Estimating soil moisture using RF signals

In this paper, we propose CoMEt, a radio frequency based approach that measures soil moisture at multiple depths underneath the ground surface without installing any objects in the soil and without making any contact with the ground surface. The main insight behind CoMEt is that the phase of an RF signal depends on its wavelength in the medium through which it is propagating, which in turn depends on the amount of soil moisture. To measure soil moisture, CoMEt leverages the phase changes across successive antennas in a receive antenna array along with the time of flight of the received signal to jointly estimate the depth of each layer of soil and the wavelength of the signal in each layer. It then uses these estimates to obtain the amount of moisture in each soil layer. We have implemented CoMEt using a software defined radio and a Raspberry Pi to measure soil moisture in real-time. We have extensively evaluated CoMEt in both indoor and outdoor environments. Our results show that CoMEt estimated soil moisture for up to three layers of soil with a median error of just 1.1%.