This paper describes a new real-time inversion method based upon multifrequency sounding schemes and variable azimuth and elevation angle backscatter ionograms for the derivation of the three-dimensional structure of the ionosphere. The inversion technique which has been developed for this purpose employs neural network models in a modular structure and is fully explained. Tests on simulated data demonstrate the potential accuracy of the method for reconstructing the electron density profile over remote geographical regions. ¿ 2001 American Geophysical Union