| Wireless sensor networks deployed throughput an indoor
environment offer the opportunity for accurate location tracking of mobile
users. Using radio signal information alone, it is possible to determine
the location of a roaming node at close to meter-level accuracy. We are
particularly concerned with applications in which the robustness of the
location-tracking infrastructure is at stake. For example,
firefighters and rescuers entering a building can use a heads-up display
to track their location and monitor safe exit routes. Likewise, an
incident commander could track the location of multiple rescuers in the
building from the command post. We are developing a robust, decentralized approach to RF-based location tracking. Our system, called MoteTrack, is based on low-power radio transceivers coupled with a modest amount of computation and storage capabilities. MoteTrack does not rely upon any back-end server or network infrastructure: the location of each mobile node is computed using a received radio signal strength signature from numerous beacon nodes to a database of signatures that is replicated across the beacon nodes themselves. This design allows the system to function despite significant failures of the radio beacon infrastructure.
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In
our deployment of MoteTrack, consisting of 25 beacon nodes distributed
across our Computer Science building, we achieve a 50th-percentile and
80th-percentile location-tracking accuracy of 1 meter and 1.7 meters
respectively when diversifying the radio signal over 16 frequencies. In addition, MoteTrack can tolerate the failure of up to 60%
of the beacon nodes without severely degrading accuracy, making the system
suitable for deployment in highly volatile conditions. We investigate in
detail MoteTrack's performance under a wide range of conditions, including
variance in the number of obstructions, beacon node failure, radio signature
perturbations, receiver sensitivity, and beacon node density.
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