The temporal correlation log for ship navigation

Modem computerised navigation systems on board ships require an accurate log for optimal performance. Traditional electromechanica1logs on board merchant ships have generally been replaced by Doppler logs, which refer the velocity to the sea-bed. However, their operational depth is limited and in deep water the velocity is referred to the water-mass some depth below the hull, where it is influenced by ocean currents. The velocity accuracy ultimately depends on the estimated local sound velocity. The acoustic correlation log has acoustic beams similar to a conventional depth-sounder, with a vertically downward propagation direction and a broad beamwidth. Compared to the Doppler log it is affected less by ship pitch and roll due to its broad beamwidth, and the velocity is virtually independent of the velocity of sound and is referred to the sea-bed to a greater depth. Temporal correlation logs use a two-hydrophone array and the time delay between the two received signals is used to calculate the velocity. The full potential of these logs for world-wide ocean navigation has not been exploited. This thesis investigates the requirements and conditions for temporal correlation logs on board ships to navigate satisfactorily on and off the continental shelf. An experimental temporal correlation log working at high-frequencies with continuous wave propagation is described. It is used to measure the velocity of a tracked platform moving across a water tank and the results are compared to their theoretical values. When fully developed, the log would be suitable for shallow-water operation, and techniques learned can be applied to a low-frequency deep-water correlation log.