Clock Synchronization and Ranging Estimation for Control and Cooperation of Multiple UUVs Gianni Cario ‡¶ , Alessandro Casavola ‡¶ , Vladimir Djapic * , Petrika Gjanci †§ , Marco Lupia ‡¶ , Chiara Petrioli †§ , Daniele Spaccini †§ , * SPAWAR Systems Center Pacific, San Diego, CA, USA vdjapic@spawar.navy.mil † Computer Science Department, University of Rome, “La Sapienza,” Italy {gjanci, petrioli, spaccini}@di.uniroma1.it ‡ Department of Informatics, Modelling, Electronics and Systems, University of Calabria, Rende, Italy {gcario, casavola, mlupia}@dimes.unical.it § WSENSE s.r.l., Rome, Italy ¶ Applicon s.r.l., Rende, Italy Abstract—This paper presents the initial implementation of an acoustic synchronization and ranging system to enable the control and cooperation of multiple Unmanned Underwater Vehicles (UUVs). Our solution is based on acoustic clock synchronization and one-way ranging. It requires minimum overhead while providing accurate and quick estimation of the relative distances among underwater nodes. The use of one-way ranging allows to scale up to large teams of UUVs and reduces the energy consumption of localization techniques. Our solution has been implemented in SUNSET, leveraging on the accurate timing information and scheduled transmissions provided by SeaModem acoustic modems. Chip Scale Atomic Clocks have been integrated in the SeaModem to overcome the typical drift of real-time clocks thus enabling accurate one-way ranging estimation during long term missions. The performance of the proposed system have been extensively evaluated in two at-sea campaigns considering different testing scenarios. We have shown that our scheme is able to maintain high ranging accuracy over time without requiring the high overhead and energy consumption of two way ranging techniques. We have also shown that the proposed scheme for acoustic synchronization is very effective in synchronizing real-time and atomic clocks of underwater nodes, whenever needed. Our results confirm that the proposed solution for synchronization and one-way ranging allows to enable the control of multiple UUVs keeping at the bay the overhead in the network and the time needed to estimate relative distances. Index Terms—UUV cooperation, Acoustic positioning, CSAC, Underwater networking, SUNSET, SeaModem. I. I NTRODUCTION Distributed control of multiple vehicles systems have been studied extensively in the past two decades. One particular technology that has matured rapidly and could offer significant merit is that of Unmanned Underwater Vehicles (UUVs) communicating through an Underwater Wireless Sensor Net- works (UWSNs). The advancements in UUVs and underwater communications technologies have allowed the researchers to move from single vehicle to networked multiple-vehicles deployments by defining algorithms which leverage teams of cooperating UUVs (and possibly other, fixed, underwater assets) to accomplish more challenging tasks. Although the UUVs technology has been widely studied in literature there is still a lack of algorithms and technologies to enable au- tonomous cooperation among surface and underwater vehicles (UUVs) in operational and commercial in-field scenarios [1], [2], [3]. When designing a cooperative network composed of a het- erogeneous set of robots, several aspects have to be considered. Among many, those related to the perception of the underwater robots to assess their positions and those of the other robots are the most challenging since the quality of the underwater acoustic communication is limited and subject to time-delay and packet losses. In particular, it requires a strong and robust communication links, interaction and collaboration among the different assets and also the design of novel algorithms to efficiently and quickly estimate the positions of all the underwater nodes, both mobile and static [4]. The relative distances among UUVs (or static nodes) can be estimated either measuring the two-way travel time (TWTT) or one-way travel time (OWTT) of the acoustic signal. Even if the advantage of the first method is that no absolute precision clock is required for the time travel measurement, TWTT does not scale well as the number of vehicles involved in the ranging process increases. Instead, the OWTT approach scales perfectly with any number of vehicles as all UUVs can get ranging updates at the same time. In fact, all the vehicles measuring the OWTT are able to estimate their relative dis- tances with respect to the sender without replying with any acoustic signal thus reducing the overhead in the network. The main drawback of this approach is that the clocks of the involved nodes have to be synchronized for the whole duration of the mission. Various time synchronization algorithms have been proposed in literature, such as for e.g., TSHL [5], MU- Sync [6], D-Sync [7], Mobi-Synch [8] and ROCS [9]. These algorithms effectively address the peculiarities of underwater domain such as long propagation delays, mobility issues and energy efficiency. However all the proposed solutions introduce a high overhead in the network due to the exchange of several acoustic messages to estimate the drift of the clocks. This makes them unfeasible for the coordination and