The waveguide invariant is classically considered as constant in shallow water and can be utilized to estimate the source range. When the environment is variable, the error between the estimated range and the true range fluctuates and becomes unstable. The fluctuation of the range estimation error is large when we use the polar coordinate transform to obtain the slope of the two-dimensional Fourier transform ridge (2DFT-PT). The lines which are parallel in the ridge are not considered and the ridge information is not utilized sufficiently. For this problem, we propose a passive ranging method by using the Radon transform to obtain the slope (2DFT-RT) and achieve a more robust result. Its capability is investigated and compared with the 2DFT-PT method via numerical simulation and the SWellEx-96 experiment.
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