Communications – electrical: acoustic wave systems and devices – Echo systems – Side scanning or contour mapping sonar systems
Patent
1998-03-17
1999-03-23
Pihulic, Daniel T.
Communications, electrical: acoustic wave systems and devices
Echo systems
Side scanning or contour mapping sonar systems
G01S 1589
Patent
active
058869506
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to synthetic antenna sonars in which the physical antenna is formed by a linear array of transducers. All the transducers are active in receive mode whereas only some of them are used for transmission.
To form the channels of such a synthetic antenna, the K.times.N signals delivered by the N transducers during K successive recurrences are combined linearly. To do this, it is necessary to know the geometry of the synthetic antenna, that is to say the position of the phase centres of the transmitting and receiving transducers, for the duration of the K recurrences in a plane Oxy where Ox is the beam axis towards the bottom (whose inclination in the elevation plane varies within the recurrence like the elevation of the bottom) and Oy is the axis perpendicular to Ox which is closest to the mean path of the platform. The deviations of the synthetic antenna with respect to the plane Oxy are small enough to be neglected in most practical cases.
Means are also available which are accurate enough for measuring the displacement of the support of the transducers along Oy, in respect of which direction less accuracy is needed than in respect of Ox. Thus, the problem of knowing the geometry of the synthetic antenna in the plane Oxy is reduced to that of knowing its geometry along the beam axis Ox.
To solve this problem the Westinghouse company has proposed applying the principle of equivalent phase centres in "Synthetic aperture beamforming with automatic phase compensation for high frequency sonars", R. W. sum of the abscissae x.sub.k of the transmission phase centre at recurrence k and x.sub.n,k of the phase centre of receiving sensor n at recurrence k, and the sum of the abscissae x.sub.k+1 of the transmission phase centre at recurrence k+1 and of receiving sensor n' at recurrence k+1 by measuring the intercorrelation delay of the two signals from sensors (n,k) and (n',k+1) chosen such that: maximum correlation for the signals received on sensor n at recurrence k and sensor n' at recurrence k+1. which is used in transmit and receive mode, and the second in receive mode only. The displacement in the Oy direction between two successive recurrences being equal to half the distance between the two sensors, relation (1) is satisfied by n=2 and n'=1. As represented in FIG. 1, this can be generalized to an array of N sensors and to a displacement .delta.y of less than half the length of the array for every Y.sub.n,k such that
Spatial interpolation over the signals of recurrence k+1 can be used to determine a sensor signal corresponding to a position y.sub.n',k+1 of its phase centre over Oy such that we have (1) (since y.sub.k+1 =y.sub.k +.delta.y and y.sub.n',k+1 =y.sub.n',k +.delta.y, (1) is equivalent to y.sub.n,k =y.sub.n,k +2..delta.y), and to then estimate the intercorrelation delay between the interpolated sensor, (n',k+1) and the sensor (n,k), which will be denoted .tau..sub.n',n,k. The spatial interpolation leads us to consider fractional indices n'. We have .tau..sub.n',n,k =.tau..sub.n',n,k,x +.tau..sub.n',n,k,y where .tau..sub.n',n,k,x depends on the abscissae x.sub.k, x.sub.k+1, x.sub.n,k, x.sub.n',k+1 which it is desired to estimate and .tau..sub.n',n,k,y depends on the ordinates y.sub.k, y.sub.k+1, y.sub.n,k and y.sub.n',k+1. The ordinates being known, .tau..sub.n',n,k,y is known. The measurement of .tau..sub.n',n,k,x =.tau..sub.n',n,k -.tau..sub.n',n,k,y is deduced from the measurement of .tau..sub.n',n,k.
We then obtain a set of pairs of transmission/reception phase centres ((n,k), (n',k+1)), an estimate of the deviation of the abscissae of which is given by:
The ordinates of the transmission phase centres y.sub.k and y.sub.k+1, and reception phase centres y.sub.n,k and y.sub.n',k+1 are given by measuring the displacement in the direction Oy obtained by the measurement means mentioned above.
It is therefore possible to estimate the shape of the synthetic antenna gradually (from recurrence to recurrence), this leading to the consideration that x.sub.k +x.sub.n,k is known, (3) th
"Thomson-CSF"
Pihulic Daniel T.
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