Space Weather & GPS

How GPS Works — and Where It's Vulnerable

Global Navigation Satellite Systems — GPS (US), Galileo (EU), GLONASS (Russia) and BeiDou (China) — work by measuring the time it takes radio signals to travel from satellites in MEO (~20,200 km altitude for GPS) to a receiver on the ground. The receiver uses signals from four or more satellites to calculate its position in three dimensions plus time.

The weak point in this system is the ionosphere — the layer of charged particles between roughly 60 km and 1,000 km altitude. GNSS signals must pass through the ionosphere on their way from satellite to receiver, and the ionosphere's varying electron density changes the speed and path of those signals. Under normal conditions, receivers apply ionospheric correction models to compensate. But during space weather events, the ionosphere can change so rapidly and unevenly that these models break down.

The Two Key Effects

1. Total Electron Content (TEC) Variations

TEC is the total number of free electrons along the signal path between a satellite and a receiver. Higher TEC means more signal delay. During a geomagnetic storm, TEC can increase dramatically (sometimes doubling or tripling) and become highly irregular — with sharp gradients between adjacent regions. This creates range errors that standard correction models cannot fully compensate for.

Under normal conditions, the ionospheric range error on a single-frequency GPS receiver is approximately 5–15 metres, largely removed by correction models. During a moderate storm (G2–G3), residual errors after correction can grow to 10–30 metres. During extreme events, positioning errors exceeding 50 metres have been observed.

2. Ionospheric Scintillation

Scintillation refers to rapid, random fluctuations in signal amplitude and phase caused by small-scale irregularities in the ionosphere. Think of it as the radio equivalent of starlight twinkling. Scintillation can cause GPS receivers to lose lock on satellite signals entirely — a phenomenon that produces sudden jumps, gaps or complete loss of position fix.

Scintillation is most severe in two regions: the equatorial/low-latitude band (roughly ±20° of the geomagnetic equator), where post-sunset plasma bubbles are common, and the high-latitude auroral and polar regions, where storm-driven particle precipitation creates intense irregularities. During major storms, scintillation effects can extend to mid-latitudes.

Who Is Affected

Aviation

Aviation relies heavily on GNSS for approach and landing procedures (SBAS, GBAS), en-route navigation and Automatic Dependent Surveillance – Broadcast (ADS-B). During space weather events, the Wide Area Augmentation System (WAAS) in the US and the European Geostationary Navigation Overlay Service (EGNOS) may issue integrity alerts that prevent pilots from using GPS-based precision approaches, forcing reversions to traditional instrument landing systems (ILS) or holding patterns. ICAO established three global space weather advisory centres (US, Europe, and an Asia-Pacific consortium) specifically to provide forecasts for aviation.

Surveying & Precision Agriculture

High-precision applications that depend on Real-Time Kinematic (RTK) or Precise Point Positioning (PPP) corrections — requiring centimetre-level accuracy — are particularly vulnerable. Storm-driven ionospheric gradients can exceed the correction capabilities of reference station networks, causing convergence times to balloon from minutes to hours or accuracy to degrade from centimetres to decimetres or worse. Professional surveyors in high-latitude regions routinely check Kp forecasts before fieldwork.

Autonomous Systems

Self-driving vehicles, autonomous drones and precision-guided munitions all depend on GNSS to varying degrees. While most autonomous systems fuse GNSS with inertial navigation (INS), lidar and visual odometry, the GNSS component provides the absolute position reference. During a severe storm, autonomous systems in open environments may experience position jumps or drift that their sensor fusion algorithms must handle gracefully — a design consideration that not all systems address adequately.

Timing-Dependent Infrastructure

Beyond positioning, GNSS provides precise timing signals used by telecommunications networks, power grids, financial trading systems and broadcast infrastructure. A sustained GNSS timing disruption during an extreme space weather event could cascade into synchronisation failures across critical infrastructure. Many organisations now maintain backup atomic clocks, but smaller operators may be fully dependent on GNSS timing.

Dual-Frequency as a Partial Solution

Modern GNSS receivers increasingly support dual-frequency operation (e.g., GPS L1 + L2, or L1 + L5). Because the ionospheric delay is frequency-dependent, measuring on two frequencies allows the receiver to compute and largely remove the ionospheric contribution in real time. This dramatically reduces TEC-related errors — but it does not fully solve scintillation, because if the signal is lost due to amplitude fading, having two frequencies doesn't help. Dual-frequency receivers are now standard in professional and aviation equipment, and are becoming more common in consumer devices.