Twinkle, Twinkle, GPS
 
June 30, 2016:  Go outside tonight and behold the stars - especially bright
stars low on the horizon. They twinkle as irregularities in Earth's atmosphere
pass by.
 
Unseen to the human eye, the same thing happens to signals from GPS, the
Global Positioning System.
 
Radio signals twinkle in much the same way as bright stars appear to do at
optical wavelengths. This can have effects on GPS, causing the signals to
brighten and fade, and reach Earth at unpredictable times. All of this could
degrade the accuracy of GPS positioning.
 
https://www.youtube.com/watch?v=UDAjxgD_lZg
 
The twinkling occurs, because signals beamed to Earth by GPS satellites pass
through a layer of Earth's atmosphere called the ionosphere. Irregularities in
the ionosphere, referred to as iononspheric depletions or bubbles in the
science community, span the hemispheres at the equator and are a major element
of the low latitude Geospace region. Dynamic and beautiful, these
irregularities form huge horseshow arcs between hemispheres with their apexes
centered on the magnetic equator.
 
Studying this phenomenon is the main reason NASA conducted a mission called
CINDI, the Coupled Ion-Neutral Dynamics Investigation beginning in 2008. The
CINDI instruments were carried into space along with other instruments on
board an Air Force Research Laboratory satellite called the Comm
nication/Navigation Outage Forecasting System, or C/NOFS. CINDI was designed
to measure ionization of the upper atmosphere-including the irregularities
that cause GPS twinkling.
 
The behavior of the irregularities responsible for the GPS twinkling turned
out to be quite surprising.
 
Rod Heelis, principal investigator for CINDI at the University of Texas as
Dallas explains: "According to conventional thinking, the ionosphere becomes
unstable shortly after the sun sets. As darkness falls, ionized atoms and
molecules begin to recombine into a neutral state. During this transition
period, 1 to 2 hours after sunset, irregularities are quite strong."
 
As the night wears on, however, those irregularities were thought to fade, and
eventually vanish around midnight.
 
"But that's not what CINDI found," says Heelis. "There were indeed many
irregularities around sunset, but they did not vanish around midnight. On the
contrary, there was another peak in irregularities during the middle of the
night. This second peak has appeared most pronounced from June through August."
 
Scientsts aren't sure yet why this second peak occurs or why it varies by
season, but Rob Pfaff, project scientist for CINDI at NASA's Goddard Space
Flight Center in Greenbelt, Maryland says "This unexpected behavior is a key
discovery. It shows that the ionosphere can still surprise us."
Researchers still have much to learn about the ionosphere and how it can
affect GPS and other satellite systems. CINDI re-entered Earth's atmosphere in
November of 2015, getting a one-of-a-kind, close-up look at the ionosphere
before it disintegrated.
 
Pfaff adds, "Towards the end of the C/NOFS mission, we had this great chance
to measure the ionosphere at much lower altitudes than we did previously. In
fact, we were able to see shear in the motions of the upper atmosphere - areas
where the ionosphere at lower altitudes flowed in the opposite direction to
that at higher altitudes. We think this shear may be one of the causes of the
GPS twinkling."
 
Next up, says Pfaff, is ICON, the Ionospheric Connection Explorer due to
launch in 2017. Led by researchers at UC Berkeley, the goal of this NASA
mission is "to understand the tug-of-war between Earth's atmosphere and the
space environment." Like CINDI before it, ICON will learn a lot about what
causes GPS twinkling-and much more.
 
Stay tuned for updates from the edge of space, at science.nasa.gov
 
 
Regards,
 
Roger

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