@inproceedings{sgem2021s05075,
  title = {AN EMPIRICAL MODEL FOR FORECASTING THE CRITICAL FREQUENCY OF THE IONOSPHERIC E-REGION OVER BULGARIA},
  author = {Rumiana Bojilova and Plamen Mukhtarov},
  booktitle = {SGEM International Multidisciplinary Scientific GeoConference EXPO Proceedings; 21st SGEM International Multidisciplinary Scientific GeoConference Proceedings 2021, Science and Technologies in Geology, Exploration And Mining},
  year = {2021},
  pages = {403-410},
  publisher = {STEF92 Technology},
  doi = {10.5593/sgem2021/1.1/s05.075}
}