Likely impact of the approaching solar maximum on GNSS surveys: Be alert but not alarmed

Global Navigation Satellite System (GNSS) signals travel about 20,000 km from the satellite to a receiver on the surface of the Earth. At the end of this journey, which only takes about 60-70 milliseconds, the GNSS signal must travel through the Earth’s atmosphere. Particularly the ionosphere, located at a height of between about 50 and 1,000 km above the surface, has a significant effect on the propagation of GNSS signals due to its high spatial and temporal variability. Most of the ionosphere is electrically neutral, but when solar radiation strikes it becomes an electrical conductor and supports the flow of electric currents. The effect of the ionosphere on GNSS signal propagation is a function of the Total Electron Content (TEC) along the signal path and the frequency of the signal. The TEC varies with time, season and geographic location. When travelling through the ionosphere, the speed of the GNSS signal deviates from the speed of light, causing measured pseudoranges to be ‘too long’ compared to the geometric distance between satellite and receiver, while carrier-phase observations are ‘too short’. The condition of the ionosphere is strongly related to the solar cycle, which shows a maximum approximately every 11 years. This paper discusses the likely effects of the maximum of the current solar cycle (cycle 24), predicted to occur in early 2013, on GNSS users. Although it is anticipated to be a smaller solar maximum than the previous peak encountered in 2000-2001, GNSS users can at times expect reduced positioning, navigation and timing performance. Particularly during enhanced ionospheric or geomagnetic storm activity caused by sudden eruptions of the Sun, increased ionospheric variability can be expected. This will also cause increased scintillation effects (i.e. rapid changes in the phase and amplitude of the transmitted signals), which adversely affects ambiguity resolution and may cause GNSS receivers to lose lock in some instances.