Satellite blackout mystery ESA Swarm

Satellite blackout mystery solved by ESA’s Swarm

The European Space Agency (ESA) has discovered why GPS navigation systems on low-orbiting satellites like its Swarm trio tend to black out when they fly over the equator between Africa and South America.

Launched in 2013 as a four-year project, the Swarm trio of satellites is designed to measure and untangle the different magnetic fields that stem from Earth’s core, mantle, crust, oceans, ionosphere and magnetosphere.

The satellites carry GPS receivers as part of their positioning system so that operators keep them in the correct orbits. In addition, GPS pinpoints where the satellites are making their scientific measurements.

However, the satellites have proven susceptible to losing their GPS connection, an issue that has occurred 166 times in just their first two years in orbit.

The project has now discovered a direct link between these blackouts and ionospheric “thunderstorms”, around 300–600km above Earth.

Claudia Stolle from the GFZ research centre in Potsdam, Germany said, “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.

“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 changes. This tends to happen close to Earth’s magnetic equator and typically just for a couple of hours between sunset and midnight.

Atoms are broken up by sunlight in the ionosphere leading to free electrons. These electrons are then scattered by thunderstorms that create small bubbles with little or no ionised material. These bubbles disturb the GPS signals so that the Swarm GPS receivers can lose track.

It transpires that 161 of the lost signal events coincided with ionospheric thunderstorms. The other five were over the Polar Regions and corresponded to increased strong solar winds that cause Earth’s protective magnetosphere to ‘wobble’.

Resolving the mystery of blackouts is not only good news for Swarm, but also for other low-orbiting satellites experiencing the same problem. It means that engineers can use this new knowledge to improve future GPS systems to limit signal losses.

ESA’s Christian Siemes said: “In light of this new knowledge, we have been able to tune the Swarm GPS receivers so they are more robust, resulting in fewer blackouts.

“Importantly, we are able to measure variations in the GPS signal which is not only interesting for engineers developing GPS instruments, but also interesting to advance our scientific understanding of upper-atmosphere dynamics.”

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