## 1 Introduction  
Wireless sensor networks (WSN) are comprised of many small devices, called motes, which are deployed over an area of interest in order to detect and monitor events, or to track persons or objects as they move through the sensed area [1]. In surveillance and tracking applications, nodes in a WSN work together to monitor the existence of intruding targets, such as persons or vehicles [2]. Since motes have limited resources, particularly computer power and energy supply, common goals of these applications are reliable detection of targets and fast event notifications with minimal resources consumption. A WSN monitoring application must periodically collect sensed data and use it to reconstruct the overall status of the monitored area through data aggregation. For this purpose, when the sensor nodes detect an event, they record it and rely on a distributed routing protocol to send the relevant information towards a base station or sink. The sink is a device with much more resources in charge of collecting the information received from the sensor nodes, processing it, and, if necessary, taking the appropriate actions. It is important that the target events are detected with an acceptable degree of accuracy. To achieve this goal, different algorithms could be used to estimate the intruder location within the sensed area. In addition, the event report should be received by the sink in a very short time, especially for time-critical events. The connectivity issues related to the transmission range of nodes should be under control, and the selected communication protocols must provide a minimum delay and packet loss. In recent years the research community has proposed several solutions for target tracking in WSNs, focusing mostly on specific elements of the architecture, such as the tracking algorithm or the communications protocol, lacking a holistic solution to the problem. To address this limitation, this paper combines a communication architecture with a tracking algorithm to achieve an efficient intruder detection and tracking system. The algorithm assumes the use of binary sensors, a mobile sink, and the IEEE 802.15.4 standard for radio communications. We also introduce the Mobile-Sink Routing for Large Grids (MRLG) algorithm, intended to support sink mobility for time-critical applications, for use along with our tracking algorithm. The MRLG routing protocol allows reducing the routing load by relying on local route recovery processes, which provides significant efficiency in scenarios with a large number of sensor nodes. Both algorithms have a low computational complexity. Many applications may benefit from the use of a mobile sink, among which are target tracking and intrusion detection applications. When an intruder is detected, the sensor nodes report an alarm to the mobile sink, which monitors the progression of the intruder and takes appropriate actions, such as sending the intruder location to the security personnel. The security personnel, endowed with a mobile terminal (sink), may move towards the intruder in an attempt to seize him, using the location feedback provided by the WSN. On the other hand, it has been shown that mobile sinks may improve the WSN lifetime by spreading the transmission overhead of nodes that are close to the sink [3]. However, the main challenge introduced by sink mobility is that the WSN must be continuously and quickly reconfigured to adapt to the topology changes, making sure that information loss is minimal. The rest of this document is organized as follows. In Section 2 we introduce the previous work. The architecture of the proposed model is discussed in Section 3. In Section 4, we present the proposedintruder tracking algorithm. The MRLG protocol is introduced in Section 5. The performance evaluation strategy is defined in Section 6. The results of the evaluation of the intruder tracking algorithm are discussed in Section 7, whereas the evaluation of the MRLG protocol is reviewed in Section 8. Finally, Section 9 concludes this paper and discusses future work.