Rigorous analytic solution is constructed for the first of 2 s-order space position and frequency coherence function (which is alternatively also termed as symmetric) of the high-frequency wave field propagating through the stochastic transionospheric channel with the inhomogeneous layer of the background ionosphere and anisotropic fluctuations of the electron density in the approximation of the appropriate Markov equation. It is obtained employing the anisotropic parabolic model of the effective structure function of the fractional electron density fluctuations with two different scales along and across the Earth's magnetic field. The explicit analytic solution is presented, which is capable of describing the coherence properties of the field for any given inhomogeneous distribution of the electron density in the background ionosphere along a path of propagation. The procedure of calibrating the spatial scales of the anisotropic parabolic model of the electron density fluctuations along and across the Earth's magnetic field lines in order to provide the adequate solution to correspond to the realistic anisotropic inverse power law spectrum of the electron density fluctuations is presented in the companion paper.