Title: Signals of Opportunity - Airborne Demonstrator (SoOp-AD)
Presenting Author: Joseph J Knuble
Organization: NASA GSFC
Co-Author(s): James Garrison

Abstract:
Root zone soil moisture (RZSM) is not directly measured by any current satellite instrument, despite its importance as a key link between surface hydrology and deeper processes. Presently, model assimilation of surface measurements or indirect estimates from other observations must be used to infer this quantity. Signals of Opportunity (SoOp) methods, exploiting reflected P- and S-band communication satellite signals, have many of the benefits of both active and passive microwave remote sensing. Reutilization of active transmitters, with forward-scattering geometry, presents a strong reflected signal even at orbital altitudes. A SoOp instrument would not be limited to operating in only a few protected frequencies and is also expected to have less susceptibility to radio-frequency interference (RFI). These unique features of SoOp circumvent past obstacles to a spaceborne P-band remote sensing mission and have the potential to enable new RZSM measurements that are not possible with present technology. In the second year of the SoOp-AD project, a brassboard analog section has been completed, along with working designs for the digital section and an antenna compatible with the NASA Langley B-200 or UC-12B aircraft. Resource requirements for the digital section FPGA are being finalized. One technical challenge at P-band is that the low transmitted data rate makes isolation of the direct and reflected signals using path delay, the conventional practice for GNSS reflectometry, difficult. Any retrieval of surface reflectivity from these measurements must account for the combination of direct and reflected signal in both the sky-view and Earth-view antennas. We have approached this challenge on several fronts; First, through null steering of the antenna. Second, through the formulation of an empirical calibration function for the reflectivity; and finally through estimating the direct and reflected signal powers as independent states in a Kalman filter. In addition to the direct-reflected interference from the same transmitter, there is also the possibility of interference due to transmissions, in adjacent bands, from other satellites in the constellation. Ground-based repeaters are also present in the S-band spectrum. Digital filters are being designed to isolate these repeaters, using simulations to verify the magnitude of their effect on the reflectivity and soil moisture retrievals. Other contributions to the error budget include the uncertainty in aircraft altitude, and the antenna pattern.