microwave reflectivity, surface roughness
A theoretical and experimental study of the microwave reflectivity of soils with varying moisture content was conducted. A system was developed to measure reflectivity over a continuous frequency range of 4 to 26.5 GHz, at incidence angles from 10° to 70°, and with both horizontal and vertical polarization. The measurements were found to be extremely accurate for smooth homogeneous surfaces, however, the effects of surface roughness were found to be more severe than predicted due to the discontinuous nature of naturally occurring rough surfaces. An algorithm was developed which used the frequency dependence of the reflectivity to estimate the effective roughness of the surface and permit correction to an equivalent smooth surface reflectivity which in turn could be related to dielectric constant or percent moisture content. For the frequency range of investigation the maximum mean square height deviation that could be accomodated was approximately one inch. From this it may be concluded that operational airborne or spacecraft sensors must operate in the 500 MHz to 1 GHz range. Even at this reduced frequency it will be impossible to neglect the effects of roughness and a diversity technique such as developed here is essential if an absolute measure of soil moisture is to be made.
Waite, W. P.; Cook, K. R.; and Bryan, B. B.. 1973. Broad Spectrum Microwave Systems for Remotely Measuring Soil Moisture Content. Arkansas Water Resource Center, Fayetteville, AR. PUB018. 175