelengths usual in mine-classifying sonar &lgr;=3.75 mm for f
o
=400 kHz), is sufficient to sum the echoes originating from F in anti-phase rather than in phase and thus to destroy the synthetic antenna. The instrumentation, which moreover is complex and expensive, does not have the required precision. Moreover, when movement errors are present, the corrections to be made to the path differences depend on the projection of these errors in the radial direction of F, which depends on the relief of the bottom. However, this remains unknown when the physical antenna is, as assumed here, a linear antenna which exhibits a conical ambiguity. Auto-focusing is then one particularly attractive solution. It makes it possible to get round the difficulties relating to the separate measurement of the movement of the carrying vehicle, of the relief of the bottom and of the profile of instantaneous ultrasonic velocity. In place of that, the combinations of these quantities are estimated, namely the differences in path length, which are required for the problem posed.
A French patent, No. 94 11464 is known, filed by ICPI of Lyons, with J. Chatillon and J. Magand [1] as inventors, which aims to define such an auto-focusing method. In contrast to the present invention, this method does not take advantage of the multi-sensor structure of the physical antenna. It assumes, moreover, that the scene imaged can be segmented into regions including a single diffuser. It is therefore inapplicable in the case, important in minehunting, where the scene imaged includes only one anechoic mine against a bottom of uniform reverberation, for example a homogeneous bottom of mud or fine sand (in this case the mine is detected by the shadow it casts). This is because even the smallest region which can be segmented in the imaged scene, which is nothing more than the resolution cell of the sonar, still contains a continuous infinity of diffusers, according to the conventionally accepted model for describing a bottom of uniform reverberation.
A U.S. Pat. No. 4,224,036, granted on Jan. 6, 1981 to Westinghouse Electric Corp. is also known, with R. S. Raven as inventor, in which is described a method of auto-focusing for a synthetic-antenna lateral sonar, which can operate even against a bottom of uniform reverberation. This method makes it possible to integrate two consecutive recurrences, the extension to the case of K recurrences being done step by step. It uses at least one transducer being shifted in reception mode between these two recurrences. The transducer is shifted relative to the physical antenna in such a way that its center of phase, defined as the place which is the geometric middle of the center of the transmission antenna and of the transducer, remains nominally fixed relative to the water. It is seen in
FIG. 1
that it has to be shifted by &Dgr;R=2 vT in the direction opposite the movement of the sonar, where vT is the nominal motion of the carrying vehicle between two recurrences. Such a motion is possible on condition that L>2 vT, that is to say that the synthetic antenna is oversampled spatially.
Under these nominal conditions, the signals at recurrences No. 1 and No. 2 from the shifted transducer are identical. In the presence of errors, a non-zero phase shift between the two complex samples relative to the echoes from the same range cell of the bottom would be observed, according to the method of this patent [2]. This phase shift would then furnish the phase correction to be applied, range cell by range cell, to the signals from all the transducers of recurrence No. 2 so as to integrate them with the signals of recurrence No. 1. It should be noted, however, that the application of the method requires preliminary readjustment, by another method not specified by the patent [2], of the paired complex samples the precision of which should be a fraction of 1/B where B is the passband of the sonar.
This method [2] also describes an extension to the case of the simultaneous use of several shifted transducers. This is because, when the synthetic antenna is sufficiently oversampled spatially, it is possible to shift at least two transducers in reception mode, to estimate at least two phase shifts according to the preceding method, and to extrapolate these multiple estimates to a phase-shifting law varying linearly along the physical antenna. This compensation law is then applied to the signals from the transducers at recurrence No. 2 in such a way as to achieve, in addition to a constant phase correction, electronic aiming-off of the physical antenna at recurrence No. 2.
Finally the method [2] also makes mention of the possibility of shifting the transmitter by 2 vT, which allows the simultaneous use of all the transducers of the physical antenna, resulting in a gain in precision. However, a problem is posed, described by the inventor of [2], when it is sought continuously to readjust a series of consecutive recurrences. Hence the backward movement of the transmitter by 2 vT between recurrences No. 1 and No. 2 makes it possible to readjust this pair of recurrences but not the recurrences No. 2 and No. 3, since the transmitter has arrived at the end stops of the physical antenna. It is only possible, in fact, to readjust every other pair of recurrences.
Another problem, not described by the inventor of [2], is that the shift on transmission cancels the nominal movement of the center of phase of the physical sonar in such a way that there is no longer spatial diversity, and thus no gain in resolution, for the coherent integration of the two recurrences thus readjusted. Auto-focusing is thus obtained at the expense of the performance of the synthetic antenna which is clearly not desirable.
The modes of shifting on transmission which are proposed in the U.S. patent application No. 9,510,953 filed on Sep. 19, 1995 in the name of Thomson-CSF [3] make it possible to overcome these two limitations. One of these modes uses two alternate auxiliary transmissions (alternating at each recurrence), which are carried out with pairwise codes distinct from each other, and distinct from the main transm
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pihulic Daniel T.
Thomson Marconi Sonar S.A.S.
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