Enigmatic GPS blackouts cleared up by satellites

Ionospheric turbulence deflected signals for a short time

The three Swarm satellites are experiencing short-term failures of their GPS signal - why, researchers have now found out. © ESA / P. Carril
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Strange failures: Researchers have discovered why the European "Swarm" satellites repeatedly lose their GPS signal for a short time. Accordingly, storm-like turbulence in the ionosphere is to blame. They cause the signals of the GPS satellites to be distracted or swallowed. This disruptive effect occurs mainly at the equator and in the first half of the night. The knowledge of the causes could also make the earthly GPS tracking more reliable.

Since 2013, a satellite trio of the European Space Agency (ESA) is orbiting the earth. One of the tasks of the Swarm satellites is to map the terrestrial magnetic field. But the analysis of the data revealed a puzzling phenomenon: Time and again, the satellites lost the signal from one or even several GPS satellites for a short time. A total of 166 times this GPS signal loss occurred in the first two years of the Swarm mission.

Cluster after sunset

But why? Claudia Stolle from the GeoForschungszentrum Potsdam and her colleagues have now examined this more closely. They found a conspicuousness: Particularly often, the satellites lost their locating signal when they flew over the equator. Much of this signal loss also occurred between sunset and midnight. There were also failures in high latitudes, but then at noon.

This spatial and temporal distribution of signal losses brought the researchers of the cause on the track. For, along the magnetic equator of the earth, local disturbances often occur in the ionosphere after sunset. During the day, gas atoms in this upper atmospheric layer are ionized by UV light from the sun to form a dense layer filled with free electrons.

Local and regional turbulence can occur in the Earth's ionosphere, visible here as changes in the electron density. NASA / JPL

Bubbles in the ionosphere

After sunset, however, so-called ionospheric currents can cause turbulence in this layer, which causes almost electron-free bubbles and holes in the ionosphere. "These ionospheric storms have long been well known, " says Stolle. "But only now we have found a direct connection between them and the loss of the GPS signal." Display

As it turned out, the bubbles and turbulence in the ionosphere ensure that the GPS signals are deflected and scattered and therefore no longer arrive at the Swarm satellite and also no longer on Earth. "This can interrupt the location of one or even more satellites for a few minutes, " explains Stolle.

Adjustments make GPS more reliable

The discovery of the cause of the mysterious failures can now help to more reliably and reliably locate GPS in low Earth orbit satellites, as well as aircraft, ships, and other terrestrial receivers to make it more stable. On the one hand, the GPS failures reveal more about when such ionosphere surges occur and what role the sun's activity plays in it.

On the other hand, GPS receivers can now be set so that they are as insensitive to such turbulence as possible. As the researchers explain, the bandwidth of the recipients plays an important role. (Space Weather, 2016; doi: 10.1002 / 2016SW001439)

(Technical University of Denmark, 05.10.2016 - NPO)