Reference total electron content: global 30-day average empirical VTEC maps for rapid assessment of weather-climate deviation. - Fron, Adam; Krankowski, Andrzej; Hernández-Pajares, Manuel; Galkin, Ivan; Bilitza, Dieter; Reinisch, Bodo; Kotulak, Kacper; Zakharenkova, Irina; Cherniak, Iurii; García-Rigo, Alberto (Geophysical Research Abstracts, 2018)