This paper describes the functional theory and design of a modular simulator to generate, in real-time, physically representative spatio-temporal sidescan sonar echo data. The seafloor topography Is generated using fractal theory and the resulting terrain is tesselated into triangular facets. Propagation of the acoustic fan beam is determined by a ray theory solution of the governing wave equation while the contribution of each facet to the recorded intensity at the sonar transducer is resolved by applying Jackson's seafloor scattering model. The computational bottleneck inherent in the discovery of illuminated facets at each ping is significantly reduced by the implementation of an optimised mesh refinement scheme intended for interactive rendering of large-scale complex surfaces described by polygonal meshes. The resulting performance increases enabled the successful integration of the sonar simulator with an existing AUV simulator. Synthetic images and the performance measures of the sidescan simulator image generation are presented. © 2005 IEEE.