Turbulences within the ionosphere may disturb GPS satellite signals

Turbulences within the ionosphere may disturb GPS satellite signals.

23.09.2016: Turbulences within the Ionosphere, the uppermost part of Earth’s atmosphere, may cause signal loss detected for Swarm satellite navigation. This result was published in the scientific journal Space Weather by a team of scientists from the GFZ section Earth’s Magnetic Field. The study is now presented as „spotlight“ at the news platform EOS by the American Geophysical Union AGU. Like this, EOS distinguishes „exciting new research“.

Since 2013 the three satellites that form the Swarm mission of the European Space Agency ESA investigate the Earth’s magnetic field and the conditions within the ionosphere. For positioning and navigation of the satellites the onboard Global Positioning System (GPS) receives signals from up to eight GPS satellites. In the past, the Swarm satellites regularly lost the signal of one or more GPS satellites around the equatorial latitudes, usually around the early evening hours. If signals of less than four GPS satellites are received, precise positioning of the SWARM satellites is impossible. The mission then runs the risk of a complete blackout of their navigation system. Between December 2013 and November 2015 this complete blackout occurred 166 times.

The GFZ scientists Dr. Chao Xiong, Prof. Claudia Stolle, and Prof. Hermann Lühr, found these GPS signal losses in equatorial latitudes highly related to irregularities within the so called ionospheric plasma (equatorial plasma irregularities EPSs). “An investigation of the SWARM data showed that the loss of GPS signals occurred when there were strong anomalies in the ionosphere that manifested in ribbon-shape around the magnetic equator”, explains Chao Xiong, the main author of the study. These irregularities reached their maximum after sunset between 7 and 10 pm local time – the time span of the disturbances of the GPS signals.

Plasma turbulences after sunset

The ionosphere begins at a height of about eighty kilometers and fades into outer space. It contains ions and free electrons because the gas atmosphere is ionized here by solar radiation, which means that ions and free electrons form a so called plasma. Within a neutral state this plasma is regularly distributed. The irregularities, that the scientists assume to be the cause for the signal loss, are expressed as a depletion in the density of ions and electrons. These irregularities usually occur after sunset, and hence the scientists consider that the reason the plasma irregularities is the special constellation between the geomagnetic equator and the day-night terminator line.

After a series of updates of the Swarm GPS receivers, that included an increase in bandwidth of the received signal, no more GPS signal losses were detected. Dr. Xiong: „We can’t tell for sure if the loss of GPS signals was entirely solved, as the sun’s activity has also declined since then, which led to generally less plasma in the ionosphere”. The scientists conclude that both, a decrease of the ionospheric plasma density as well as a higher receiving band width of the receivers can reduce the probability of signal loss of the GPS satellites. (ak)

Xiong, C., Stolle, C., Lühr, H., 2016. The Swarm satellite loss of GPS signal and its relation to ionospheric plasma irregularities. Space Weather, 14. DOI: 10.1002/2016SW001439

>> EOS Research Spotlights