This paper presents the design and implementation of a low-cost indoor motion detection and localization system using three HB100 continuous-wave (CW) Doppler radar sensors. The proposed system employs synchronized multi-channel data acquisition using a standard PC sound card as a shared-clock analog-to-digital converter (ADC), enabling accurate timedifference-of-arrival (TDOA) estimation. Doppler velocity is extracted using FFT-based spectral peak detection, while twodimensional target localization is achieved through hyperbolic TDOA equations solved by the Chan algorithm. Experimental evaluations demonstrate an average localization error of 0.12 m. The results confirm that low-cost Doppler radar sensors, when combined with appropriate synchronization and signal processing, can provide an effective indoor motion-tracking solution

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