The development of small low-cost autonomous underwater and surface vehicles has increased the need for underwater wireless communication and ranging to support swarm operations for collaborative data collection efforts. Specifically, newly available low-cost underwater and surface vehicles make the realization of swarm formation cost effective. However, their mission coordination involves the use of expensive acoustic modems whose price may be higher than that of the vehicle itself. In this paper, we describe and evaluate a low-cost software-defined acoustic modem developed with only off-theshelf components, able to perform one-way travel-time ranging. The modem is specifically designed for dense mobile networks deployed in shallow water environments, such as rivers, lagoons and lakes. The modular software design of the modem allows us to easily configure parameters such as modulation and coding schemes, scheduling algorithm, source power, carrier frequency and bandwidth. In this paper we evaluate, in a shallow environment, the modem performance in terms of packet detection ratio, packet delivery ratio, and one-way travel-time ranging.

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The features of the underwater acoustic channel are remarkably dependent on the considered scenario; for instance, the link quality differs significantly in shallow water with respect to deep water, and series of events such as the presence of rain or ships passing nearby, changes of temperature and wind strength, can change drastically the channel conditions observed in a certain area in different seasons and even during the same day. Mathematical models that consider these parameters exist, but are either very computationally demanding, like the Bellhop ray tracer, or not sufficiently accurate, like the Urick model that often exhibits optimistic results. In this paper, we discuss the development of a statistical channel model based on the analysis of real field experimental data and compare its performance with the other channel models available in the DESERT Underwater network simulator.