A model, based on the principle of volume scattering and the assumption of locally frozen-in refractive index irregularities, is proposed for mesosphere-stratosphere-troposphere (MST) radar observations in the spaced antenna mode. Autospectrum, cross-correlation function, cross-spectrum, and coherence function for the received signals are derived and related quantitatively to the atmospheric parameters. The model predicted results are compared with observational results. It will be shown that under observational and atmospheric conditions with appropriate time and spatial scales the model is consistent with experimental results. Those conditions and scales are discussed in the paper.