Localization is of great importance in mobile and wireless network applications. TDOA is one of the widely used localization schemes, in which a to-be located object emits a signal and a number of receivers record the arriving time of the signal. By calculating the time difference of different receivers, the location of the object is estimated. In such a scheme, receivers must be precisely synchronized, even slight noises are completely unacceptable for centimeter-level localization. Previous studies have shown that existing time synchronization approaches for low-cost devices are insufficiently accurate and basically infeasible for high accuracy localization. In our scheme (called Whistle), several asynchronous receivers record a target signal and a successive signal that is generated artificially. By two-signal sensing and sample counting techniques, high time resolution can be achieved. This design fundamentally changes TDOA in the sense of releasing the synchronization requirement and avoiding many sources of inaccuracy. We implement Whistle on commercial off-the-shelf (COTS) cell phones. Through extensive real-world experiments in indoor and outdoor, quiet and noisy environments, the mean error is 10~20 centimeters in a 9m ˜ 9m ˜ 4m_3 3D space.
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