Statistical parameters of the harmonic signal and the beat signal of CWFM radio altimeter scattered from a rough cylindrical surface are derived in the Kirchhoff approximation. Theoretical derivations of the paper are proved via computer simulation. Principal outcome of the paper is a concept of the double dimensionality of the altimeter beat signal. Scattered form a rough surface, this signal is a random field evolving the «fast» and «slow» times. In the «fast» time the time delay of altimeter signals is measured, its range equals to the modulation period, and its quantum is the DSP sample period. In the «slow» time a vehicle with the airborne altimeter moves, the Doppler frequency shift manifests itself in this time, and its quantum is the modulation period. As a result of the 2D nature of the beat signal, its power spectrum should also be treated as a 2D function, which support belongs to the plane «range estimation beat frequency - Doppler frequency». In case of a random cylindrical rough surface the beat signal spectrum looks like a «ridge» on the plane and follows the line connecting Doppler and range estimation beat frequencies of the surface specular points. Spreading of the beat signal spectrum due to the surface roughness occurs for both frequencies. But if the spectrum spreading along the range estimation beat frequency depends only on the standard deviation of the surface random heights, the spreading along the Doppler frequency depends on either the surface correlation length (in case of small surface roughness) or the mean slope of the surface (in case of large surface roughness). The concept of the 2D spectrum presented in the paper permits to reconsider the transformation of the conventional 1D beat signal spectrum that is used in modern altimeters for the height estimation. This spectrum is a projection of the 2D spectrum onto the plane, which trace corresponds to the range estimation beat frequency. That is why spreading of the 1D beat signal spectrum that introduces additional estimation errors is a result of the said location of the 2D spectrum on the plane «range estimation beat frequency - Doppler frequency». The suggested concept can find practical implementation because it improves the accuracy of physical representation of the wave scattering on rough surfaces for the CWFM radio altimeters.