The ionosphere is a layer of Earth’s atmosphere spanning 60 to 1,000 kilometers above the surface. Intense periods of solar activity create fluctuations in the ionosphere’s charged state and create challenges for RF systems such as high-precision GNSS applications like Real-Time Kinematic (RTK) positioning. During the 2025 solar maximum, understanding how to mitigate these disturbances is crucial to achieving high accuracy operations.
The 11-Year Solar Cycle
Solar activity follows a roughly 11-year cycle. Solar Cycle 25, which began around 2019, is expected to peak in 2025. When solar cycles “peak” or reach their maximum, there is an increase in solar flares, coronal mass ejections, and enhanced ionospheric activity.
In turn, the ionosphere is more dynamic and unpredictable as we reach a solar cycle maximum. Sudden disturbances can occur within minutes that create spatial variations over short distances. This is problematic for RTK positioning technology, which generally assume similar ionospheric conditions exist between a fixed base receiver and a roving receiver. The good news is that we know the solar cycle patterns and therefore should be able to correct these assumptions.
Ionosphere Effects on GNSS Signals
GNSS signals traveling through the ionosphere encounter free electrons, creating both delays and refraction. The magnitude of the delay along the signal path fluctuates based on solar activity, time of day, season, and geographic location, as well as the frequency of the signal itself.
GNSS signal delays introduce errors varying from several meters during quiet conditions to tens of meters during peak solar activity. For standard GNSS receivers, built-in models can correct much of this error. However, RTK systems rely on precise carrier phase measurements and differential corrections from nearby base stations.
Non-Uniform Ionospheric Effects
The ionosphere does not affect GNSS signals uniformly across geographic regions. There are several factors contributing to this variability.
Latitude: Equatorial regions experience the worst ionospheric effects due to intense solar heating and strong magnetic field interactions. High-latitude areas face different challenges from aurora and polar cap events during geomagnetic storms.
Time of Day: Ionospheric activity peaks during local afternoon hours when solar radiation is strongest. Nighttime generally sees reduced but more variable conditions.
These non-uniform effects result in RTK uncertainties that can vary dramatically based on location and timing. Shortened baseline lengths and longer convergence times for RTK base stations are recommended as mitigating factors during disturbed conditions.
Post-Processed Kinematics (PPK) as a Remedy
Given the challenging ionospheric conditions expected during the 2025 solar peak, Post-Processed Kinematic (PPK) positioning offers several advantages over real-time RTK:
Enhanced error detection: PPK processing allows for thorough quality control and outlier detection that is not possible in real-time. Signal data is analyzed to identify and remove periods of poor satellite geometry or ionospheric disturbances.
Multiple processing strategies: PPK typically includes longer observation periods and advanced ambiguity resolution to mitigate ionospheric effects.
Integration with precise products: PPK can incorporate precise satellite orbits, clock corrections, and use global ionospheric models that become available hours after observation.
As we navigate the solar peak - in the current cycle and in the future - the ionosphere will present challenges for RTK positioning as discussed above. Industry professionals should implement strategic mitigation to maintain accurate navigation through periods of intense fluctuations within the ionosphere.