This book addresses problems of GNSS performance support under geomagnetic storms and solar radio bursts. It analyses both physical and radio-engineering sources of GNSS performance deterioration caused by geomagnetic storms, solar radio bursts and peculiarities of the polar and equatorial ionosphere. The book takes into consideration both standalone GNSS and differential GNSS. Based on experimental data analysis, it presents a systematic approach to maintaining reliable GNSS performance despite the Space Weather impacts.