Ocean Sensor Networks Time Synchronization

With the increasing development of science and technology, wireless sensor networks have been known by people gradually. China as a huge marine power country with a vast coastline, in the sea fishery resources investigation and biological information acquisition, seabed mineral exploration, military reconnaissance and other marine applications, all of them need the help from wireless sensor network in the sea, therefore, research on marine wireless sensor network has important academic significance and broad application prospect.Time synchronization is the supporting technology of sensor network. Unifying the local time of each node in the network makes the nodes accomplish the tasks coordinately. It is the foundation to imply data fusion, location tracking, transmission scheduling and other functions in wireless sensor network. The underwater environment use acoustic communication, which has a very high propagation delay and other uncertain factors, resulting in many mature time synchronization algorithms on land which use radio frequency communication cannot be applied to the water directly. So the time synchronization algorithm for wireless sensor network in the sea should consider the propagation delay and node mobility of ocean environment’s effects on synchronization precision, which is one of the main research contents in this paper.This paper proposes two kinds of time synchronization algorithm for different application environment. First, Energy attenuation-based time synchronization algorithm called "Ea-Sync" is proposed for the ocean networks which is not huge but has a strict need for precision. It analyzes relationship between the node movement and propagation delay from the point of synchronization information’s energy attenuation, with the assistance of mobile beacon node, nodes exchange information twice to complete the node’s time synchronization. It also uses the hierarchical structure of acoustic velocity to design the trajectory of mobile beacon node and reduce the influence of the synchronizing accuracy caused by the variety of acoustic velocity. Second, for the large scale ocean sensor networks, Beacon Node based Clock Synchronization called "B-Sync" is designed for the coverage area of beacon node; Dynamic Node assisted Clock Synchronization called "D-Sync" is designed for the area that beacon node could not be covered by regarding the general node which is synchronized as the beacon node; In order to reduce the influence of acoustic velocity change, Layering Clock Synchronization called "L-Sync" is proposed. In the simulation, those two algorithms are compared with the existing time synchronization algorithms. The limitations and deficiencies of other algorithms is analyzed and the superiority of the algorithm is verified obviously from the initial synchronization and clock drift rate, the distance between nodes, node mobility rate and energy consumption point of view.