A system, method and apparatus tracks targets (e.g., tumors) during treatment (e.g., radiation therapy) using a radar motion sensor by generating a microwave signal, radiating the microwave signal to a subject, and receiving a modulated microwave signal from the subject. The modulated microwave signal is processed to provide a subject motion information using a sensor having an arctangent-demodulation microwave interferometry mode. A location of a target on or within the subject is determined based on the subject motion information and a three-dimensional model for the subject and the target. One or more control signals are generated based on the location of the target, and the treatment device is controlled using the one or more control signals to treat the target on or within the subject.