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Structural health monitoring calls for sensors that are low-cost, low-profile, and power-constraint. It also requires the sensors to form a network to accurately monitor the low-frequency response that often occurs in many civil structures such as long-span bridges. However, the existing sensors are either not practical for wireless implementation, does not have enough accuracy, or are not cost-effective. This paper presents a multi-function interferometric radar sensor that can easily form a smart sensor network by the merit of the ZigBee mesh networking function integrated in the sensor. The radar sensor, integrated with a micro-controller, works in the arctangent-demodulated interferometric mode to monitor the structure's displacement with an accuracy of sub-millimeter. Experimental results show that the smart sensor network using the multi-function radar sensors serves as a good alternative for monitoring structural health.