Mitchell, C. N., and P. S. J. Spencer. “A Three-Dimensional Time-Dependent Algorithm for Ionospheric Imaging Using GPS”. Annals of Geophysics 46, no. 4 (December 25, 2003): 687–696. Accessed May 2, 2024. https://www.annalsofgeophysics.eu/index.php/annals/article/view/4373.