The 2017 Eclipse ‘seen through the eyes’ of a GPS receiver
CLEMSON, South Carolina – As Clemson University went dark on Aug. 21, 2017, for “The Great American Eclipse,” a GPS antenna poised on top of Kinard Hall monitored the ionosphere, the layer of Earth’s atmosphere that is charged by solar and cosmic radiation.
The antenna was part of a study – led by principal investigator Kshitija Deshpande and undergraduate student Nicolas Gachancipa from Embry-Riddle Aeronautical University – to understand how a natural phenomenon, like the total solar eclipse, might affect GPS capabilities.
The answer: not much.
The results showed a reduction in the ionosphere’s total electron content (TEC) leading up to totality. Values for scintillation – the turbulence that GPS signals experience as they pass through the ionosphere – were the same as on a typical day.
Together, the data demonstrated that GPS signal strength was lower during the eclipse, but it was not significant enough to have a noticeable effect for most users. This reduction is explained in more detail by a national experiment that tested other variables beyond total electron content and scintillation.
“Many rockets, missiles and airplanes use GPS to navigate, and if they lose the signal, that’s a problem. So many scientists became interested in modeling ionospheric disturbances. Scientists today are still trying to figure out how to predict the ionosphere, like how we predict the weather,” said Gerald Lehmacher, an associate professor in the department of physics and astronomy at Clemson.
Lehmacher worked with the researchers to install the GPS antenna at Clemson, which was in a prime position – almost dead-center within the path of totality – to help scientists take strides toward understanding ionospheric modeling.
“Radars and satellites have been monitoring the ionosphere since the beginning of the space age. Variations in the ionosphere are important for the navigation and communication of rockets and satellites and they have triggered the new field of space weather research,” Lehmacher said.
During daytime, atoms present in the ionosphere are heated and ionized by the ultraviolet rays of the sun, causing the atoms to split and release free-floating electrons. This is called ionization, and it results in an ionosphere that is crowded by electrons, making it difficult for GPS signals to transmit to receivers on Earth.
“At some point – usually at night when levels of solar radiation are reduced – these electrons and excited atoms will come together and recombine to become neutral again,” said Steve Kaeppler, an assistant professor and ionospheric scientist in physics and astronomy at Clemson.
For the Aug. 21 eclipse, the researchers from Embry-Riddle were interested in testing how scintillation values would change with a total solar eclipse – a concept that has not been studied extensively in the past. Values for total electron content, which Gachancipa predicted would decrease by 60 percent during totality, were also gathered.
At approximately 1:42 p.m. EDT, when the moon was partially blocking the sun, Gachancipa’s predictions rang true as the team noted a drop in the number of electrons in the ionosphere. Interestingly, the GPS satellite positioned in the darkest part of the moon’s shadow, named PRN 12 in the study, recorded the lowest number of total electrons of any satellite studied.
However, where Gachancipa expected that scintillation strength associated with the eclipse would decrease, the data showed that scintillation values on Aug. 21 didn’t differ much from those recorded on a normal day.
Lehmacher did not find this surprising. “Scintillations can be understood as turbulence in the ionosphere, and it’s just that the disturbance of the eclipse wasn’t violent enough to be considered turbulence,” he said.
Instead, Lehmacher said that conditions such as gravity waves and traveling ionospheric disturbances (TIDs) played the larger role in the reduction of GPS signal strength.
The national experiment that occurred on eclipse day – including over 2,000 Global Navigation Satellite Systems (GNSS) receivers in the United States – set out to look for these TIDs, which are waves in the ionosphere that originate from some sort of forceful impact.
“When you hit on something, like a desk, you create sound waves that radiate off of it,” Lehmacher said. “In this case, the eclipse created a hole in the ionosphere, and as it moved through the ionosphere, it also created waves. It’s analogous to a boat in the water. When a boat is in the water, it technically makes a hole in the water. As it moves, it has to push the water in front of it. The same thing happened with the eclipse – the hole in the ionosphere had to push the ionosphere in front of it.”
Enhanced TID activity during the Aug. 21 eclipse – with waves traveling at the speed of the moon’s shadow both prior to and following the depletion of electron content around the time of totality – were noted in the national study. The possible presence of gravity waves was also mentioned.
“Gravity waves are just temperature and pressure waves that are traveling through the atmosphere. All kinds of pulses can create gravity waves. For example, thunderstorms are like explosions on the bottom of the atmosphere. Any kind of weather system, even mountains that are in the way – they can launch gravity waves that propagate upward, all the way into the ionosphere,” Lehmacher said. “The eclipse doesn’t change the weather much. It just changes a little bit of this cold wind and cold air on the ground, and they wanted to know if this change was enough to create gravity waves.”
All of this, Lehmacher and Kaeppler said, further progresses ionospheric modeling, which thereby benefits anyone who uses a device powered by GPS navigation.
“For the university, the eclipse was a great opportunity to be a part of this chain of imagers, scientific experiments and citizen science. It was a unique chance to be able to watch it, do something and capitalize on it scientifically,” Kaeppler said.