Solar Burst Impacts GPS

April 27, 2007  - By
Image: GPS World

On December 6, 2006, the sun emitted a burst of radio energy that impacted the performance of GPS receivers all over the sunlit side of the Earth. That the sun produces radio emissions is not surprising. What is surprising is that on this day they were extremely powerful. The sun continuously emits energy across a broad region of the radio spectrum. The flux density of these emissions is normally fairly low and contributes imperceptibly to the background radio noise collected by GPS receiver antennas.

However, when a solar flare occurs, it is often accompanied by very powerful bursts of radio energy. Although they are more numerous near the peak of the solar sunspot cycle when the sun is more disturbed, solar flares and their associated strong radio bursts can occur at anytime – including near the current sunspot minimum. Still, the December 6 solar radio burst came as a surprise. It was one of the largest on record and had an impact on all GPS receivers on the sunlit side of the Earth, including most of North America, South America, and the Pacific Ocean. The added noise significantly reduced carrier-to-noise-densities (C/N0 – a measure of the strength of received signals) at both the L1 and L2 frequencies by as much as 15 dB-Hz. This resulted in receivers losing lock on some satellites for many minutes, particularly those at low elevation angles with low C/N0 values before the burst’s arrival. Those receivers closer to the sub-solar point were typically affected more than those further away as more or the burst energy was picked up by the receiver antennas.

Nevertheless, it appears that a lot of single-frequency receivers continued to provide navigation solutions with as few as four satellites — and even three in 2D mode — and the noise burst went unnoticed by most users of such receivers. However, many dual-frequency receivers used for high-accuracy applications including those at reference stations suffered significant signal losses, particularly at the L2 frequency. As well, military receivers in some sectors lost the ability to navigate. A “widespread loss of GPS” in the Four Corners region of New Mexico and Colorado was reported by military authorities. Several aircraft reported losing lock on GPS signals with the number of tracked satellites dropping from 7-9 to 1 or even none!

Alessandro Cerruti, a graduate student at Cornell University, is among a group of scientists and engineers studying the effects of this and other solar radio bursts on the operation of GPS receivers. He has examined the data provided by the receivers in the International GNSS Service (IGS) network on the sunlit side of the Earth. The number of stations providing data at both frequencies on at least four satellites dropped from more than 120 to below 60 during the burst. The timing of the drop-outs coincides with the power of the burst which is shown in the lower panel.

The burst power was recorded at the Owens Valley Solar Array (OVSA) in California’s high desert. Operated by the New Jersey Institute of Technology’s Center for Solar-Terrestrial Research, OVSA records solar radio emissions at over a range of frequencies and polarizations including right-hand circular polarization (RHCP) at 1.6 GHz, very near the GPS L1 frequency. As the plot shows, noise power exceed one million solar flux units at the peaks of the burst, making this burst one of the largest on record.

Alessandro Cerruti has also looked at data from the Wide Area Augmentation System (WAAS) which is very robust and although WAAS continued to operate throughout the period of the burst, signals at the WAAS reference stations suffered significant degradations as elsewhere. The C/N0 values for PRN 4 as recorded at the Houston reference station on both the L1 and L2 frequency for a quiet day and on December 6. The drop in C/N0 values during the burst is very dramatic.

Mitigation. What can be done, if anything, to mitigate the effects of solar radio bursts? As the bursts are broadband noise, it is difficult for a receiver to discriminate them from GPS signals. Some antenna designs, such as choke rings, attenuate signals arriving at low elevation angles, so if the sun is low in the sky at the time of a burst, receivers with such antennas will be less impacted than those with conventional antenna designs. And as a receiver loses track primarily on satellites at low elevation angles, having more satellites at higher elevation angles will also help. So receivers operating with a mixed constellation of GPS and GLONASS or GPS and Galileo satellites should be better able to weather a solar radio burst than those operating with GPS alone. Similarly, a larger GPS constellation by itself would help.

Modernization. Stronger transmitted signals from future GPS satellites might allow receivers to continue tracking even low-angle satellites during a large burst. Newer signal formats, which could be tracked at lower C/N0 values, would also help receivers to contend with the sun’s outbursts. Even current receiver technology developed for anti-jamming protection and for indoor GPS use would allow receivers to track to much lower C/N0 values and perhaps sail through even very strong solar radio bursts.

As we approach the peak of the next sunspot cycle in 2012, we can expect more solar radio bursts. Some forecasts peg the next peak at 30–50 percent stronger than the last one as measured by the fraction of the sun’s visible hemisphere with sunspot activity. Will future solar radio bursts have as dramatic an effect as the burst of December 6, 2006? Time will tell.

 — Richard Langley

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About the Author:

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.

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