Signal Fluctuations in the BIFI Range

Short‐term and long‐term fluctuation studies have been made in the BIFI Shallow X, Water Acoustic Range, which is 37 kyd in length. It is shown how time‐smear analysis may be used to predict the extent of short‐term signal fluctuation by taking into account the energy content of the “front” and “tail” of the received signal. The method is applied to an actual received signal with good results. Long‐term fluctuation, such as that due to tide, is considered in terms of normal mode theory, using the results of the time smear analysis to provide an estimate of the relative strengths of the modes. When applied to the BIFI range this method predicts that tidal effects will cause negligible fluctuations in the received signal, due to the absence of modes other than the fundamental at this large range. This prediction was verified by experiment. At shorter ranges, the interaction of several modes will cause relatively large fluctuations in the signal as the water depth changes during the tidal cycle.