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A sensor network system has been developed for tracking people's locations in workplaces as part of a ubiquitous network system for providing context-aware services in daily activities. Since the installation of such a sensor is desired any place within its target domain with few limitations, it must operate by battery for a relatively long time, e.g., one month. To satisfy this requirement, we designed a batteryoperated sensor node based on ZigBee technology and extended its operation period by developing a exible sleep control protocol and a high-accuracy time synchronization mechanism between sensor nodes to reduce power consumption. From simulations based on actual data collected, we conrmed that a sensor node located in a hospital's medical ward can work over 21 days using four AA Ni-H batteries.

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