Localization is an important aspect in the field of wireless sensor networks that has attracted significant research interest recently. The interest in wireless sensor network localization is expected to grow further with the advances in the wireless communication techniques and the sensing techniques, and the consequent proliferation of wireless sensor network applications. This chapter provides an overview of various aspects involved in the design and implementation of wireless sensor network localization systems. These can be broadly classified into three categories: the measurement techniques in sensor network localization, sensor network localization theory and algorithms, and experimental study and applications of sensor network localization techniques. This chapter also gives a brief introduction to the other chapters in the book with a focus on explaining how these chapters are related to each other and how topics covered in each chapter fit into the architecture of this book and the big picture of wireless sensor network localization.
Distributed sensor networks have been discussed for more than 30 years, but the vision of wireless sensor networks (WSNs) has been brought into reality only by the recent advances in wireless communications and electronics, which have enabled the development of low-cost, low-power and multi-functional sensors that are small in size and communicate over short distances. Today, cheap, smart sensors, networked through wireless links and deployed in large numbers, provide unprecedented opportunities for monitoring and controlling homes, cities, and the environment. In addition, networked sensors have a broad spectrum of applications in the defence area, generating new capabilities for reconnaissance and surveillance as well as other tactical applications (Chong & Kumar, 2003).
Localization (location estimation) capability is essential in most WSN applications. In environmental monitoring applications such as animal habitat monitoring, bush fire surveillance, water quality monitoring and precision agriculture, the measurement data are meaningless without an accurate knowledge of the location from where the data are obtained. Moreover, the availability of location information may enable a myriad of applications such as inventory management, intrusion detection, road traffic monitoring, health monitoring, reconnaissance and surveillance.
WSN localization techniques are used to estimate the locations of the sensors with initially unknown positions in a network using the available a priori knowledge of positions of a few specific sensors in the network and inter-sensor measurements such as distance, time difference of arrival, angle of arrival and connectivity. Sensors with the a priori known location information are called anchors and their locations can be obtained by using a global positioning system (GPS), or by installing anchors at points with known coordinates, etc. In applications requiring a global coordinate system, these anchors will determine the location of the sensor network in the global coordinate system. In applications where a local coordinate system suffices (e.g., in smart homes, hospitals or for inventory management where knowledge like in which room a sensor is located is sufficient), these anchors define the local coordinate system to which all other sensors are referred. Because of constraints on the cost and size of sensors, energy consumption, implementation environment (e.g., GPS is not accessible in some environments) and the deployment of sensors (e.g., sensors may be randomly scattered in the region), most sensors do not know their own locations. These sensors with unknown location information are called non-anchor nodes and their coordinates need to be estimated using a sensor network localization algorithm. In some other applications, e.g., for geographic routing in WSN, where there are no anchor nodes and also knowledge of the physical location of a sensor is unnecessary, people are more interested in knowing the position of a sensor relative to other sensors. In that case, sensor localization algorithms can be used to estimate the relative positions of sensors using inter-sensor measurements. The obtained estimated locations are usually a reflected, rotated and translated version of their global coordinates.
In this chapter, we provide an overview of various aspects of WSN localization with a focus on the techniques covered in the other chapters of this book. These chapters can be broadly classified into three categories: the measurement techniques in sensor network localization, sensor network localization theory and algorithms, and experimental study and applications of sensor network localization techniques.