Artist’s representation of the signals from GPS satellites being interrupted when Swarm satellites fly into strong equatorial plasma irregularities.
Caption: Signals (yellow lines) from GPS satellites can get interrupted when lower-orbiting satellites like Swarm fly into strong equatorial plasma irregularities. The green line represents a sample in situ electron density profile measured by the Swarm satellites during one of these disruptions—note the high small-scale fluctuations. The two peaks are the major maxima of background electron density, with the minimum in between, representing the crests and the trough of the well-known ionospheric equatorial ionization anomaly.  The image is a modification of an artist’s representation of the Swarm satellites orbiting Earth. Credit: ESA/AOES Medialab
Source: Space Weather

The European Space Agency’s Swarm satellites were launched in 2013 to study Earth’s magnetic field; the spacecraft allow researchers to study both magnetic signals from inside the Earth and space weather in Earth’s ionosphere and magnetosphere. These three satellites, called Swarm A, B, and C, make precise measurements of the magnetic and electric fields around Earth while using eight-channel GPS receivers to keep close track of their locations and navigate their orbits.

Sometimes, however—usually in the evening hours—the Swarm satellites lose signal from one or more GPS satellites; once a Swarm satellite is tracking less than four GPS signals, its navigational system can suffer or even fail completely. Over the first 2 years of the Swarm mission, the three satellites have lost GPS signal on all eight receiver channels 166 times.

A new study by Xiong et al. connects these losses of GPS signal to equatorial plasma irregularities (EPIs), which occur when the electron density in the ionosphere’s F region undergoes large, rapid changes. EPIs mostly occur close to Earth’s magnetic equator, which might explain why the loss of GPS signal to Swarm satellites also occurred most frequently when the satellites were at low latitudes.

Nearly all of the 166 loss of GPS signal events adhered to this pattern, and the researchers confirmed that the 161 low-latitude events coincided with EPIs that caused significant depletion of the ionospheric electron density. The other five events at high latitudes corresponded to polar patches or increased geomagnetic activity. In addition, when a Swarm satellite flew through an area of strong EPIs, it experienced loss of GPS signal for at least one channel about 95% of the time. These results indicate that EPIs play a critical role in causing loss of GPS signal for satellites in low Earth orbit.

The authors noted other distinct patterns. Swarm B, for example, underwent fewer GPS signal loss events than Swarm A or C:  Swarm B’s orbit keeps the satellite 50 kilometers higher above Earth than Swarm A and C, which means it inhabits an area with lower electron density. The authors indicate that the lower background density limits the magnitude of local density depletions, protecting the satellite from the most intense EPIs.

This finding suggests that satellites flying at higher altitudes could be at lower risk for loss of GPS signal. Also, the bandwidth of the Swarm satellites’ GPS receivers was updated in 2015, which may help decrease issues with GPS signal in the future. With further study and more data over time, researchers may soon determine how best to limit loss of GPS signal, whether that is through further changes to the GPS receivers or adjusted orbits. (Space Weather, doi:10.1002/2016SW001439, 2016)

—Leah Crane, Freelance Writer 


Crane, L. (2016), What causes GPS signal loss on satellites like Swarm?, Eos, 97, Published on 23 September 2016.

Text © 2016. The authors. CC BY-NC-ND 3.0
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