An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> III. RAY-TRACING MODELS <s> Abstract : Interest has arisen in the last few years to develop analytical models of the undersea acoustic environment which are suitable for use on digital computers. To realistically represent the sound-field structure which exists in the ocean, one must consider the variation of sound speed in two dimensions: depth and range. A computer program, TRIMAIN, was written in Fortran 4 in which the sound-speed field for a given region is divided into triangular segments of the range-and-depth plane. Four types of intensity calculations are available: a completely random phase summation, a completely coherent phase summation, a statistical influence over depth, and an average over a convergence zone. Additional output options are ray depth distributions, ray printplots, and Calcomp ray plots, including plots of input sound speeds and of bathymetric profiles. <s> BIB001 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> III. RAY-TRACING MODELS <s> Abstract : Ray equations based on an acoustic Hamiltonian, and formulated by Jones et al. (1986) for spherical coordinates in NOAA's three dimensional ray tracer HARPO, are adapted to ellipsoidal coordinates in the oceanic waveguide. The ensuing modified HARPO is used to model very long range (up to antipodal) acoustic paths in which the difference between geodesics and great circles is measurable. The eventual objective of this modeling is to extract the predictable part of the travel time trend and fluctuations along several long paths that will be used to monitor hypothetical global warming effects. The requirements for easy assimilation and representation of realistic environmental inputs are discussed. These requirements, when coupled with the possibility of classical chaos in the ray paths, dictate a new software architecture. We use the existing software, however, to breadboard and test features of new ray tracers in the global boundary layer, and to support the experimental design of a forthcoming pilot experiment that will use a transmitter located at Heard Island in the Indian Ocean near Antarctica. <s> BIB002
Simulators would always be chosen based on the parameters required for a user going to perform an experiment. Due to complex underwater environment, acoustics simulations are time consuming; therefore, to provide precise acoustics simulations a vital and accurate model is required. The thorough description of how sound propagates through the ocean medium is finely described by ray tracing models  . For short-range calculation, where the sound speed profile (SSP) and the bottom profile are considered flat, a single profile program can be used. However, for long-range calculations, where both the SSP and bottom profiles vary, an advance allpurpose program is required BIB001 . To determine the ray coordinates, solution of ray equations is the basic requirement of ray tracing  . A full 3D modeling is important for rays that travel several kilometers with vertical fluctuations. A tracer should be very fast and flexible simultaneously to handle propagations timely BIB002 . This section provides several ray tracing models that are available for underwater simulations. All the following models were developed to tackle the hardest problem of ray tracing i.e., the calculation of eigenrays along with other challenges  .