Continuous-wave software-defined radar interferometry

This paper presents an experimental implementation of continuous-wave interferometric radar based on low cost USRP platform and computer. The purpose of the proposed system is to measure small displacements of the monitored target over a long time period. These measurements are extremely useful in the field of civil engineering. Common applications are landslide monitoring or bridge and building deformation analysis. The proposed solution transmits a monochromatic radio wave towards the monitored target and measures the phase shift suffered by the reflected wave. The measured phase shift is stored in a file on the hard drive of the computer. After a certain period of time, spanning from minutes to years, specific to the application, with the system placed in the same position, a second phase shift measurement needs to be performed, storing a new result. The system then computes the difference between the two stored phase shifts and determines the displacement. As expected for all interferometric measurements, due to the cyclic nature of the phase shift, the displacement must be smaller than a certain fraction of the wavelength of operation; otherwise the result will be ambiguous. In our case the displacement must be smaller (in module) than a quarter of the transmitted wavelength. Due to the fact that in a software-defined radio the frequency is user selectable in a very wide range, the system can be configured to adapt to the magnitude of the expected displacement. All of the above-mentioned assertions have been validated experimentally.