SwimSafe: A Smartphone-Based Acoustic System for Water-Depth Detection and Hazard Alert

Water depth is a crucial factor affecting the safety of underwater activities. However, the lack of portable and cost-effective depth estimation solutions persists. While existing technologies can estimate the distance to the water surface (DS), accurate water-depth measurement remains challenging due to the variability and location dependence of the distance to the underwater bottom surface (DB). In this article, we introduce SwimSafe as the first system to implement DB measurement using a single-commercial smartphone. SwimSafe utilizes acoustic sensors on the smartphone to estimate the swimmer’s depth relative to the pool bottom and provides hazard alerts when the swimmer enters a deep-water hazardous area. To ensure precise depth estimation, we develop a novel echo-ranging method capable of effectively handling various interferences. Specifically, we design an ultrasonic hybrid signal and develop a frequency compensation method for the underwater Doppler frequency shift. In addition, we adopt signal separation, noise removal, and ID coding schemes to improve the system’s robustness against various interferences, including the multipath effect, the influence of multiple smartphones, and water flow. Furthermore, we employ vibration feedback for deep water hazard alerts to reduce the risk of injury or death during underwater activities. Extensive depth estimation experiments have been conducted under various conditions. The results indicate that the SwimSafe system is capable of real-time depth estimation, achieving an average error of 0.08 m in an outdoor lake with a depth of 2 m, while maintaining an alert success rate of over 98%, effectively enhancing the safety of underwater activities.