Michael D. Collins
B. Edward McDonald
W. A. Kuperman
Naval Res. Lab., Washington, DC 20375
Kevin D. Heaney
Planning Systems, Inc., McLean, VA 22102
The parabolic equation (PE) method is efficient for solving range-dependent sound propagation problems, including relatively small-scale three-dimensional problems. Global-scale three-dimensional problems are well beyond current computer resources because the size of the computational grid increases with range. The adiabatic normal mode approximation, which ignores the effects of mode coupling, is applied to reduce problems of this class to a practical size. Each of the complex modal coefficients is a function of longitude and latitude and is determined by solving a two-dimensional PE. The wave number appearing in the PE, which is a function of longitude and latitude, corresponds to the modal eigenvalue. An interesting description of global-scale three-dimensional propagation can be obtained by solving this two-dimensional problem for a small number of modes. This approach is orders of magnitude faster than the three-dimensional PE even when all of the modes are considered. This model should be applicable to analyzing results from global-scale experiments such as the Perth to Bermuda transmissions [J. Acoust. Soc. Am. 71, 51--60 (1982)] and the Heard Island Feasibility Test [Baggeroer and Munk, Phys. Today 45(9), 22--30 (1992)].