The Global Positioning System (GPS) is a satellite-based location system, which
is widely used for location determination, navigation and time synchronization.
Because GPS signals are transmitted at relatively low power levels and over great
distances, the received GPS signal is relatively weak. Moreover, inside buildings or
in urban canyons, the GPS signals are further attenuated by walls, roofs and other
objects. Thus, the reception of GPS signals is not reliable in indoor or obstructed
environments.
An accurate and reliable indoor positioning system is critical to provide navigation
support to rst responders in emergency scenarios as well as to minimize
the rescue time of injured personnel. Moreover, Location Based Services (LBS)
drive the development of indoor positioning techniques, mainly due to the need to
provide service continuity inside buildings. Location based services are typically
used with mobile devices in pedestrian mode for applications such as locationbased
advertising, people or pet tracking and friend-nder functionality. All these
applications require continuous, seamless and ubiquitous positioning, which GPS
is not capable of delivering.
In the last ten years several techniques have been proposed to solve the problem
of indoor positioning. Some depend on the presence of a known network of
base stations such as Wireless Local Area Networks (WLANs) access points, Bluetooth
nodes or digital television towers. Other short-range technologies depend
on specialized transceivers specically designed for positioning e.g. GPS pseudolites
and repeaters, active Radio Frequency Identication (RFID) or Ultra-Wide
Band (UWB). However, no single positioning technology is able to provide high
availability and precision. For this reason, the trend is toward hybrid solutions,
i.e. the combination of dierent positioning methods. The seamless cooperation
between dierent technologies is required in order to provide location capability
whenever a clear view of the sky is obstructed.
While dead-reckoning is not a standalone positioning technology (it needs a
known start reference position) it is the most promising option when other infrasiii