Most ultrasonic transmitters (pingers) used for behavioral surveys of aquatic animals send a variety of information by changing the interval of the pulses. The reflection of a pulse generally occurs at the sea surface and/or the bottom. Reflection is a critical problem in considering the measurement error generated by the arrival-time delay of the reflected pulse. We conducted a field experiment to evaluate the effect of reflection, and discussed ways to prevent the effect. In this paper, we

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... d a tracking-type ultrasonic telemetry system. The pinger of the system measures its depth by changing the interval of two pulses. In order to evaluate the paths of the reflected pulses, we measured the difference in arrival time between a direct pulse that propagated the direct path to the hydrophone and reflected pulses. We observed the variation in arrivaltime difference in field experiments. Results showed that reflected surface pulses easily affected the system. Improvements to the ultrasonic telemetry system helped to prevent the effect of reflection, and we demonstrated the effectiveness of the improved system in an additional field experiment. The improved system could measure the depth data without the effects of reflection. We also discuss a more optimal configuration for preventing the measurement error from the additional experimental data.