P-Band Signals of Opportunity: A New Approach to Remote Sensing of Root Zone Soil Moisture.

Root zone soil moisture (RZSM), is an essential variable in agriculture. Passive and active microwave techniques are the most mature for soil moisture remote sensing, however their penetration depth is proportional to wavelength, typically around 5~cm at L-band (e.g. SMOS or SMAP). Measuring RZSM (the top meter) requires lower frequencies. (VHF and P-band), presenting significant difficulties for spaceborne measurements, due to the required antenna size, presence of radio-frequency interference (RFI), and competition for spectrum allocations. Signal of Opportunity (SoOp) is a new approach to remote sensing, re-utilizing powerful signals in bands allocated for communications. Reflectometry uses a forward-scatter geometry, presenting a high signal to noise ratio and different response to vegetation than either backscatter or radiometry. This paper will present results from an airborne experiment (October 17-26, 2016) in the Little Washita watershed in Oklahoma. A geostationary communication satellite, transmitting a 25 KHz data signal on a 260.375 MHz carrier was used as the signal source. An observable, computed from the complex correlator outputs, can be solved for reflectivity and phase. Reflectivity is then related to the dielectric constant and, through established semi-empirical models, temperature, salinity, soil texture, and soil moisture. Receiver channel gain variations can be cancelled through antenna swapping and noise injection. Presently, we are comparing reflectivity retrievals with in situ measurements from permanently installed soil moisture sensors in the ARS mircronet. We expect to present results showing a comparison between the corrected reflectivity retrievals and these measurements. A fixed tower experiment is being planned for the Summer 2017 and preliminary results will also be presented. These airborne experiments are intended to evaluate the feasibility of RZSM remote sensing from satellite. The small power requirements of a passive SoOp receiver also make it an ideal payload for unpiloted aerial vehicles (UAVs).