Radiative transfer theory is an important method to treat multiple scattering in medium consisting of random discrete scatteries. Classic books on the subject include Chandrasekhar ; Ishimaru ; Case and Zweifel . The theory has been extensively applied [Tsang et al., 1977; Tsang et al. 1985; Fung, 1994; Ulaby et al. 1990; Burke et al. 19791.