Calculation of transmission loss in range dependent ocean environments usually involves CPU-intensive models based on normal mode or parabolic equation techniques which offer little insight to the problem. In this paper we present a new approach based on hybrid ray and mode concepts. A set of incoherent intensity formulae is given for different components of the propagation. The key points are the conservation of energy flux and the exploitation of the ray invariant to model weak range dependence. Comparisons against benchmark solutions demonstrate the accuracy of the model and the immediate interpretation of its results in terms of the propagation components. The model, dubbed INSTANT, is extremely fast at all frequencies and is suitable for research, assessment and operational purposes.
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