A method for inversion of backscatter ionograms is described. Source data for the method consist of leading edge of backscatter ionograms from all azimuthal sectors of the backscatter sounder and the quasi vertical incidence (QVI) ionogram. Such data are routinely collected at over-the-horizon radar (OTHR) installations operated by the U.S. Navy. The output is a three-dimensional model of the ionosphere for the coverage area of the sounder. The model produced is consistent with all the input data. The method is based on the Newton-Kontorovich method for nonlinear operator equations and Tikhonov's regularization technique. Theory of the method and principles of its numerical realization are described. The method is applied to data acquired by the backscatter sounder operating at the Chesapeake, Virginia, OTHR facility. Maps of plasma frequency at a constant altitude over a geographic region that covers a 64¿--80¿ azimuthal sector out to distances of more than 2000 km are presented. The backscatter ionogram inversion technique suggested makes it possible for OTHR backscatter sounders to provide real-time monitoring of the ionosphere over a large geographical area. ¿ 1998 American Geophysical Union