Indoors Localization Using Mobile Communications Radio Signal Strength

Abstract

Radio frequency (RF) indoors localization is adopted by automated guided vehicles (AGVs) positioning due to availability of communications framework sub-system (e.g. ZigBee wireless network) in the entire working system. AGV (i.e. a type of wheeled mobile robot) communications sub-system can therefore support RF localization hardware without additional cost. Mobile communications for indoors environments have many applications and are generally implemented with a personal digital assistant (PDA) for people to exchange information efficiently. In this perspective, examples of applications of RF indoors localization are resources (e.g. products at an automatic warehouse) or people (e.g. doctors in a hospital). The main problem to overcome corresponds to radio signal strength which is difficult to relate to distance to transmitter in indoors environments due to obstacles and objects that cause multi-path, interferences, noise, etc. (Azenha et al., 2010). As radio signal strength is measured with noise, this fact leads to fluctuations on its values which then require filtering (e.g. low-pass filtering, Kalman filtering). At the present, research is being made in order to develop low-cost navigation hardware such as inertial navigation systems (INSs). INSs are composed of inertial sensors such as accelerometers and gyroscopes (Fu & Retscher, 2009). Namely, low-cost gyroscopes with a drift below 1 degree/hour have been described. Position is computed according to double time integration of acceleration and orientation is computed according to time integration of angle rate provided by a gyroscope. Therefore, in indoors environments, INS can become an aiding scheme to the dead-reckoning algorithm (Borenstein et al., 1996; Azenha & Carvalho, 2008b) in the near future. Dead-reckoning is the most adopted scheme for indoors localization, because other systems such as global positioning system (GPS) do not work indoors. Dead-reckoning can use accelerometers and gyroscopes for INS navigation or rotary encoders and gyroscopes or magnetic compasses for wheeled AGVs indoors navigation. RF localization schemes are also being developed for indoors localization purposes, because they increase system efficiency in terms of its lower cost and they can have sufficient accuracy characteristics. RF indoors localization methods are therefore means of attenuating AGV dead-reckoning navigation errors (Azenha & Carvalho, 2007b; Azenha & Carvalho, 2008a; Azenha et al., 2008; Park et al., 2009). Dead-reckoning (from sailing: deduced reckoning) navigation method makes use of odometry and heading measurement signals. Dead-reckoning is prone