Bojilova, Rumiana, and Plamen Mukhtarov. “Preliminary Results of Methodology for Forecasting the Minimum Virtual Height of Ionospheric F-Region for Sofia City”. Proceedings of the Bulgarian Academy of Sciences 76, no. 12 (December 21, 2023): 1860–1867. Accessed October 27, 2026. https://proceedings.bas.bg/index.php/cr/article/view/440.