Satellite engineers have puzzled over why GPS navigation systems on low-orbiting satellites sometimes black out when they fly over the equator between Africa and South America. The European Space Agency's Swarm satellites have now pegged "thunderstorms" in the ionosphere as the reason.

The agency's three Swarm satellites—which measure the Earth's magnetic fields—carry GPS receivers as part of their positioning system so that operators can keep them in their correct orbits. However, the satellites sometimes lose their GPS connection—an event that occurred 166 times during the Swarm trio's first two years in orbit.

The ESA's Swarm satellites lost their GPS connection 166 times in their first two years in orbit. Image credit: ESA.“Ionospheric thunderstorms are well known, but now we have been able to show a direct link between these storms and the loss of connection to GPS," says Claudia Stolle, of the Faculty of Science at the University of Potsdam, which made the discovery. “This is thanks to Swarm because it is the first time that high-resolution GPS and ionospheric patterns can be detected from the same satellite.”

These thunderstorms occur when the number of electrons in the ionosphere undergoes large and rapid change. This tends to happen close to the Earth’s magnetic equator and typically for a couple of hours between sunset and midnight.

As its name suggests, the ionosphere is where atoms are broken up by sunlight, which leads to free electrons. A thunderstorm scatters these free electrons, creating small bubbles with little or no ionized material that disturb the GPS signals so that the satellites' GPS receivers can lose track.

Research determined that 161 of the lost signal events coincided with ionospheric thunderstorms. The other five were over the polar regions and corresponded to an increase in strong solar winds that cause the Earth’s protective magnetosphere to "wobble."

Resolving the mystery of blackouts is relevant not only for Swarm, but also for other low-orbiting satellites experiencing the same problem. ESA says that engineers will be able to use this new knowledge to improve future GPS systems to limit signal losses.