It is well known that the electron density in the ionosphere varies with the time of the day and the geographical location, giving rise to horizontal ionization gradients responsible for asymmetric ray paths, out of the great circle plane reflections and other tilted propagations (i.e., ducted modes and chordal modes). Simple ionospheric models, assuming spherical symmetry, must then be modified in order to take into account, at least, gross horizontal gradients. A monostatic backscatter sounder, with angle-of-arrival (azimuth and elevation) measurement capability, is very well suited for horizontal gradients measurements. This paper describes a new real-time inversion method, using constant frequency, variable azimuth, and elevation angle backscatter ionograms for the derivation of mesoscale, or average, horizontal ionization gradients. The inversion process which has been developed for that purpose is fully explained. Tests on model data demonstrate the potential accuracy of the method and its stability to measurement errors. Finally, limitations and possible improvements to the current method are discussed.¿ 1997 American Geophysical Union