## 1 Introduction  
Underwater Wireless Sensor Networks (UWSNs) have gained researchers' much attention in the past few years due to their great potential utility in many applications, such as ocean resource exploration, marine environment monitoring, ocean target surveillance, submarine tracking and disaster prevention [1; 2]. In these applications, sensor node's accurate and reliable locations are required. In addition, some network system functions, e.g., network topology management and design of network communication protocols, also need sensor nodes' location information to achieve. Thus, the secure localization problem becomes one of the most important and fundamental issues in UWSNs. Many localization algorithms have been proposed for UWSNs [3]. However, few of them take location reliability or security into consideration, while UWSNs are always deployed in unattended and even hostile environment. Ensuring node safety is a basic and essential knowledge to improve node location accuracy and reliability. Therefore, in this paper, we study both node security and localization accuracy in the proposed secure localization algorithm.  
A large number of security mechanisms have been proposed to ensure safety of sensor nodes. Traditional mechanisms, e.g., cryptography and authentication, can well resist external attacks, but cannot eliminate insider attacks effectively. In addition, the traditional mechanisms are not energy efficient for energy and resource limited UWSNs. Relatively speaking, trust management has been recently suggested as an innovative and energy efficient security mechanism. Although trust management mechanisms are widely studied in Internet, Aodhoc networks, P2P networks, social networks and Terrestrial Wireless Sensor Networks (TWSNs) for network security [4; 5], they cannot be directly applied to UWSNs due to the unique characteristics of underwater environment, e.g., inevitable node movement, variable propagation delay, instable acoustic channel, poor underwater link quality and limited channel bandwidth. These factors mainly impact the success communication rates and the accurate trust assessments of underwater sensor nodes. This highlights the fact that it is critical to design a new trust model for UWSNs considering the unique characteristics of the acoustic communication channel and underwater environment.  
In this paper, we propose a Collaborative Secure Localization algorithm based on Trust model (CSLT) for UWSNs. CSLT first uses trust model to ensure node safety and avoid the influence from malicious nodes, which ultimately reduces unknown nodes' localization error and enhances localization accuracy. Then, based on the collaboration of sensor nodes, localization ratio and localization accuracy can be further improved. The proposed CSLT consists of the following five sub-processes: trust evaluation of anchor nodes, initial localization of unknown nodes, trust evaluation of reference nodes, selection of reference node, and secondary localization of unknown node. In the first sub-processes, each anchor node pretends to be an unknown node to ask for localization and evaluate trust for each other. Only trusty anchor nodes can be used to localize unknown nodes. Then, the unknown nodes fail to be localized in the initial localization process can ask for secondary localization. Before the secondary localization, the trust values of reference nodes are calculated based on the cloud theory. Only trusty reference nodes are chosen to further localize the rest of unknown nodes until all the nodes are successfully localized.  
The rest of this paper is organized as follows: Section 2 reviews existing related works. Section 3 presents the network model, the assumptions and the overview of CSLT. Section 4 describes the proposed trust model in detail. Section 5 gives a detailed analysis of simulation results. And finally, Section 6 makes a conclusion of this paper and discusses our future research works of secure localization in UWSNs.