## 1 Introduction  
When applying a sensor network to fields that involve emergency events, such as battlefield surveillance, medical healthcare, illegal smuggling, _etc._, the primary concern is to preserve all valuable data acquired from the targeted area without any losses. For instance, sensor nodes have been deployed in the military area, and each sensor node was equipped with a sound sensor. The sound sensors can detect the sound made by the soldiers and military vehicles in a limited sensing range. The sensor network needs to discover any unusual sound made by enemies; hence, it is crucial to have low-power, new protocols, methodologies and structures for transferring data and information in a network with full sensing coverage capability for an extended working period. In recent years, the goal of constructing wireless sensor networks (WSNs) provides an _ad hoc_ communication model serving in a specific region with mission-critical applications. WSNs consist of a great number of sensor nodes with wireless communication capability. With the advantage of integrated circuits and wireless communication technology, wireless sensor nodes have been manufactured using low-cost and low-power design for practical applications [1,2]. Due to the limited energy resources of sensor nodes, many previous studies, such as routing algorithms, coverage control, power management, node localization, and medium-access control, have been proposed to deal with the limited energy issue [3]. In many applications, WSNs are organized as clusters, which have been widely studied in recent years. The clustered architecture decreases the opportunity of communication overhearing and power dissipation of sensor nodes. The clustered architecture groups up sensor nodes that are nearby. The sensed data is sent to a cluster head for data fusion and aggregation. Thus, the size of the sensed data sent to the sink can be reduced, and the energy consumption of sensor nodes is further reduced. The clustered architecture has been proven to be successful in saving energy and prolonging the network lifetime [4, 5, 6]. In addition to energy efficiency, it is critical to maintain sensing coverage over the entire targeted area. The coverage preservation is a basic requirement for fulfilling the quality of service (QoS) in many mission-critical applications, such as battlefield or border surveillance [7]. Any hole that occurs in the coverage of a given network might be fatal and not be tolerable [8]. The network designed for mission-critical applications using WSN technologies exploits the features of _ad hoc_ networking. The primary goal of the mission-critical network is to prevent the sensed data from being routed through sparsely populated areas covered by a small number of sensor nodes [9]. The idea behind this approach is that the nodes in the sparsely populated areas are less used as packet routers. Thus, these nodes can utilize their energy resources to collect data for a longer working period. Furthermore, in a mission-critical application, dynamic deployment of sensor nodesfor the rapid exploration of the emergency area is essential. The sensor nodes should be able to be rapidly deployed without concerning the network topology that influences the sustenance of full sensing coverage. In this study, we integrate energy-efficiency and coverage-preserving techniques in a cooperative manner. A novel energy-aware coverage-preserving hierarchical routing protocol (referred as ECHR) is presented to maximize the working time of full coverage in a given WSN regardless of the deployment patterns of the sensor nodes. The basic idea of the proposed ECHR algorithm is to take the remaining energy of the nodes as well as the coverage redundancy of its sensing ranges into consideration when selecting a root node. Intuitively, the sensor nodes deployed in a densely populated area have a higher probability to be selected as the root node in each round.