This work presents a complete in-door microwave localization system based on six-port interferometry. Using two planar fan beam antennas and an interferometer with power detectors, the angle of arrival (AOA) from a 2.4 GHz ISM-Band transmitter is determined. With two receivers the position of the transmitter is computed from the direction angles and displayed on a PC. The manufactured prototype detects and localizes a transmitter inside a room with an accuracy of about 10 to 20 cm. It is proposed to use the system for the telemetry control of small unmanned aerial vehicles inside closed buildings where GPS systems are not available.