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The paper describes research into local positioning in a confined space using ultrasonic signal propagation. The work is targeted at the realization of a neutrally buoyant flow follower to be injected into a process vessel. The flow follower emits an ultrasonic signal which is detected by four receivers located on the inside wall of the vessel. Spread spectrum techniques, such as binary phase shift keying, have been borrowed from GPS to explore improvements in noise immunity. Thresholding and correlation techniques have yielded 3D positional accuracy of the order of centimeters in a laboratory-based steel vessel with dimension about lm.