## 3 Collaborative Secure Localization Algorithm
### Overview of Secure Localization Algorithm CSLT  
Before introduce our proposed algorithm, some definitions are presented at first. Definition 1. localization beacon. In the process of localization, anchor nodes broadcast information to help unknown nodes with localization. All the broadcasted packets, which include coordinate information, ID information, trust values, _etc._, are named as localization beacons. Definition 2. upgrade anchor node. A successfully localized unknown node can work as an anchor node to localize neighbor nodes. In this paper, the trusty and successfully localized unknown node can be selected as an anchor node, we named it as an upgrade anchor node. Definition 3. reference node. All the anchor nodes and upgrade anchor nodes are collectively named as reference nodes. They can help unknown nodes with localization. In order to maximize localization ratio and improve localization accuracy of underwater sensor nodes, we propose a multi-anchor nodes collaborative localization (MANCL) algorithm in [29]. The MANCL algorithm divides the whole localization process into four sub-processes: (1) unknown node localization process; (2) iterative location estimation process; (3) 3D Euclidean distance estimation process; and (4) 3D DV-hop distance estimation process. In the first sub-process, anchor nodes broadcast their coordinates periodically. All the unknown nodes that receive the coordinates can estimate their distances to the corresponding anchor nodes. If an unknown node receives four (or more than four) non-coplanar coordinates from different anchor nodes, the unknown node can calculate its position based on a multilateral localization method. However, in large-scale UWSNs, not all the unknown nodes can be successfully localized in the first sub-process. For example, some unknown nodes may not receive enough (at least 4) beacons from anchor nodes, or receive beacons from coplanar anchor nodes. In this case, the unknown nodes cannot be localized. In order to localize all the unknown nodes in the UWSN, we propose use other distance estimation methods and localization algorithms to help unknown nodes with localization. First, the successfully localized unknown nodes are used to further localize their neighbor nodes in the second sub-process. The successfully localized unknown nodes with higher trust values can be chosen as upgrade anchor nodes; Then, the upgrade anchor nodes within two hops are used to help residual unknown node with localization during the rest of localization sub-processes; In the third sub-process, the improved 3D Euclidean distance estimation process which consists of two mechanisms (a communication mechanism and a vote mechanism) is proposed to localize unknown nodes. In the communication mechanism, non-localized unknown nodes use localized sensor nodes within communication range to estimate their coordinates. In the vote mechanism, neighboring anchor nodes and upgrade anchor nodes vote to determine the coordinates of non-localized sensor nodes. Finally, in the 3D DV-hop distance estimation process, the two-hop anchor nodes are used to calculate coordinates of unknown nodes. Simulation results show that, the proposed MANCL algorithm can perform better than related works with regard to localization ratio, average localization error, and energy consumption. However, how to evaluate trust values for sensor nodes are not discussed in [29]. Therefore, in order to improve location security of underwater sensor nodes, we propose a trust model for sensor nodes' trust evaluation and apply the trust model in the secure localization.