A Doppler Effect Based Framework for Wi-Fi Signal Tracking in Search and Rescue Operations

We consider rescue missions in postdisaster scenarios with particularly difficult environments where no infrastructure is available. Given the increasing popularity of smartphones and wearable devices, this paper proposes a rescue system which uses the Doppler effect to determine the direction of Wi-Fi signals emitted from disaster survivors' mobile devices to help rescuers quickly locate the survivors. First, we investigate the impact of the search and rescue environment on the direction-finding accuracy of Doppler effect to identify the major challenge and several implementation issues of the system. Then, to address the major challenge of Doppler shifts being too small, we propose an algorithm, which consists of three mechanisms, to solve the problem with the objective of maximizing the direction-finding accuracy. These mechanisms improve the direction-finding accuracy via eliminating the frequency fluctuation as much as possible and improving the sensitivity on small frequency shifts. Also, an active detection scheme is proposed to ensure that the survivors' devices emit steady and continuous Wi-Fi signals, along with a decision logic to minimize energy consumption by the active scheme. We implement the rescue system as a mobile application on Android smartphones and conduct extensive experiments in real-world environments. Results show that the proposed system can reduce rescue times by up to half while consuming reasonable amounts of energy from survivor smartphones.