# An Energy Efficient Sink Location Service for Continuous Objects in Wireless Sensor Networks
## 1 Introduction  
The amount of damages from either disasters or accidents is influenced by the level of the prevention and countermeasures system. Especially, in case of the distant areas from the social infrastructures, such as a mountainous region and a forest area, a prevention system exerts a stronger influence then the case of a downtown area because of their low accessibility. As a promising solution for a prevention system in inhospitable areas, Wireless Sensor Networks (WSNs) have been receiving significant attention. A WSN consists of low-cost and small sensor nodes and is able to provide sensing information to remote users by self-organizing manner. These advantages enable construction of high-level prevention systems using WSNs with low-price. This paper is the part of an effort to facilitate advanced disaster prevention and environment monitoring system using WSNs. Concretely, we focus on increasing the usefulness by reducing energy consumption in communication parts. After sensor nodes are deployed in interested field, they organize a network by self-organizing manner. The information about the base station, called a sink, is embedded into sensor nodes before deployment or advertised on the network after deployment. Once an event is occurred, the sensor nodes exchange messages with each other to elect a reporting node, called a source. The source generates a data packet and sends it to the sink by multi-hop transmission. These features leverage the applications of sensor networks ranging from military surveillance to habitat monitoring [1; 2]. Object tracking is one of the important applications in WSNs where sensor nodes detect where and when the object exists. There have been many researches on object tracking with sensor networks. Target objects are divided into two categories: individual objects and continuous objects. The individual objects usually have very small size comparing with the large area with sensor network deployed. An individual object is sensed by one or few sensor nodes. For example, an individual object might be an animal in habitat monitoring or an intruder in military surveillance. In contrast with the individual object, a continuous object is a large-scale phenomenon which is originated from a specific point (e.g., a spark in the forest or a crack on the gas pipe) and spreads continuously. Then, the continuous object occupies large area and it is detected by many sensors simultaneously. The continuous object might be fire in forest fire monitoring system and harmful gas in leakage alarm system. Therefore, continuous object tracking is suitable for prevention of disasters and environment monitoring. Figure 1 shows the forest fire monitoring system as an example of the continuous object tracking. Once forest fires brake out in the sensor field, detecting information is reported to the control center. Then, the fire fighters start extinguishing the fire. During extinguishing process, latest detecting information should be reported to the control center as well as the fire fighters. The fire fighters can move to strategic locations by means of the information from the sensor network. Besides, the information is provided to the mountain climbers as well. The climbers can find fire escape routes by means of the information. In this scenario, the nodes detecting fire are sources. The fire fighters and climbers act as mobile sinks which are mobile communication entities whereas the control center is a static sink which is located at fixed location. In the scenario described above, the applications should satisfy the following requirements:  
* Sink mobility support: In case of a static sink, it is possible that providing sensor nodes with its location information as a priori knowledge because its location is fixed. On the other hand, a mobile sink has to notify its location to the network periodically because it moves continuously.