# A Mobile Anchor Node Assisted RSSI Localization Scheme in Underwater Wireless Sensor Networks
## 1 Introduction  
With the rapid development of communication and sensor technology, underwater wireless sensor networks (UWSNs) have become a fast-growing field [1]. UWSNs have a wide range of applications, such as pollution monitoring, off-shore oil exploration, and oceanography data collection [2; 3; 4]. In UWSNs, localization is a fundamental issue, because only the data associated with the location of the sensor node can be meaningfully interpreted by data users [5]. Compared with the terrestrial localization issue, underwater localization has some fundamental differences. First, the Global Positioning System (GPS) technology is unavailable in the underwater environment, because of the strong attenuation of radio waves in water [6]. Thus, the underwater node cannot directly locate itself through the GPS. Second, due to the large propagation delay in underwater acoustic channels, the cost of maintaining clock synchronization is high and underwater nodes are usually clock asynchronous [7]. Third, the transmitting energy consumption of the underwater sensor node is several times or even hundreds of times the receiving energy consumption. To save energy, the underwater sensor node should reduce message transmission during the localization process [8]. The localization method based on Received Signal Strength Indicator (RSSI) does not require clock synchronization, which avoids the high cost of maintaining clock synchronization [9]. Localization methods require anchor nodes to provide reference locations [10]; since the cost of the anchor nodeis much higher than that of the ordinary sensor node, it is expensive to deploy a large number of static anchor nodes in UWSNs [11]. Hence, using a small number of mobile anchor nodes to assist localization is an economical method [12]. Localization schemes [13; 14; 15] jointly use the RSSI and one mobile anchor node to reduce location cost. [PERSON] [13] proposed a mobile anchor node-assisted localization scheme called Perpendicular Intersection (PI). By comparing the received RSSI values on a node, this scheme achieved the locating of nodes using the geometric relationship between the node and the trajectory of the mobile beacon. In this scheme, the error of determined projection in the localization process led to the error of the node location. To improve location accuracy, [PERSON] et al. [14] adopted a piecewise polynomial curve fitting method to reduce the error of determined projection. [PERSON] et al. [15] applied the polynomial curve fitting method and the derivative method to reduce the error of determined projection. Compared with the scheme in [13], the schemes in [14; 15] improve the location accuracy. However, there are two points worthy of further study in [13; 14; 15]: (1) To improve location accuracy, is there any other better way to reduce the error of determined projection than existing schemes? (2) In existing schemes, the anchor node needs to travel at least two trajectories to locate the sensor node. Can the anchor node only travel one trajectory to shorten the location time? Based on the above two considerations, in this paper, we propose a mobile anchor node assisted RSSI localization scheme in UWSNs. First, to improve location accuracy, we design a support vector regression (SVR) based interpolation method to estimate the projection of sensor nodes on the linear trajectory of the mobile anchor node. The proposed method increases the accuracy of the nonlinear regression model of noisy measured data and synchronously decreases the estimation error caused by the discreteness of measured data. The SVR based interpolation method achieves an improvement in location accuracy by reducing the error between the determined projection and the actual projection. Second, we develop a curve matching method to obtain the perpendicular distance from sensor nodes to the linear trajectory of the mobile anchor node. The location of the sensor node can be calculated based on the projection and the perpendicular distance.