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GPS ‘sees’ the Great American Eclipse

August 25, 2017  - By

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.)

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About the Author: Richard B. Langley

Richard B. Langley is a professor in the Department of Geodesy and Geomatics Engineering at the University of New Brunswick (UNB) in Fredericton, Canada, where he has been teaching and conducting research since 1981. He has a B.Sc. in applied physics from the University of Waterloo and a Ph.D. in experimental space science from York University, Toronto. He spent two years at MIT as a postdoctoral fellow, researching geodetic applications of lunar laser ranging and VLBI. For work in VLBI, he shared two NASA Group Achievement Awards. Professor Langley has worked extensively with the Global Positioning System. He has been active in the development of GPS error models since the early 1980s and is a co-author of the venerable “Guide to GPS Positioning” and a columnist and contributing editor of GPS World magazine. His research team is currently working on a number of GPS-related projects, including the study of atmospheric effects on wide-area augmentation systems, the adaptation of techniques for spaceborne GPS, and the development of GPS-based systems for machine control and deformation monitoring. Professor Langley is a collaborator in UNB’s Canadian High Arctic Ionospheric Network project and is the principal investigator for the GPS instrument on the Canadian CASSIOPE research satellite now in orbit. Professor Langley is a fellow of The Institute of Navigation (ION), the Royal Institute of Navigation, and the International Association of Geodesy. He shared the ION 2003 Burka Award with Don Kim and received the ION’s Johannes Kepler Award in 2007.