RayPath is a sophisticated tool, primarily designed for use by sailors at sea, which enables the operator to make simple predictions of active sonar performance. The app requires the input of either a Temperature-Depth (BT) profile or a Sound Velocity-Depth (SV) profile. These can be either created manually as an ASCII file using any standard text editor on a PC or can be input manually from within the app. From knowledge of the BT or the SV profiles, and a number of user input sonar and environmental parameters, the app calculates and plots the paths taken by acoustic rays through the water mass. Besides calculating the acoustic rays, the app is also capable of making predictions of detection performance by calculating the Signal Excess, that is, the strength of the returning sonar echo, at the input to the receiver, above the echo strength required for a 50% probability of detection. In order to make these calculations the app takes into account the signal losses due to absorption in the water, due to interactions with the surface and bottom of the ocean and due to signal spreading over the transmission path. Unlike some less sophisticated programmes, RayPath directly estimates the spreading losses from a knowledge of the ray-paths, rather than simply making the assumption that the losses follow either spherical or cylindrical spreading laws. This allows the programme to predict convergence zones and other phenomena. The calculations performed also require a knowledge of the noise environment and sonar specific parameters such as sonar source level, detection threshold and others. Again, these parameters can be input and adjusted from within the app and can also created off-line on a PC or Mac and transferred to the app via iTunes.