## 1 Introduction
### Cooperating Autonomous Maritime Vehicles as a Cyber-Physical System  
There are several ways to describe CPSs according to the existing literature. For [PERSON] et al. [1], they are physical and engineered systems that operate in a way that their elements are controlled,monitored, coordinated and integrated by an underlying core of computing and communication operations. According to [2], they imply a tight integration of physical, communication and computation elements ranging from medical devices to transportation and energy systems. All in all, although CPSs can be described in many different ways, they usually imply a collection of devices distributed in a certain location that interact with the environment by means of software-based commands that are sent and received with a minimal interaction from a human end user. This is no exception regarding the integration of communications at the data level regarding maritime robotics. This latter kind of devices can be better described as the usual vehicles manufactured to perform over- and subsea operations. Several examples of them are: Autonomous Underwater Vehicles (AUVs), Autonomous Surface Vehicles (ASVs), Unmanned Surface Vehicles (USVs) or a Remote Operated Vehicles (ROVs). Some examples of the operations they will be involved in are navigation in unknown underwater environments with simultaneous data gathering [3], river tracking and navigation while observing as much as possible The International Regulations for Preventing Collisions at Sea (COLREGs) [4], as components of distribution management systems with aerial vehicles [5], and as hardware components for underwater sensors networks [6]. Having a myriad of these vehicles working cooperatively under a common mission for all of them can be regarded as a CPS, where several physical elements are integrated in a deployment where vehicles gather data and are controlled and/or monitored by an entity responsible for taking decisions or designing the policies of that system. Unfortunately, a CPS with those features also presents important challenges that must be addressed, both related to the maritime environment (low bandwidth for data transmissions, unreliable channel for information exchanges, environment with a high degree of unpredictable features such as seawater composition, underwater currents, etc.) and to the distributed nature of the system (communications among heterogeneous hardware elements deployed in a certain environment). Among other pieces of work, these challenges are stated in [7], where it is mentioned how the movement added to the underwater nodes by submarine currents makes underwater routing very unreliable, and [8], where it is explicitly claimed that \"_Underwater acoustic communications are notoriously prone to interferences, disruption and unpredictable delays. Furthermore, the available bandwidth is usually very limited and the latency is large_\". As in any other CPS, information must be transferred among the elements used to collect information and interact with the application domain where those devices are included. This is a mandatory feature for the kind of environment that is described in this manuscript because there are several use cases where different types of information must be exchanged:  
1. If an infrastructure that requires pillars or columns has been installed in the sea (for example, an offshore windmill or an oil platform), maintenance operations can make use of a CPS composed by several AUVs to monitor those pillars searching for cracks or other signs of structural fatigue in a cheaper--and less dangerous for human lives--manner than if there were human divers participating in the surveying operations. 2. In a similar way, should there be a need to explore any subsea, pipe-like structure (telecommunication cables, underwater pipelines) it can be done in a more efficient manner by deploying several underwater vehicles.