The present invention relates to a direction determining method and apparatus for close-range sonar systems having a wide aperture angle and high azimuth and radial resolution. In the case of short pulses, the radial resolution is defined as c/2B, where c is the sound velocity in the medium and B is the pulse bandwidth.
In systems of this type, it is common to employ an arrangement of spatially arranged acoustic-electric transducers to determine the direction of incidence of sonar signals. While this procedure basically serves for the determination of a direction in one plane, it can also be used for the determination of a direction in two planes.
It is known to determine the direction of incidence of sonar echoes by providing a staggered time delay of the electrical signals from the individual transducer groups and adding these signals in the correct phase relation. The sum signal is a measure for the echo amplitude from a certain direction of incidence, the particular direction being determined by the selection of the delay times. If this method is used for a plurality of directions, it is called beam scanning.
It is also known to simplify this principle by employing a phase shift to correspond to the delay times, a process which is briefly called phase compensation.
Since in addition to the direction of incidence, the distance, or range, of the reflecting object is also to be determined, the emitted pulses have a finite duration and a predetermined bandwidth. A phase compensation will then, however, reach its limits. In sonar devices for close ranges it is generally necessary to have a high distance resolution, a wide angle resolution and a wide aperture angle. Phase compensation is permissible only for the region ##EQU2## where .alpha..sub.o /2 is the maximum angle between the normal to the transducer array and an object point, c is the speed of sound, D is the length of the transducer array and B is the pulse bandwidth.
Phase compensation for a plurality of receiving directions can be realized for distant targets with a Fourier transformation of the received signals from the individual groups of transducers u.sub.n and for the short-range field with an additional multiplication by a phase factor, as described in the publication Acoustical Holography, Vol. 2, pages 136, 137, Plenum Press, 1970. Such a discrete Fourier transformation can be effected electrically. Alternatively, it can be effected optically in a holographic process. The equipment costs for this are substantially less than for an exact delay time compensation system.