## 3 Greedy Scanning Data Collection Strategy  
### Network Model  
Data gathering strategies using mobile sink can prolong the network lifetime. However, for event area changing continuously situation, normal data gathering strategies with mobile sinks are not suitable. Especially, when the events involve animals migrating, enemy troops' intrusion, and fire monitoring of forest, the events area appears dynamically, and regionally. Our GSDCS is designed for this situation. The typical application for this situation is animals monitoring. In this kind of application scenario, the wild animals have many group behaviors in many cases [44; 45; 46]. Hence, we set our application scenario as social animals monitoring in a field. Then the event area is local, so we use a circular to represent the event area. As the animals may migrate or interest of monitoring may change, the location of circular event area may change with time. As illustrated in Figure 2, sensor nodes are deployed randomly in a rectangle area. The network is divided into grids, consists of \(N\) nodes, and one mobile sink, which collects data from the whole network. The number of virtual grid cells are \(M\). All nodes are well connected, and the network is full covered. Sensor nodes are static and location aware (i.e., equipped with GPS-capable antennae). Each node has the same initial energy, sensing radius \(r_{s}\), and communication radius \(R\). To guarantee the full cover of network, \(N\geq Ma^{2}/\pi r_{s}^{2}\) is necessary. However, in the simulation or practical application, the \(N\) adopted is much larger than \(Ma^{2}/\pi r_{s}^{2}\). Hence, we assume that each virtual grid cell always has sensor nodes. The mobile sink is not constrained by energy and moves without predefined trajectory. The trajectory of mobile sink is constructed dynamically according to the current status of network. The situations of obstacle or multi-path are not discussed in this paper. In our GSDCS, the notation and corresponding definition is shown in Table 2.  
Our GSDCS is composed of three phases:  
* Network initialization: To achieve the goal of updating sink location in a local area, the network area is divided into virtual grids. Nodes in the same grid form a group. The initialization process includes the grid structure establishment, head nodes election and neighbor table establishment. The details are discussed in Section 3.2.
* Data collection: In this phase, the way of data routing is proposed first. With the cooperation of virtual grid structure, this data routing way can deliver sensory data to the mobile sink easily. Whereafter, we talk about the moving pattern of the mobile sink with which energy consumption of network can be balanced. To minimize the energy cost in updating sink location information, the local updating with virtual grid structure is discussed at the end of this phase.
* Head nodes re-election: The long time working of head nodes may lead to network energy being unbalanced and lifetime being decreased. To improve the performance of the network, the re-election of head nodes is discussed in Section 3.4.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Notation** & **Definition** \\ \hline \(r_{s}\) & Sensing radius of nodes \\ \(a\) & Side length of a grid cell \\ \(v\) & Speed of the mobile sink \\ \(N\) & Number of nodes deployed in network \\ \(R\) & Communication radius of nodes \\ \(L\) & Longitudinal length of network \\ \((x_{0},y_{0})\) & Initial coordinate of the mobile sink \\ \((x_{i},y_{i})\)\((1\leq i\leq N)\) & Coordinate of node deployed in network \\ \hline \hline \end{tabular}
\end{table}
Table 2: Notations and definitions.