Backscatter ionograms (BI) and quasi vertical incidence (QVI) ionograms are routinely collected at over-the-horizon radar (OTHR) installations operated by the U.S. Navy. The BI provides information about the downrange ionosphere up to several thousand km from the sounder. In this paper, a method for the quantitative extraction of this information by means of a leading edge inversion technique is described and applied to data acquired by the BI sounder operating at the Chesapeake, VA OTHR facility. Because this sounder is used to sample 8 azimuthal sectors, maps of plasma frequency at a constant altitude can be derived over a geographic region that covers a 64¿ angular sector out to distances of ~2000 km. The unique capability of the OTHR BI sounders to provide realtime monitoring of the ionosphere over a large geographical area is demonstrated. ¿ 1998 American Geophysical Union