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   
      
   --- DB 3.99 + Windows 10   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)