Distance measurement between smartphones within an ad-hoc camera array, using audible PRBS

An approach for measuring the distance between two smartphones is presented in this paper. The method uses each smartphone’s microphone(s) and speaker(s) to concurrently emit and record audio in order to calculate the sound propagation delay and hence distance. Each device in turn emits a different audible pseudo-random binary sequence (PRBS) - specifically, a maximum length sequence (MLS). Each device captures both emitted signals in one continuous recording. The propagation delay between the devices is calculated by comparing their respective recordings, and in particular the temporal positions of the emitted signals within each recording. Each device emits one of the signals, records both signals, and then sends its recording to a master device for analysis, which is performed by a custom web application and is therefore independent of operating system. A mean error of 32.29 mm was found in initial testing, which was conducted using Samsung Galaxy A10 devices running Android 10. The key innovation in this method is that it requires no clock time synchronisation between devices because the distance is determined by comparing inter-transmission delays in the two recordings. Potential future improvements are discussed, including how to take into account the exact locations of each phone’s microphone and speaker to increase accuracy.