Why GNSS Can Fail During Geomagnetic Storms

Conceptual diagram: space-weather conditions, receiver measurements and aircraft response are related but distinct layers.

How geomagnetic storms affect GNSS signals

Geomagnetic storms can disturb the ionosphere through which satellite-navigation signals travel. The resulting errors and tracking difficulties depend on the disturbance, location, signal frequencies and receiver. NOAA’s planetary Kp index describes geomagnetic activity; it does not certify the position accuracy of a particular drone. Treat space-weather information as an environmental input, then examine the aircraft’s navigation quality and apply its documented operating procedures. A Kp number alone is not a universal permission to fly or instruction to land. [S01] [S02]

What actually changes in the signal path?

GNSS receivers estimate position using measurements from satellites whose signals cross the ionosphere. The first-order ionospheric effect depends on total electron content and signal frequency. Code and carrier-phase measurements are affected differently. Dual-frequency observations can remove much of the first-order effect, but that is not the same as immunity to every disturbance or loss of tracking. [S03]

The practical distinction is between an environmental event, a measurement problem and an aircraft response. A storm can change the environment without producing an observable problem on every receiver. A receiver can produce a degraded position without the aircraft immediately declaring it unusable. An aircraft can also reject a position for reasons unrelated to space weather.

A useful investigation therefore preserves all three layers rather than collapsing them into “GPS failed because Kp was high”.

Why a Kp-to-accuracy table is misleading

Kp is an index, not a metre-error scale. Its global and time-aggregated character cannot describe the exact signal path between a moving aircraft and every satellite. A table promising “Kp 5 means 10 metres of error” introduces a numerical relationship that the index does not provide. [S01]

The same objection applies to a green indicator at low Kp. Buildings, antenna installation, multipath or equipment faults can still undermine navigation. GPS.gov distinguishes the performance of signals in space from the accuracy achieved by a user’s equipment. [S04]

Evidence	What it contributes	What it does not establish
Space-weather report	Environmental context and timing	Aircraft-specific positional accuracy
Satellite count	Number of reported observations or tracked signals	Independent proof of a trustworthy solution
Receiver quality and integrity flags	Receiver’s assessment of its measurements	A complete explanation of the underlying cause
Aircraft estimator and flight logs	How navigation data affected the system	Attribution without supporting investigation

A worked interpretation—not a flight clearance

Consider an illustrative log set. A public report identifies elevated geomagnetic activity. A receiver later records reduced carrier-to-noise density on several signals, while its estimated horizontal uncertainty rises. The aircraft log shows a navigation warning during the same interval.

Those observations justify examining a possible environmental contribution. They do not prove it. Check whether the antenna view changed, a high-current payload switched on, the aircraft moved near reflective structures, or other receivers in the area showed a similar pattern. Preserve timestamps and the receiver’s exact definitions of its quality fields.

Conversely, a stable receiver during a storm does not disprove the storm. It establishes only that the recorded receiver outputs remained within the observed conditions. An engineering report should state the evidence at that level of precision.

What should be recorded?

THL’s suggested investigation record contains the receiver and firmware version, antenna installation, time reference, approximate location, operating mode, tracked signals, quality flags, estimator warnings and relevant changes in equipment state. Record any available carrier-to-noise-density values with their unit, dB-Hz. Do not replace them with an unrelated analyser’s “dB above noise floor” reading.

Preserve original logs before smoothing or plotting. A minute-average graph can conceal short losses of tracking that matter to the aircraft. Include both the complete timeline and a closer view of the event, with any missing samples explicitly marked.

This is a reporting method, not a universal list of dispatch limits. The limits must come from the aircraft’s supported procedures and the operation’s applicable requirements.

Where a visibility calculator helps

A geometry tool can estimate which satellites lie above a chosen elevation mask. That can help identify a planned period with a poor view of the sky. It does not model ionospheric scintillation, receiver tracking, local interference or the integrity of a position solution.

An almanac-based geometry estimate is a planning aid, not a space-weather risk score. Read it alongside How Many Satellites Does a Drone Need? and GNSS Jamming vs Spoofing Explained.

The useful conclusion

Replace a single-colour Kp rule with a traceable question: what environmental condition was present, what did the receiver measure, and what did the aircraft do? That separation makes both pre-operation planning and post-event reporting more defensible without pretending that one public index can describe an entire navigation system.