ECE: Electrical & Computer Engineering
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Special Feature: SPACE WEATHER

The Effects of Space Weather on Satellite Navigation

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Solar Wind

The solar wind interacting with Earth's magnetic field and upper atmosphere can disrupt satellite navigation systems. Brent Ledvina studies these effects to develop the technology to predict and avoid these disruptions.

Brent Ledvina studies the effect of space weather phenomena on satellite navigation systems. "The primary science," he says, "is looking at radio wave propagation issues and how the chaotic ionosphere (the ionized part of the Earth's upper atmosphere) can disturb radio waves via refraction, reflection, diffraction, and absorption."

His work recently focused on the mid-latitudes between 22 degrees and 60 degrees latitude. "Previously, that part of the ionosphere was considered absolutely boring. Nothing was happening there. Or so we thought," he says. Historically, the interesting phenomena occurred at the equator and near the poles, he explained. Then, public data from surveyed GPS receivers started to reveal some eccentricities of the ionosphere in the mid-latitudes.

"We didn't see it until we had the GPS data from all over the U.S. It is not only interesting scientifically, but potentially has an impact on society and the technologies we rely on," he says.

Ledvina

The solar wind interacting with Earth's magnetic field and upper atmosphere can disrupt satellite navigation systems. Brent Ledvina studies these effects to develop the technology to predict and avoid these disruptions.

One of the main eccentricities is the occurrence and dynamics of large plasma densities in the ionosphere, he explained. Plasma is the fourth state of matter &ndash an ionized gas where the electrons in an atom or molecule separate from the nucleus. On a grand scale this leaves a soup of positively charged ions and negatively charge electrons in the upper atmosphere. The large plasma densities generate instabilities, which in turn can cause errors in the received signals from the GPS.

"Above the Midwest, for example, there is essentially a very benign low-density ionosphere, then during a space weather event, a wall of density can form and flow through Canada towards the North Pole. We do not understand these plasma flow dynamics. They cause perturbations to the position solution output by our GPS receivers. The real problem is, the GPS receivers weren't designed to deal with such a large gradient in electron density," Ledvina says.

Why does position matter? The Federal Aviation Administration (FAA) has a mandate to allow commercial planes to land using GPS. "Getting an incorrect position could be devastating."

Another interesting phenomenon being studied is solar radio bursts. "These can cause noise in the GPS transmission band, making the signal-to-noise ratio go down, thus making a GPS signal difficult to track. It's like the noise on your TV set when your cable is out and somebody turns on the microwave or blender," Ledvina explains.

A solar radio burst in the fall of 2006 was an order of magnitude larger in strength than ever predicted. "It knocked out a large percentage of the science-grade GPS receivers on the daylight side of the U.S. for several minutes. We assumed we understood the worst case and it turned out we had no clue," he says.

To study these effects, Ledvina builds instrumentation to collect and analyze the data. GPS is used to measure the line-of-sight integrated plasma density from a receiver on the ground to a GPS satellite. Using arrays of receivers, he is involved in mapping the ionospheric content.

One project involves putting GPS receivers in Chile and Colombia to understand the conjugate effects of the Rayleigh-Taylor instability on the plasma densities of the equatorial ionosphere. A simple explanation of the Rayleigh-Taylor instability is the situation in which a heavier fluid is on top of a lighter fluid, an obviously unstable situation, which typically results in chaotic mixing of the two fluids. By observing from two locations on opposite sides of the Earth's magnetic equator, the team will be able to study the instability's effects as they propagate down the geomagnetic field lines.

Another project involves building a commercial-grade GPS receiver based on software-radio technology. "The GPS receivers we build will be able to work easily with the European, Russian, Chinese and other satellite navigation systems. The commercial market doesn't produce receivers that operate well during challenging space weather events," he says. With more robust receivers, the team will be better able to measure the extremes of space weather.

Ledvina is also upgrading and developing ECE's GPS course and laboratory. Virginia Tech is the first U.S. university to have a simulator for Galileo &mdash the European GPS system that will go online in the next 4-7 years.