Abstract
This paper explores the problem of severe multipath propagation of underwater acoustic signals in a swimming pool. The problem appeared in a study that examined a system used to signal emergency situations (i.e., pre-drowning symptoms detected by a wearable device on a pool user’s wrist) and locate the signal source. A swimming pool acoustic environment is characterized by the presence of large flat reflecting planes surrounding a small volume of water. The reflections are numerous and much stronger than in typical hydroacoustic applications. In this paper, we attempted to create a model of the swimming pool response, one that is suitable for simulation experiments with detection and localization of emergency signals. Then, we explore the possible remedies for the localization system, applied on the transmit side (waveform design) and on the receive side (receiver placement and signal processing). Finally, we present an algorithm for object localization, considering the possible reflections with a multi-hypothesis approach.
Highlights
The multipath propagation problem appeared in connection with a study of an underwater signaling system designed for the safety of public swimming pools
The system’s concept required reliable detection of a distress signal as well as accurate measurements of the signal’s arrival time. The latter feature was intended to be used to locate the signal source with the time difference of arrival (TDOA) technique
The TDOA technique for source localization is based on measuring signal arrival times at separate receivers
Summary
The multipath propagation problem appeared in connection with a study of an underwater signaling system designed for the safety of public swimming pools. One known method is the usage of special wristbands, which monitor the depth and time of staying underwater Such a wristband generates an alarm signal upon detecting an emergency (i.e., potential user drowning). Many pulse copies overlay within the compression side lobe area, making constant false alarm rate (CFAR) processing difficult Another problematic situation that we observed, in practice, occurred when the straight path to a given receiver exhibited high attenuation. When the signals arrive via different paths—directly or with a reflection—the measurements at one or more sensors may correspond to the reflection If it is mistaken for the direct path measurement, the location result is wrong. The original conference paper [8] deals with modeling the pool response and the reception of the signal, whereas the present article adds to the description of the TDOA methodology modification
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