Localisation systems

We present the common techniques and technologies that enable location identification in a certain area. Localization in wireless networks is the process of finding a physical location in an automated manner using wireless communication.

Practically, localization can be a stand-alone application (e.g. inventory tracking in a distribution center), or it can provide support to the network service (e.g. routing). Today, such applications have evolved into real-time location systems (RTLS) using a wide range of wireless technologies.

Location systems can have various classifications:

• They can be based on the type of distance measurement technique employed as: range based and range free. The former systems make use of range-based measurements for measuring absolute distance/angle and the latter makes use of techniques that give relative location of the objects.
• Localization techniques can also be classified as centralized or distributed. The centralized approach depends on sensor nodes transmitting the data to a central server, where the computation is performed to determine the location of each node.
• Another way to look at the location systems is based on the type of coordinate it outputs. An absolute coordinate system has global coherence and is desirable for most situations, being aligned to popular coordinate systems used as commercial and military references, such as GPS. Relative positioning establishes positions that are relative to a system that is local to the network and can possibly be arbitrarily, but still provide network wide coherence.

There are 3 steps involved in any localization system namely, distance measurement to anchors or beacons, position estimation and position refinement, which is an optional step. The choice of the techniques and technology affects the granularity and accuracy of the location information.

The ranging technology forms the heart of any range based localization system. There are several range based techniques such as Time-of-arrival (TOA), Time difference of Arrival (TDOA), Angle-of-arrival (AOA), and Received Signal Strength Indication (RSSI). The TOA and TDOA make use of signal propagation time for finding the range of distance. To augment and complement TOA and TDOA, AOA allows nodes to estimate and map relative angles between the neighbors. RSSI makes either theoretical or empirical calculations to convert the signal strength measurements to distance estimates. Range free schemes make use of some algorithm that calculates the distance in terms of e.g. hop count to anchor nodes.

The next step after the determination of the distance is the location estimation. There are different methods for estimating the location. Triangulation uses the geometric properties of triangles to compute object locations. It is a positioning procedure that relies on angle measurements with respect to the known landmarks. Triangulation is subcategorized into lateration—using distance measurements— and angulation—using angle or bearing measurements. Trilateration uses ranges to at least three known node position to find the coordinates of unknown nodes.

The trilateration procedure starts with an a priori estimated position that is later corrected towards the true position. Triangulation has been extensively used for both navigation and geodetic purposes, because measuring angles is easier and more accurate than measuring distances. Also transformation from triangulation to trilateration is possible with simple transformation methods. Least square algorithm is used to derive position estimation from collection of reference points and their associated ranges.

To conclude, designing a location system for a particular environment presents difficulties when the system is applied to other environments. Despite the plethora of established location technology, there is no single location technology that may be relied upon in all environments to provide accurate location information. Clearly ”No one size fits all”, there may not be a single best technology. However each of the techniques has its own pros and cons.

Reference

Muthukrishnan, K. and Lijding, M.E.M. and Havinga, P.J.M. (2005) Towards Smart Surroundings: Enabling Techniques and Technologies for Localization. In: 1st Int. Workshop on Location- and Context-Awareness (LoCA), Oberpfaffenhofen, Germany. pp. 350-362. Springer-Verlag. ISSN 0302-9743 ISBN 3-540-25896-5