Acoustic pulse compression using passive phase-conjugate processing

Passive phase-conjugate (PPC) processing consists of using the first arrival, from a stream of pulses that have traversed a complex refractive medium, as a filter for later pulse arrivals. Its connection with true ‘‘active’’ phase conjugation lies in invoking reciprocity for the backpropagation. When the intervening acoustic medium changes slowly, the PPC processor creates an unambiguous temporal peak even when a complex multipath environment separates the acoustic source and receiver. Temporal sidelobes in the PPC processor output can be suppressed by increasing the number of receivers and coherently summing their output. Recent deep-ocean acoustic propagation measurements (IWAC ’90) are used to assess PPC processing in a fading multipath acoustic channel. For a range of 272.3 km at 460 Hz, the compressed peak persists on average for about half an hour. Fading of the PPC processor peak is found to be consistent with parabolic-equation second-moment theory for wave propagation in a random medium.