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 [1960]; Ishimaru [1978]; Case and Zweifel [1967]. The theory has been extensively applied [Tsang et al., 1977; Tsang et al. 1985; Fung, 1994; Ulaby et al. 1990; Burke et al. 19791.