ESA's Swarm satellite trio has discovered why GPS navigation systems on low-orbiting satellites sometimes black out when they fly over the equator between Africa and South America. Swarm itself has experienced this anomaly numerous times before its scientists finally discovered what's causing it.
Swarm was launched in 2013 on a mission to measure and untangle the different magnetic fields that stem from Earth’s core, mantle, crust, oceans, ionosphere and magnetosphere.
As with many satellites, ESA’s three Swarm 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, sometimes the satellites lose their GPS connection. In fact, during their first two years in orbit, the link was broken 166 times.
Swarm scientists discovered there is a direct link between those anomalies and ionospheric thunderstorms, around 300 - 600 km (190 - 370 miles) above Earth.
Although atmospheric thunderstorms are well known, this is a first direct link between these storms and loss of connection to GPS.
Atmospheric 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.
As its name suggests, the ionosphere is where atoms are broken up by sunlight, which leads to free electrons. A thunderstorm scatters these free electrons, creating small bubbles with little or no ionised material. These bubbles disturb the GPS signals so that the Swarm GPS receivers can lose track.
Signals (yellow lines) from GPS satellites can be interrupted when lower-orbiting satellites like Swarm fly into equatorial plasma irregularities. The green line is a sample electron density profile measured by the Swarm satellites during one of these events. Credit: ESA–ATG medialab (GFZ annotated)
The study investigated conditions for loss of GPS signals observed by the Swarm satellites during a 2 year period, from December 2013 to November 2015.
"Our result shows that the Swarm satellites encountered most of the total loss of GPS signal at the ionization anomaly crests, between ±5° and ±20° magnetic latitude, forming two bands along the magnetic equator, and these low-latitude events mainly appear around post sunset hours from 19:00 to 22:00 local time," study authors said.
By further checking the in situ electron density measurements of Swarm, scientists found that practically, all the total loss of GPS signal events at low latitudes are related to equatorial plasma irregularities (EPIs) that show absolute density depletions larger than 10 × 1011 m−3; then, the Swarm satellites encountered for up to 95% loss of GPS signal for at least one channel and up to 45% tracked less than four GPS satellites (making precise orbit determination impossible). For those EPIs with density depletions less than 10 × 1011 m−3, the chance of tracked GPS signals less than four reduces to only 1.0%.
Swarm also observed total loss of all GPS signal at high latitudes, mainly around local noon, and these events are related to large spatial density gradients due to polar patches or increased geomagnetic/auroral activities.
"We further found that the loss of GPS signals were less frequent after appropriate settings of the Swarm GPS receivers had been updated. However, the more recent period of the mission, e.g., after the GPS receiver settings have been updated, also coincides with less severe electron density depletions due to the declining solar cycle, making GPS loss events less likely."
The red dots in the map show where the Swarm-C satellite lost its GPS connection between launch in November 2013 and March 2015. These losses in tracking signal were down to equatorial plasma bubbles. The green line denotes the geomagnetic equator. Credit: NASA blue marble/GFZ Potsdam/ESA
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.
Christian Siemes, who works at ESA on the mission, 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.”
“What we see here is a striking example of a technical challenge being turned into exciting science, a true essence of an Earth Explorer mission such as Swarm," ESA’s Swarm mission manager, Rune Floberghagen, added.
“These new findings demonstrate that GPS can be used as a tool for understanding dynamics in the ionosphere related to solar activity. Perhaps one day we will also be able to link these ionospheric thunderstorms with the lightning we see from the ground.”
- The Swarm satellite loss of GPS signal and its relation to ionospheric plasma irregularities" - Chao Xiong, Claudia Stolle and Hermann Lühr - Space Weather - An AGU Journal - August 2016 - DOI: 10.1002/2016SW001439
- Swarm reveals why satellites lose track - an article published by ESA on October 28, 2016
Featured image credit: ESA–ATG medialab (GFZ annotated)