Do Solar Flares or Ionospheric Disturbances Affect RTK?

Yes, and significantly. Ionospheric disturbances caused by solar flares or geomagnetic storms are one of the primary factors degrading RTK measurement accuracy. Under normal conditions, RTK delivers horizontal accuracy of 1-3 cm. During a severe geomagnetic storm (Kp index above 7), the error can exceed 50 cm, and carrier phase initialization may become entirely impossible.

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How RTK Uses GNSS Signals

RTK (Real-Time Kinematic) measures the carrier phase rather than pseudorange. This is what enables centimeter-level accuracy, but it also makes the method sensitive to any delay or distortion in the signal path.

A satellite signal passes through the ionosphere at altitudes of 80-1000 km and through the troposphere up to 12 km. Ionospheric delay depends on the number of free electrons along the signal path, quantified by the TEC (Total Electron Content) parameter. The higher the TEC, the greater the delay and the worse the positioning accuracy.

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What Happens During a Solar Flare

An X-class solar flare emits a powerful X-ray burst. It ionizes the upper atmosphere within minutes, sharply increasing TEC. The effects on GNSS fall into two categories:

• Direct ionization - TEC temporarily spikes, delays become unstable and difficult to predict
• Geomagnetic storm - 1-3 days after the flare, the solar wind compresses the magnetosphere, causing longer-lasting disturbances lasting from several hours up to 2-3 days

During so-called D-layer radio blackouts (SWPC D-Region Absorption Events), L1/L2 signals can be completely absorbed in the sunlit hemisphere for 10-30 minutes.

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Single-Frequency vs Dual-Frequency Receiver: A Critical Difference

A dual-frequency receiver computes the ionosphere-free linear combination (LC = 2.546 * L1 - 1.546 * L2), which neutralizes up to 99% of first-order ionospheric delay. A single-frequency receiver has no such capability and relies on the Klobuchar model, whose error grows 5-10 times during disturbances.

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The Effect of Baseline Length

In RTK, the difference in ionospheric delay between the base and rover is very small over short distances. For baselines up to 10 km, the residual ionospheric error is typically less than 1 ppm. At 30-50 km it already reaches 5-15 ppm and can increase sharply during a disturbance event.

A practical rule: when working during a period of elevated solar activity or ahead of an expected geomagnetic storm, shorten the baseline to a minimum or switch to Network RTK (NRTK) with a modeled ionospheric correction.

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Forecasting and Monitoring

Solar Cycle 25 reached its maximum approximately in 2025-2026, increasing the frequency of X-class events. For surveying and construction work requiring critical accuracy, it makes practical sense to monitor:

• The Kp index (NOAA Space Weather Prediction Center) - values above 4 already warrant heightened attention
• SWPC Real-Time Solar Wind - rising proton density 30-60 minutes ahead of a geomagnetic storm
• HDOP and the number of active satellites in the field controller - a sudden drop in these values is an indirect indicator of ionospheric disturbance

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Practical Recommendations

A few simple measures significantly reduce the risk of gross errors:

1. Use a dual-frequency or multi-frequency receiver for any work with a tolerance below 5 cm
2. Watch the PDOP: values above 3.5 put RTK accuracy in question even during a quiet ionosphere
3. Avoid critical measurements within 48 hours of a major X-class or M5+ flare
4. With a fixed base station, compare its position before and after the session: a drift exceeding 2 cm signals an ionospheric disturbance

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Conclusion

RTK is vulnerable to solar activity, but the degree of impact depends on the equipment, baseline length, and nature of the disturbance. Dual-frequency systems with short baselines handle most events with acceptable accuracy degradation. Single-frequency solutions under the same conditions can produce a false Fix with an undetectable error of 20-50 cm, which is far more dangerous than losing the Fix entirely.