Title: Signals of Opportunity Airborne Demonstrator (SoOp-AD): Results of First Field Experiment
Presenting Author: James L Garrison
Organization: Purdue University
Co-Author(s): Jeffrey R. Piepmeier (1), Manuel A. Vega (1), Yao-Cheng Lin (2), Manuel A. Vega (1), Matthew Fritts (1), Cornelis F. Du Toit (1), Benjamin Nold (2), Joseph Knuble(1) (1) NASA Goddard Space Flight Center (2) Purdue University

Abstract:
Preliminary results from the first airborne campaign to evaluate P-band reflectometry for soil moisture remote sensing are presented. Signals of Opportunity Airborne Demonstrator (SoOp-AD) is a brass-board P-band reflectometry instrument developed under the 2013 NASA Instrument Incubator Program. P-band (230-270 MHz) radiation is expected to penetrate 10-20 cm into the soil, offering the possibility of directly measuring Root-Zone Soil Moisture (RZSM). For comparison, passive and active L-band observations are generally accepted as penetrating only the top 5 cm of the soil. Space borne RZSM remote sensing is infeasible with existing microwave techniques due to the large antenna required, interference from communication links, and lack of spectrum allocations. RZSM, defined as water content in the top meter of soil, where it is absorbed by plant roots, is an essential variable for hydrology, agriculture, meteorology and climate change studies. Soil reflectivity is estimated from the cross-correlation of direct and reflected signals. The long wavelength and narrow modulation bandwidth (25 KHz) will introduce significant interference between direct and reflected signals, which must be removed through a model inversion process. Key technological advancements on the roadmap to a spaceborne instrument are demonstrated in SoOp-AD, including an FPGA-based correlator array and "smart antenna" null-steering. Preliminary results from the first airborne tests of SoOp AD, conducted around the USDA Agricultural Research Service (ARS) Micronet in Little Washita, OK, will be presented. These experiments demonstrate functionality of the correlator array, advancing the instrument to TRL-5. Data confirms the assumption of coherent scattering and the early reflectivity measurements clearly show the water-land transition over Lake Ellsworth and exhibits variation over the land area. Antenna null-steering methods to enhance isolation of the direct and reflected ray paths are now being evaluated and the reflectivity retrievals will be compared to in situ soil moisture measurements at multiple depths.