Recent results on soil moisture remote sensing using P-band signals of opportunity

Root zone soil moisture (RZSM) is an essential variable in meteorology, hydrology, and agriculture. Current methods in passive and active microwave remote sensing at L-band (e.g. SMOS or SMAP) are limited to a sensing depth of less than 10 cm. Observing RZSM (water in the top meter of soil) will require lower frequencies (P-band), presenting significant difficulties for a spaceborne instrument, due to the required antenna size, the presence of radio-frequency interference (RFI), and competition for spectrum allocations (in the case of active radar). Bistatic radar using Signal of Opportunity (SoOp) (e.g. digital satellite transmitters) offers the possibility of remote sensing using powerful signals already occupying bands allocated for communications. This paper will present early results from the first airborne campaign to study P-band reflectometry for soil moisture, conducted in Oklahoma between October 17–26, 2016. A geosynchronous communication satellite, transmitting a 25 KHz data signal on a 260.375 MHz carrier was uses at the signal source. Preliminary assessment of the data showed high reflectivity over a lake and consistent results between subsequent overflights of the watershed. Recently, a tower experiment has started using a 20 MHz wide signal centered at 370 MHz. Linearly polarized measurements were made over bare soil between 25-May-2017 and 8-June-2017 observing a strong reflected signal. Corn is being planted on the field and measurements will be made throughout the growing season.