Broadband Source Ranging In Shallow Water By Using The Generalized Array Invariant

The invariant principles of waveguide are the interesting problems of ocean acoustics at present. The array invariant can also be used to realize source ranging as same as the waveguide invariant. Compared with the traditional three-elements passive ranging method, the array invariant theory involves the effects of sound propagation in waveguides, and has efficient capability of source ranging in shallow water therefore. Be different from matched field processing, the array invariant method is not susceptible to environment parameters, and can be available even without a priori environmental knowledge.In this paper, the array invariant ranging method is developed on the basis of previous research done by Sunwoong Lee and Nicholas C.makris . the main results are as follows:First, the physical mechanism of array invariant is clearly explained. The arriving time and elevation angle are used to describe the multi-modal propagation and single-mode dispersion in shallow water, and the author presents a new different derivation, which can clearly illustrate the essence of array invariant. The physical meaning of array invariant is the absolute traveling time of the signals from source to receiver under the average sound speed. So the value of array invariant is the key point of passive source ranging, which can be gotten by the derivative of the arriving time against the corresponding cosecant function of elevation angle.Second, the concept of generalized array invariant is proposed. The array invariant is a constant for fixed source-receiver geometry when the waveguide is perfect., however, for the more realistic waveguides ( for example there is a big perturbed sound speed profile in water, or the sea bed's effect can't be ignored), the array invariant varies with mode and frequency, which is defined as generalized array invariant. When the sound speed profile is complex or known, the ranging accuracy can be improved by correcting the generalized array invariants to a constant.Third, the inner relationship between generalized array invariant and generalized waveguide invariant is built. Both of the two invariant concepts are the reflections of the dispersion characteristic in waveguides, and can be expressed as functions of normal mode elevation angle. The generalized array invariant is the ratio of the difference of arriving time to the corresponding difference of cosecant of elevation angle, where the generalized waveguide invariant is the product of sine of two different elevation angles. The new formula is helpful to understand or estimate the waveguide invariants.The source ranging method discussed in this thesis is based on the fact that the sound mainly propagates in the water. It is concentrated on the effect of perturbed sound speed profile but not the sea bed. When applying the array invariant principle to source ranging, it is mainly considered using the conventional short-time beamforming technology on horizontal and vertical arrays to get the elevation angles of different signal components, and at last to convert to the values of generalized array invariants. The simulation results verify the validity of ranging method .