The two-dimensional linear theory of radial correlation reflectometry applicable for the case of low turbulence level, when the probing line is observable in the spectrum of reflected wave, is developed. It is shown that the small angle scattering in the plasma volume makes a significant contribution to the fluctuation reflectometry signal complicating the turbulence correlation length measurements. The conditions under which the cut off backscattering plays the dominant role and the correlation length estimation is possible are discussed.