Whitepapers

The delay through the ionosphere is not fixed and will change based on the time of day, year, and location. The elevation angle between the receiver and satellite also impacts the magnitude of the delay. A high-elevation signal will take the shortest path through the ionospheric layer of the atmosphere as the path is perpendicular to the ionosphere. A low elevation signal will pass through the ionosphere at an angle and thus experience a much higher delay. In the absence of a geomagnetic storm, the ionosphere is correlated with solar activity and hence the peak delay is in the early afternoon, with a lower delay overnight. Equatorial Effects During the evening hours around the geomagnetic equator, plasma rises in the ionosphere. This can lead to instability within the ionosphere and result in scintillation. This is an effect where the GNSS signals are impacted by varying electron densities in the ionosphere, which can result in very rapid phase and amplitude change, leading to poor tracking, complete loss of lock, and/or carrier phase cycle slips. When the instability occurs, it may be limited to certain regions or bubbles of the ionosphere and thus only a subset of satellites may be affected. In South America, where many of our agriculture and mining customers operate, scintillation occurs an hour or two after sunset and will typically last for 4-5 hours. It also follows an annual cycle with most disturbance between September and March and severity dependent on the 11 year solar cycle. Polar Effects Sunspots can eject material from the sun that travels a few 100 km/s to a few 1,000 km/s. This phenomenon is called Coronal Mass Ejection. If the material is ejected with an Earth-bound trajectory, it will typically take a few days to reach Earth. Due to the Earth's magnetic field, it tends to travel to the poles, where it can significantly impact the ionosphere at either pole. In addition to impacting GNSS performance, this can sometimes be observed as the Northern (or Southern) lights, a phenomenon referred to as aurora borealis (or aurora australis). During more intense storms, the Northern lights can extend to the continental US, and the impact on the ionosphere can affect GNSS signals at lower latitudes. While a loss of lock and cycle slips can occur in the polar region, data typically shows less severe amplitude scintillation compared to the equatorial regions, with limited cycle slips and less disruption. Global Effects Although most noticeable disturbances occur around the geomagnetic equator and northern latitudes, we have also observed an increase in the ionospheric delay measurement globally as we approach the solar cycle maximum. While dual and triple frequency techniques are leveraged to mitigate these effects by using an ionospheric free combination, this also increases measurement and position noise. With the potential for large solar storms to cause disruptions in mid-latitude operations, ionospheric protection has become a critical global requirement for GNSS receivers. Impact on GNSS Operations As mentioned above, ionospheric disturbance can lead to poor signal tracking and in some cases complete loss of lock on the GNSS satellite. Disturbances can also be localized, resulting in RTK algorithms experiencing difficulty when base and rover measurements are affected differently. The agriculture and mining industries are the largest RTK user base that experience disruptions from high ionospheric activity. These operations depend on the availability of centimeter level positions. Machines cannot operate safely or effectively if the accuracy is determined to be outside the threshold limits. Productivity is reduced and the economic impact can be substantial. 7

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