GNSS in Earth Science Research: GNSS-RO, GNSS-R, and GNSS-GR


GNSS and the 25th Solar Cycle

We are experiencing more frequent and intense solar storms as the Sun approaches the peak of its eleven-year cycle. This heightened solar activity, in addition to pushing auroras towards mid-latitudes, poses risks to the global navigation satellite systems we depend on every day. In this post, we will examine where we are in Solar Cycle 25 and the network of government agencies tracking the Sun’s impact on Earth before discussing how more intense space weather impacts the GPS and other GNSS constellations.

The Sun and its cycles

The Sun’s churning plasma creates a dynamo that generates large-scale magnetic fields that flip polarity every eleven years. This process is marked by a corresponding rise and fall in the number of sunspots and eruptive events driving Space Weather , including:

  • Solar flares: These are intense blasts of ultraviolet and X-ray light that last minutes or hours.
  • Coronal mass ejections (CMEs): A sudden eruption of solar energetic particles (SEPs) — electrons, protons, and heavier ions — several times larger than the Earth.

Predicting the scale and duration of solar cycles falls to an international panel of representatives from the National Oceanic and Atmospheric Administration (NOAA), National Aeronautics and Space Administration (NASA), and the International Space Environment Service (ISES), a 23-member multinational coordinating group. They predicted Solar Cycle 25 would be like its relatively mild predecessor.

While Solar Cycle 25 may be generating fewer sunspots than the historical average, it has outpaced the previous cycle and generated spectacular events. So far, this cycle has generated more than 900 M-class and X-class solar flares, with more than 15% occurring in the first five months of 2024. By 2023, this cycle was already generating half a dozen CMEs.

In early May 2023, the active region 3664 unleashed a cascade of flares and CMEs, peaking with a G5-class geomagnetic storm and an 8.7-magnitude X-class flare. In early June, another region sent a cloud of SEPs toward Earth, triggering the strongest solar radiation storm in since Solar Cycle 24. Fortunately, these extreme events did not cause long-term or systemic problems, but they did create isolated issues. For instance, farmers using John Deere tractors for reported positioning errors that interfered with their Spring planting.

Monitoring solar activity

Space weather monitoring and forecasting is a global collaborative effort led by the United States. NOAA is America’s lead civilian agency for space weather, responsible for making and publishing forecasts based on data collected on Earth and in space. NOAA’s National Weather Service issues the forecasts, while its National Environmental Satellite, Data, and Information Service maintains space weather instruments on NOAA’s fleet of Earth-observing satellites.

The defense and intelligence communities receive space weather forecasts and warnings from the United States Space Force’s 557th Weather Wing, which also operates several solar observatories.

Federal research agencies also contribute to America’s space weather monitoring efforts. NASA heliospheric science missions contribute observations from unique vantage points in space. For example, the Solar Dynamics Observatory (SDO) has a geosynchronous orbit that rarely blocks the Sun and supports high-bandwidth downlinks. The Department of the Interior’s United States Geological Survey maintains a network of geomagnetic observatories that measure space weather’s impact on our planet’s magnetic fields.

Solar activity impacts on GNSS performance

When the flares and CMEs erupting from the Sun follow a trajectory intersecting with Earth, they trigger a series of atmospheric and electromagnetic effects that can disrupt GNSS reception.

Most of these impacts happen in the ionosphere, a shell of electrons and ions 80 to 1000 kilometers above the Earth’s surface. GNSS satellites have 20,000-kilometer orbits, so their signals transit the entire ionosphere. Shifting electron densities alter these signal paths, resulting in scintillation similar to how stars visibly twinkle due to changes in atmospheric density. Normally, these conditions are well modeled, which allows GNSS receivers to compensate for the effect.

A solar storm’s SEPs and energetic photons will penetrate Earth’s magnetic field and impact the ionosphere. The stronger the storm, the deeper its effects penetrate. These charged particles ionize atmospheric particles to increase electron densities.

The supercharged regions of the ionosphere will absorb radio wavelength photons blocking low and medium-frequency radio signals, especially in polar areas. GNSS constellations use signals in the 1 GHz range, so they do not suffer from these radio blackouts.

However, solar storms also unpredictably change electron densities at different altitudes to significantly increase scintillation. When GNSS signal paths deviate enough from predictions, a receiver’s models cannot compensate. It may report positions with errors measured in tens of meters or fail to lock onto a satellite’s signal at all.

Multi-frequency and constellation receivers are more resilient to a solar storm’s effects since they combine several signals to smooth out the variations. GNSS constellations can become more resilient to space weather impacts through ground and satellite-based augmentation. For example, the European Geostationary Navigation Overlay Service (EGNOS) receives GPS and Galileo signals at precisely located ground stations in Europe, calculates positioning corrections, and broadcasts the results from geostationary satellites.

Outlook for Solar Cycle 25

The latest forecast from NOAA’s Space Weather Prediction Center includes a higher and later peak for Solar Cycle 25 than initially expected. Sunspot activity may not decline until sometime between November 2024 and March 2026 and will continue for several years after that.

The strong solar events in the first half of 2024 did not cause systemic issues. Still, more CMEs and flares will head Earth’s way in the coming months, with the potential to disrupt GNSS performance for users who depend on highly accurate positioning data.

June 21, 2024