Over the past years, wireless sensor nodes (WSN) in border surveillance and their application in a numerous other scenarios have been researched worldwide. One of the most interesting scenarios for application of WSNs related to its use in the monitoring systems areas. WSN is a unique system that can used for the surveillance of critical areas such as borders, private properties or even rails. In this paper will be made an analysis of WSN application in border surveillance, its functions and system architecture.
II. Existing border patrol techniques
Border patrol systems and techniques have recently gained interest to address the concerns about national security. One of the major challenges concerning the protection of long stretches of borders is the necessity for intensive human involvement in patrolling locations. With the invention of different electronic patrol techniques, this involvement helped to decrease that involvement. Thus, Unmanned Aerial Vehicles (UAVs) allows to minimize human involvement in border patrol due to their large coverage of areas and high mobility of those devices. UAVs have recently been used to automatically detect and track illegal border crossing. With the help of UAV significant human resources can be redirected to decision management activities and information processing based on the data from these devices .
To complement the UAV activities, recently, Fiber Optic Sensors (FOSs) are introduced. Seismic sensors are equipped with FOSs so that they can measure pressure waves in the earth caused by intruders .
Compared to the wired sensors, Unattended Ground Sensors (UGSs)  provide higher system robustness. UGSs have been intensively used for military Intelligence Surveillance and Reconnaissance (ISR) applications. UGSs can detect vibration/seismic activity or magnetic anomaly, which indicate that people or vehicles are crossing the border .
While scalar sensors such as vibration sensors are important to detect an intrusion, these sensors provide limited information to classify the intruder. To this end, surveillance towers equipped with video monitors and night vision scopes provide high accuracy in human detection and keep false alarms to a minimum .
III. Wireless sensor Node (WSN) in Border Surveillance
The role of WSN in border surveillance, as in most WSN applications, focuses on information gathering from various types of sensors: seismic, thermal cameras, and motion detectors. Some advanced WSN process these raw data and send an abstracted alarm or aggregated data to the command center, which, in turn, takes the appropriate defense action. Many researchers from different organizations have suggested solutions for border surveillance problems . In this section, we will be reviewed deployment methodology, their primary objectives and working principles.
IV. Deployment Methodology
A wireless sensor node is a popular solution when it is difficult or impossible to run a mains supply to the sensor node. Considering that wireless sensor node often is placed in a hard-to-reach location, regular battery changings can be costly and inconvenient. An important aspect in the deployment of a wireless sensor node is ensuring that there is always adequate energy available to power the system. From that point, accumulation of the local energy to power the wireless sensor node is desirable .
V. Primary Objectives for Deployment
Wireless sensor nodes in border surveillance must be deployed in a way that leverages the overall design goals. Therefore, most of proposed schemes aimed to cover next goals: increasing the coverage area, prolonging the network lifetime, achieving strong network connectivity, and/or boosting the data fidelity. Secondary objectives, such as node failure tolerance and load balancing must be also considered. The main object is to maximize the deployment objectives with the least resources, such as number of nodes. It should be noted that meeting the design objectives with the use of random deployment schemes is the most challenge, meanwhile, during spontaneously deterministic placement can theoretically meet main objectives .
A. Area Coverage
Area of coverage is one of the most important parameters. The assessment of the WSN coverage area is based on particular sensor model, used metric to measure the collective coverage of deployed WSN. Optimized sensor placement is not an easy problem. The complexity is often introduced by the quest for employing the least number of sensor for meeting the application requirements and by uncertainty in a sensor’s ability to detect an object due to distortion that may be caused by terrain or the sensor’s presence in a harsh environment .
B. Objects detection
Object detection and tracking is one of the challenging and non trivial applications for Wireless Sensor Network in which network of wireless sensors are involved in the task of tracking a moving object. For that functions there are several factors that need to be considered when developing algorithms for detection of the moving objects including single vs. multiple targets, stationary vs. mobile nodes, target motion characteristics, efficient energy demands and network architecture .
The target detection is performed with the use of Received Signal Strength Indicator [RSSI] method. This method performs estimation of the distance between two sensors through the measurement of the signal power which is transmitted from the sender to receiver. In theory, the signal strength is inversely proportional to squared distance, and with a known radio propagation model it can be used to convert the signal strength into distance . The main advantage from such method is low price, because most receivers can estimate the received signal strength. However, there may be minor inaccuracies regarding distance estimation due to noise and interference. It should be noted that considering its low cost, it is possible that a more sophisticated and precise RSSI application, for example with better transmitters can become the most popular technology to estimate distance between sensors. A sender node sends a signal with a determined strength that fades as the signal propagates. The bigger the distance to the receiver node, the lesser the signal strength when it arrives at that node .
C. SYSTEM ARCHITECTURE
In this section, we will present the global architectural issues of the network that will be used by our border surveillance system. We will first, present the hierarchy of the nodes used and the functionalities assured by each type of node. Then we will present the global network architecture and the disposition of the nodes in the monitored area. Equally, WSNs are dynamic in the sense that radio range and network connectivity changes by time; sensor nodes die and new nodes can be introduced to the network. Though WSNs are more induced, denser, and may not operate with suffer redundant information. In this paper will be reviewed hierarchical structure of WSN .
A. Node Hierarchy
All the sensors that are deployed in the sensor field construct a logical tree. In this node hierarchy system consists of two nodes: one represent parental node and other represent leaf node. In such hierarchy, data packets from a leaf node transferring to parent nodes. Responding on that signal a receiver node receives signal from the child node and sends information to receiver’s parent node after comparing that signal its own possessed data. The main advantages, of why such hierarchy was chosen lies in the fact that it requires less energy power, if to compare with topologies, because flooding is not required for data interaction .
Figure 1. Tree topology
B. The Network topology
The hierarchy among the nodes will be based on their capabilities: base stations, cluster heads and sensor nodes. Base stations collect sensor readings and perform precise operations on sensor nodes function, as well as manage the network. Sensor nodes are deployed around one or more hop neighborhood of the base stations . They create a compact network where a cluster located within specific area may provide similar or close readings. Nodes with better resources, cluster heads, may be used to collect and merge the traffic of border surveillance system is usually sufficient to reach base stations. Thus, data flow in current network will be: (1) pair-wise (unicast) among pairs of sensor nodes and from sensor nodes to base stations, (2) group-wise (multicast) within a cluster of sensor nodes, and (3) network-wise (broadcast) from base station to sensor nodes of border surveillance system .
Fig. 2. Network Topology of Wireless Sensor System
Wireless sensor nodes represent an emerging technology that is used in border surveillance and intrusion detection applications. The main advantage of using wireless sensor nodes in such areas is the high spatial and temporal data resolution results from deploying numerous groups of low-cost sensor nodes along the surveillance area.
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