An introduction to GRAB eigenrays and CASS reverberation and signal excess

The Comprehensive Acoustic System Simulation (CASS) with the Gaussian Ray Bundle (GRAB) eigenray propagation model is an active and passive range dependent propagation, reverberation, noise and signal excess model being adopted as a Navy standard. This paper will present a simple description of how GRAB computes eigenrays and how CASS computes monostatic reverberation and signal excess. GRAB calculates the eigenrays that connect the source and target by first tracing user selected test rays. The GRAB test ray amplitude differs from classical ray theory in that it is Gaussian distributed in depth, contributing some energy to all depths. GRAB sorts the test rays into families with like number of boundary interactions and turning points and then power averages the ray properties for each family to generate an eigenray representative of that ray family. The Comprehensive Acoustic System Simulation computes range dependent reverberation for monostatic and bistatic transmitter/receiver to target scenarios. The reverberation is calculated in the time domain centered about the receiver. CASS accounts for all possible combinations of eigenrays that connect the transmitter to the scattering cell and the scattering cell to the receiver. The signal excess is calculated by summing all possible signal eigenray path combinations at a given range and selecting the peak signal to noise/reverberation level.