GPS World

Tag: TEC

  • GPS ‘sees’ the Great American Eclipse

    GPS ‘sees’ the Great American Eclipse

    The eclipse across America on Aug. 21 was not only a magnificent visual event, it was also observed indirectly by the impact that it had on the propagation of radio signals — including those of global navigation satellite systems.

    There was a decrease in the number of free electrons in the part of the Earth’s ionosphere along the eclipse path where sunlight was temporarily blocked by the moon. While not as significant as the daily variation as day turns to night, the effect was clearly seen in the signals received on the ground from GPS satellites.

    GPS signals are routinely used to monitor the behavior of the ionosphere. The density of electrons in the ionosphere affects the speed of propagation of radio signals and this effect is slightly different at different frequencies.

    By combining measurements made on the L1 and L2 legacy signals transmitted by all GPS satellites using high-grade receivers, scientists and engineers can measure the total electron content (TEC), which is the number of electrons in a column with a cross-sectional area of one meter squared along the path of the signal from satellite to receiver.

    This value can then be projected to the vertical direction using a simple equation. Given the large number of electrons in the column, we measure the TEC in TEC units (TECU), where 1 TECU = 1016 electrons per square meter.

    TEC time series from two continuously operating GPS monitoring stations near the path of totality, BREW at Brewster, Washington, and NISA at Boulder, Colorado, show a small dip of about 2 TECU or so around 18:00 UTC on Aug. 21, coincident with the timing of the eclipse. These time series are illustrated in FIGURES 1 and 2. Also shown in the figures are the time series for the day before, Aug. 20, which just show the normal diurnal ionospheric variation.

    Figure 1. Time series of vertical total electron content observed using all GPS satellites observed at Brewster, Washington, on Aug. 21, 2017, the day of the eclipse (in blue) and the time series from the previous day, Aug. 20., 2017, for comparison (in red).
    Figure 2. Time series of vertical total electron content observed using all GPS satellites observed at Boulder, Colorado, on Aug. 21, 2017, the day of the eclipse (in blue) and the time series from the previous day, Aug. 20., 2017, for comparison (in red).

    The effect of the eclipse was also be seen in the real-time correction data transmitted by the U.S. Wide-Area Augmentation System (WAAS) using geostationary satellites.

    WAAS provides enhanced accuracy, integrity and availability for GPS single-frequency users using a network of dual-frequency GPS receivers all across North America. Corrections include a grid of ionospheric propagation delay values, updated every 5 minutes, which are used to account for the delay in receiver measurements.

    FIGURE 3 shows part of the grid transmitted by WAAS and the path of totality across the U.S. Three of the grid points are close to the path and the time series of delay values of these points are shown in FIGURE 4.

    Figure 3. Map showing the locations of a subset of the grid points used for the WAAS ionospheric delay corrections highlighting the three grid points close to the eclipse path of totality used to examine the effect of the eclipse along with one grid point far removed from the path for comparison.
    Figure 4. Time series of ionospheric vertical delay values of three WAAS ionospheric grid points along the eclipse path of totality on Aug. 21, 2017, along with the values from a grid point far removed from the path.

    We see clear dips in values of up to about 50 centimeters. This is equivalent to what we see in the TEC time series from the BREW and NISA monitor stations since 1 TECU equates to 16 centimeters of propagation delay at the GPS L1 frequency.

    Furthermore, the times of the dips correspond to the times of totality as the eclipse quickly moved across the country from west to east. Also shown for comparison in Figure 4 are the delay values for a grid point far removed from the path of totality, which show only the normal diurnal variation.

    Not only does a total eclipse mesmerize the general public, it excites many scientists and engineers, too. A number of university research groups organized special eclipse observing campaigns to collect data from GPS receivers as well as other ionospheric monitoring tools to better understand exactly how the ionosphere reacts to a total eclipse of the sun.

    And although we expect future analysis of the data will show features of great interest to science, the immediate results from the total eclipse of Aug. 21 show no significant impacts on the position, navigation and timing service GPS provides.

    GPS “weathered” the eclipse with flying colors.

    (Attila Komjathy, Siddharth Krishnamoorthy, Anthony J. Mannucci, Lawrence C. Sparks, Lawrence E. Young and Giorgio Savastano from the NASA Jet Propulsion Laboratory operated by the California Institute of Technology; Gerald W. Bawden from NASA HQ Earth Science Division; and Hyun-Ho Rho and Richard B. Langley from the University of New Brunswick, Fredericton, Canada, contributed to this article.)

  • NASA describes expected impact of total eclipse on GPS

    NASA describes expected impact of total eclipse on GPS

    NASA has issued a statement to let the GPS community know what to expect when the total solar eclipse takes place across America on Aug. 21.

    On Aug. 21, the eclipse will cross all of North America. Anyone within the path of totality will see the moon completely cover the sun, and the sun’s tenuous atmosphere — the corona — can be seen.

    Observers outside this path will still see a partial solar eclipse where the moon covers part of the sun’s disk.

    A map of the United States showing the path of totality for the August 21, 2017 total solar eclipse. (Image: NASA)

    For NASA, the eclipse provides a unique opportunity to study the sun, Earth, moon and their interaction because of the eclipse’s long path over land and coast to coast. Eleven NASA and NOAA satellites, as well as the International Space Station, more than 50 high-altitude balloons and hundreds of ground-based assets, will take advantage of this rare event over 90 minutes, sharing the science and the beauty of a total solar eclipse with all.

    Via live streams and a NASA TV broadcast, NASA will bring the Aug. 21 eclipse live to viewers everywhere in the world.

    Below is the statement from NASA regarding GPS.

    NASA Note on the Aug. 21 Solar Eclipse and Its Effect on GPS Users

    FOR THE GPS COMMUNITY

    From ionospheric point of view, the expected effect of solar eclipse is a significant reduction in solar EUV ionization (solar EUV radiation is blocked) and thus in the amount of ionospheric total electron content (TEC) with respect to nominal conditions along the eclipse path.

    Some observations also show wave-like TEC perturbations in small magnitude (~1 TECU) during eclipse as shown in the attached reference. The wave-like perturbations appear to be the effect of atmospheric gravity waves or traveling ionospheric disturbances (TIDs) that might be triggered during eclipse.

    The TEC decrease would reduce ionospheric-induced delay of GPS signals. The small-magnitude TIDs won’t cause any major effects on GPS signals. These should not cause loss of GPS signals.

    I have not seen any reports about ionospheric scintillation observations during eclipse (I might have missed them). It would be interesting to analyze GPS data along the path of upcoming August eclipse to see if any scintillation events could be triggered.

    We have some GPS data processing tools at JPL and can contribute to this analysis.

    FOR THE GENERAL PUBLIC

    A solar eclipse occurs when the Moon passes between the Sun and the Earth, thereby totally or partly obscuring the image of the sun for a viewer on Earth. There is a region of Earth’s upper atmosphere, called the ionosphere which affects radio waves, including GPS.

    The ionosphere consists of “ions,” a shell of electrons and electrically charged atoms and molecules. Because ions are created through sunlight interacting with the atoms and molecules in the very thin upper atmosphere, the density (thickness and consistency) of the ionosphere varies from day to night.

    The ionosphere bends radio signals, similar to the way water will bend light signals. That is why you can hear AM radio broadcasts from far away at night. Also, ham radio operators rely on the ionosphere to bounce their signals from their station to the far reaches of the globe.

    Since GPS is a radio signal, its measurements are slightly impacted by ionosphere changes, resulting in small increases in position error. For all except very precise GPS users, these changes are negligible.

    Note that a total eclipse of the Sun is similar to our day-night cycle, only much faster. So, while the ionosphere will be more dynamic during an eclipse, it will not cause a loss of the GPS signal.

    In summary, while any effects from the eclipse are of scientific interest, GPS service should not be adversely affected by the Aug. 21 solar eclipse.

    Ionospheric effects should not be confused with those from solar flares (a brief eruption of intense high-energy radiation from the sun’s surface) that can cause significant electromagnetic disturbances on the earth, impacting radio frequency communications/transmissions (including GPS signals) and power line transmissions. Solar flares are not produced because of an eclipse.

    NASA has funded 11 studies in a range of heliophysics disciplines; work at MIT Haystack Observatory and Virginia Tech will make extensive use of GPS receivers to study the effects of the total eclipse on the Earth’s ionosphere.

    (NASA acknowledges the expertise of Larry Young and Xiaoqing Pi of NASA’s Jet Propulsion Laboratory for content, and AJ Oria of Overlook Systems Technologies for the coordination and editing of these statements.)