This paper presents a new technique for the mapping of ionospheric electron density distributions for determining ambient HF radio propagation characteristics necessary for applications such as over-the-horizon-radar (OTHR). The multisource volumetric inversion technique (MSVIT) combines information from conventional ionospheric probes used for OTHR, such as quasi-vertical incidence and oblique backscatter ionosondes, with total electron content data obtained by computerized ionospheric tomography (CIT), to form three-dimensional maps of ionospheric electron density distribution. MSVIT is an iterative reconstruction technique that draws on the wide-area coverage of VHF-UHF data from a CIT imaging system colocated with an OTHR system to enhance the reliability of sector-wise localized predictions of HF propagation conditions. A detailed analysis of the MSVIT algorithm together with simulations using the international reference ionosphere model of 1990 and experimental data from recent CIT campaigns shows that MSVIT has considerable potential for improving the quality of OTHR operations. ¿ 1998 American Geophysical Union