Researchers have been puzzled by regular blackouts in the GPS navigation system on the European Swarm satellites where DTU Space leads the international scientific cooperation. The loss of signals occurs in particular when the satellites pass the equator between Africa and South-America. The reason is wild space weather.
The strange blackouts on the satellites can now be explained by Swarm mission scientists: there is a direct link between these GPS blackouts and ionospheric ‘thunderstorms’ that appears frequently in space in this area some 300 to 600 kilometers above Earth.
“These ionospheric thunderstorms are well known, but it’s only now we have been able to show a direct link between them and the loss of the GPS. This is possible because the Swarm satellites provide high resolution observations of both phenomena at one spacecraft,” said Prof. Claudia Stolle from GeoForschungsZentrum Potsdam in Germany and a Swarm project scientist.
“These storms would typically occur for one or two hours between sunset and midnight, and cause loss of the GPS signal from the satellites for some minutes”.
Claudia Stolle and her colleagues have now published their findings in the scientific journal Space Weather. Swarm is a constellation of three identical satellites launched by the European Space Agency (ESA) to track and study the Earth’s magnetic field. DTU Space professor Nils Olsen at DTU is a chief scientist on the ESA Swarm mission and DTU Space has been involved in the project from the beginning.
The signal is bend away
The ionosphere is the part of the upper atmosphere where atoms are ionized by sunlight, leading to free electrons.
An ionospheric thunderstorm appears as a turbulence in the distribution of these electrons. The ‘storm’ tears away or scatters the electrons and creates small ‘bubbles’ with little or no ionized material. At these areas the GPS’ electromagnetic waves are bent and scattered so the signal to the satellite is lost.
GPS receivers onboard the Swarm satellites or at the ground receive electromagnetic waves from transmitting GPS satellites some 20,000 km further away in space to determine precise positioning. Without a continuous contact to these GPS satellites, a precise positioning of satellites as well as cars or mobile phones becomes impossible.
Usually the GPS blackout is fixed after a blackout by requiring the GPS signal on the Swarm satellites. But engineers and scientists are striving to develop more efficient GPS systems that are less affected by ionospheric disturbances.
A great tool for measuring
The link between GPS blackouts and ionospheric thunderstorms has turned out to be an excellent tool to investigate the systematic irregularities in the ionosphere.
This makes it possible to model and analyze these variations that can occur due to changes in the solar magnetic activity. By looking at changes in the GPS signal it is possible to calculate backwards and determine variations in the ionosphere for a certain level of this solar activity.
“The GPS has become a very convenient scientific tool for us in addition to being a navigation and positioning instrument for the satellite,” said prof. Stolle who is an expert in the ionosphere and geomagnetism.
Swarm’s mission is to track and study the Earth’s magnetic field with unprecedented precision and resolution. The magnetic field shields us from damaging radiation from Space. Earth’s magnetic field is created by a giant dynamo in the molten outer core of Earth 3,000 km beneath our feet. In addition there are contributions from magnetized rocks in the Earth’s crust and external sources like ionospheric thunderstorms in space.
”It’s important to understand these external sources to get a complete picture of the Earth magnetic field. This study looks at one of these factors,” explains Claudia Stolle.
Improved GPS systems in the future
With the new findings engineers can gain knowledge that might be helpful to improve future GPS systems and limit loss of signal. This is valuable not only for space scientists but also in aviation and the maritime area where precise navigation is crucial.
The Swarm satellites were launched in 2013. Two of them – Alpha and Charlie – fly in tandem at about 450 km from Earth and descends over time. The third, Bravo, is in orbit 500 km from earth and slightly drifts away from the two others in its orbital plane.