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The design and implementation of a three-dimensional (3D) location system to provide accurate location information is proposed in this paper. The location information is computed by two techniques. Both Trilateration technique and the Time Difference of Arrival (TDOA) are applied to three sets of collected distance measurements and to obtain accurate distance measurements between the Transmission Processing Unit (TPU) and the Distance Sensor (DSR) module. TPU works as the beacon transmitter of infrared (IR) and ultrasonic signals, while DSRs act as fixed reference points of the locating system and are generally attached to the ceiling and wall of the room. Each DSR is a receiver of IR and ultrasonic signals. Up to 250 DSRs are arranged and connected together on a single RS-485 bus called Sensor Grid (SG). Located at the end of the RS-485 bus, the Sensor Processing Unit (SPU) collects and sends data produced by DSR to TPU using Zigbee modules in the construction of a Wireless Sensor Network (WSN). Both Trilateration technique and Kalman filter are implemented to improve the accuracy of the location system. The 3D system is able to provide accurate locations for path tracking in robot navigation experiments conducted at a large indoor area.