Polarization discrimination ratio approach to retrieve bare soil moisture at X-band

Abstract

An observation has been carried out at different incidence angle by X-band scatterometer for like polarizations (i.e., Horizontal-Horizontal and Vertical-Vertical) to retrieve the soil moisture of bare field. To retrieve the soil moisture with minimizing the surface roughness effect on the soil moisture a polarimetric discrimination ratio (PDR) (PDR = (/spl sigma//spl deg/ VV - /spl sigma//spl deg/ HH)/ (/spl sigma//spl deg/ VV + /spl sigma//spl deg/ HH) where /spl sigma//spl deg/ is scattering coefficient for VVand HH-polarization) approach is proposed. Generally in microwave applications, polarization is sensitive to the size, shape and orientation of target elements, whereas frequency functions primarily as a size filter with important consequences regarding attenuation. In the polarization, the horizontal polarization gives a measure of the horizontal dimension of the emitting elements, while the vertical polarization essentially gives a measure of the vertical dimension. The power observed to the system is a complicated function of each particle size, shape and density, therefore we have normalized the value of HH- and VV-pol to the range -1/spl les/PDR/spl les/1 to partially minimize the effect of orientation and shape on dielectric properties of the scattering elements and results mainly the effect of dielectric (i.e., moisture content) on /spl sigma//spl deg/ . The angular dependence of PDR on moisture is observed and it was found that the suitable incidence angle to observe the soil moisture at X-band with PDR is 35/spl deg/. An empirical relation has been developed between PDR and soil moisture to retrieve the moisture content at X-band. The results are validated with another set of data. The retrieved values of soil moisture are in good agreement (standard error-1.02) with observed values of soil moisture. The significance of the results has been tested by F-ratio test and quite significant results were obtained. This type of results are helpful to use the microwave techniques to assess the soil moisture by the air-borne or space borne sensor and propose the suitable angle of incidence to observe the soil moisture with minimum effect of surface roughness at X-band.