Initial results of land-reflected GPS bistatic radar measurements in SMEX02

To investigate scattering of Global Positioning System (GPS) signals from terrain and the potential for remotely sensing soil moisture with the L-band GPS bistatic radar concept, a prototype GPS bistatic radar participated in airborne measurements during the Soil Moisture Experiment 2002 (SMEX02). A 12-channel GPS navigation receiver, modified to perform bistatic radar measurements, was mounted on the NCAR C-130 aircraft to make co-located measurements with other instruments. Narrow pulse returns and comparison of the reflected GPS signal-to-noise ratio (SNR) measurements to digital imagery and cover maps indicated that the scattering was most likely quasi-specular, originating from a small footprint on the order of the first Fresnel zone (∼30 m). Temporal changes were observed in the measured signals and were expected to be proportional to varying soil moisture content. To investigate this effect, the bistatic signal measurements were interpolated to a spatial grid to produce daily maps of relative change of surface soil moisture over the study region. The maps of the study region showed a transition from dry surface soil moisture conditions to wet conditions following precipitation events occurring in the middle of the study period. Additionally, the maps showed the scattered power increased in areas with localized rainfall relative to areas without precipitation. Comparing the GPS-reflected SNR measurements with L-band brightness temperatures measured coincidently by the PALS radiometer showed good agreement in the trend measured by the two sensors. The scattered signal measurements were also compared with in situ soil moisture measurements and found to follow the general soil moisture trend as a function of time. These initial results from the first controlled experiment of GPS bistatic radar for soil moisture remote sensing indicate that the technique is sensitive to temporal and spatial variations in soil moisture.