> FROM THE NEWS
>
> Big Solar Storm, Little GPS Effect
> Inside GNSS
> 27 January 2012
>
> Although it was billed as the strongest for the past eight years and coincided nicely with
> the American Meteorological Society (AMS) annual meeting in New Oreans, this week's solar
> storm apparently had a limited effect on GPS receivers and users.
>
> A large solar flare began at about 10:38 p.m. January 22, according to the space weather
> Laboratory at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Less than two
> days later, the resulting coronal mass ejection (CME) collided with Earth's magnetic field,
> producing a variety of ionospheric effects.
>
> NOAA's Space Weather Prediction Center rated the solar radiation storm as the largest since
> October 2003, categorizing it as a "strong" -- or S3 (with S5 being the highest) -- storm.
>
> In anticipation of the storm's arrival, the National Oceanic and Atmospheric Administration
> (NOAA) issued a warning to the aviation community. As a precaution, several airlines
> including Delta, Air Canada, and Qantas rerouted a handful of flights that crossed polar
> regions, where solar storm effects can be most pronounced and affect HF communications and
> GPS signals used for navigation.
>
> However, because the CME struck the Earth's atmosphere at an angle, rather than head on,
> the effects were weaker than they could have been and caused little noticeable effect on
> GPS satellites or receivers.
>
> The storm did, however, produce impressive display of "Northern Lights" -- aurora borealis --
> and presages increasing activity expected from the sun as it approaches the solar maximum
> in the current sunspot cycle.
>
> Three space weather phenomena account for most GNSS errors: total electron content
> (TEC)-induced delays, scintillation, and solar bursts. As GPS signals propagate through
> the ionosphere, their speed and direction change in proportion to the varying electron
> density or TEC along the line of sight between the receiver and the satellite. This affects
> the GPS range observable, adding a delay to the code measurements and an advance to the
> phase measurements.
>
> Scintillation, a distortion of radio waves' phase and amplitude, creates rapid variations
> in signal power, reducing the received power and phase coherence of the GNSS signals, which
> can cause a loss of lock on the signal. Solar radio bursts begin with a solar flare that
> injects high-energy electrons into the solar upper atmosphere, producing RF noise that can
> degrade GNSS signals.
>
> A recent AMS report highlighted the vulnerability of GPS-dependent critical infrastructures
> to disruptions caused by solar flares and other space weather events and offered recommendations
> for making GPS service more robust.
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