A system, including: a radar including a transmitter and a receiver, the transmitter configured to transmit an electromagnetic wave; a reflector placed a predetermined distance away from the radar, the reflector configured to reflect the transmitted electromagnetic wave back to the receiver; and a control unit coupled to the radar and configured to measure a magnitude of the reflected electromagnetic wave received at the receiver from the reflector by measuring Doppler signal corresponding to the predetermined distance, wherein the control unit is configured to measure a reference value of the magnitude of the reflected electromagnetic wave when no intrusions are occurring, and wherein the control unit is configured to trigger an alarm when the magnitude of the reflected electromagnetic wave changes from the reference value.