Building redundancy and resiliency against scintillation

GNSS signals are broadcast from satellites and travel thousands of kilometres through Earth’s atmosphere to reach user equipment. While they travel, these signals can be affected by ionospheric activity, or scintillation, and cause positioning errors or outright failure of positioning systems. How can farmers build their system to withstand this interference and continue working?

Below we will explain what the ionosphere is, how it affects GNSS signals, and what users can do to strengthen their systems against positioning errors caused by scintillation. 

How the ionosphere impacts GNSS

The ionosphere is one layer of Earth’s atmosphere found between 50 to 1,000 kilometres above Earth's surface. There are high concentrations of ions and free electrons in this layer that react to the Sun’s radiation.

When there are periods of high solar activity like in a solar flare, the density of the ionosphere changes and fluctuates temporarily. These fluctuations can occur over a broad region or as small an area of 100 kilometres. Ionosphere density changes like these delay GNSS signals so it takes longer for them to arrive to user equipment, or may completely overwhelm the signals that they are no longer usable..

Changes in the ionosphere’s density, called scintillation, is unpredictable. Scintillation can occur worldwide, but is often concentrated along the geomagnetic equator in areas like South America and Southeast Asia, or in high latitudes.

Scintillation can occur daily, frequently in the evenings, and also increases during spring and fall equinoxes. These changes are also tied to the Sun’s own cycles that occur every 11 years – as we enter solar cycle 25, we are seeing an increase of solar activity that result in scintillation being stronger and more frequent. Regardless of the cause, worldwide users of GNSS are reporting increased positioning challenges resulting from ionospheric scintillation. 

What can scintillation look like

Satellite signals travel through the ionosphere to antennas on Earth. When scintillation is present, GNSS receivers may have intermittent reception of signals, loss of precision resulting in position jumps and overall unavailability of positioning.

When operations rely on GNSS positioning, scintillation can result in significant working delays, hazardous risks and downtime.

For operations that must continue 24/7/365, or operate during periods when scintillation is more frequent, the interference effects can cause large positioning errors, like those seen in the above graph. 

Scintillation impacts in agriculture

Precision agriculture applications rely on GNSS positioning to maximise yields, resources and inputs. Crop-specific inputs, like seed, fertilizer or agrichemicals, can be placed in specific locations for optimal efficiency and lower environmental impact. When GNSS is disrupted by scintillation – a frequent event for farms in Brazil, for example –  operations come to a halt waiting for high accuracy and reliable positioning to resume, causing costly downtime and inefficient use of resources. In an industry that is seasonal and has very limited time windows for each part of the agricultural lifecycle, this can be highly detrimental. As a result, agriculture operations in regions with high scintillation trend toward using a PPP solution as it typically performs better than RTK during scintillation, as seen in the above graph. 

Lessening the impact of scintillation and the ionosphere

While there are different methods to reduce the impact of ionospheric errors, leveraging a combination of different solutions creates the most redundant system.

• Precise point positioning (PPP): PPP solutions (such as TerraStar-C PRO) are globally available and don’t require local base station infrastructure. Instead, satellite corrections are generated from a network of global reference stations and transmitted to the user’s rover receiver via geostationary satellites or the Internet, and the receiver leverages multi-frequency measurements to directly observe and remove ionospheric errors.Additional advancements to receiver firmware provide further improvements to PPP accuracy and availability during scintillation.
  
  This is why PPP tends to perform better than RTK during scintillation, like the performance seen in the previous graph. RTK relies on assumptions of errors identified in the area between the rover receiver and base stations, and these assumptions become unreliable for scintillation.
• Additional signals and frequencies: When you use positioning technologies that are compatible with multiple constellations and multiple signal frequencies, your system becomes compatible with every possible signal available. With more signals, your system is better able to identify if and when scintillation is occurring, and is strengthened against intense ionospheric activity. 
• Alternative scheduling: While solar storms are not always predictable, regular occurrences of scintillation are. It often occurs in the evenings and increases during spring and fall equinoxes. Where possible, plan operations ahead so you can avoid times of high scintillation is one method to lessen the impacts of ionospheric activity. View the forecast map to help with planning. 

As the ionosphere becomes more active in solar cycle 25, it’s vital to understand these effects in order to mitigate the impact on applications and end-users. There are many resources available to learn about scintillation, other effects that cause positioning errors, and how to build system resiliency against them. 

Continue learning about space weather and its impact on GNSS in this webinar from Hexagon’s Autonomy & Positioning division and Inside GNSS.