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We study the relevance of highly precise indoor localization techniques for quadrocopters and present an ultrasonic sensor system that achieves excellent localization performance, although the system has to rely on the limited computation resources of sensor nodes used for time-of-flight-based localization. Quadrocopters, i.e., flying four-rotor robots, are on-board sensor controlled systems. In comparison to classical monorotor objects (helicopters), the quadrocopters can be piloted with much lower effort. However, lateral drifts cannot be compensated for only referring to the built-in sensors. Nonetheless, the detection of such drifts is strongly necessary for indoor operation-without any corrections, the quadrocopter would quickly cause a collision. To compensate for the dislocation, we developed an indoor-localization framework for time-of-flight-based localization using ultrasonic sensors. It is optimized for use in sensor nodes with low computational power and limited memory. The system is designed for high scalability and to provide high accuracy, even in the case of erroneous measurements. The developed hardware platform is very lightweight to be carried by mobile robots and flying quadrocopters. Based on our real-time localization system, position controller and navigation functionality can be implemented.