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In rescue scenarios, real-time requirements are one key issue when using wireless sensor networks (WSNs) for tracking and monitoring of rescue forces. If a node detects an alarm condition, the alarm message must be delivered to the base station in time. To guarantee the timely delivery the whole system architecture has to fulfill real-time requirements. There are only a few real-time architectures which can be used for sensor networks. These architectures are too generic to comply with the other requirements of a WSN architecture, like RAM usage and energy awareness. In this paper, we present FireKernel, a real-time micro kernel designed for WSN operating systems with a special focus on hard real-time requirements and strict energy management. It offers a preemptive real-time scheduler, mutexes and synchronous message passing for interprocess communication (IPC). The scheduler is prioritybased and uses no periodical timers. As part of the kernel we introduce a tickless timer system. We have implemented and tested the kernel on MSP430 and ARM7TDMI based sensor nodes. We discuss the kernel's architecture, implementation and resource usage. Further, we present benchmark results for interrupt latency and energy consumption of the timer architecture.