Soil moisture estimation using continuous wave radar

This work introduces SoilCW, a low maintenance and cost-effective soil moisture monitoring system that uses compact and affordable continuous wave (CW) radar technology. By employing passive metal reflectors to eliminate the need for battery-powered underground sensors, SoilCW helps reduce deployment costs and minimizes the risk of soil contamination. SoilCW operates on the principle that the phase change of radar echoes is related to the moisture content of the soil through which the electromagnetic (EM) wave propagates. To accurately capture the phase changes caused by soil moisture, the SoilCW uses two auxiliary frequencies close to the main radar frequency to eliminate the impact of ground reflection and addresses the phase ambiguity that can occur due to significant variations in signal propagation velocity within the soil. This design utilizes only a narrow bandwidth and does not require an antenna array, commonly needed in existing works. The prototype of SoilCW was developed using a software-defined radio (SDR) board, and extensive evaluations were conducted in laboratory environments, with sandy soil and potting mix loam, as well as field environments with clay soil, and considered various surface coverings such as mulches and rocks. The results show that CW radars are promising for low-cost and accurate soil moisture monitoring. The overall mean absolute error is approximately 1.91% under laboratory conditions, and the results obtained from the field experiments are comparable to those of dedicated industrial grade soil sensors.