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Space Weather Effects on GPS and WAAS

isr assisting the faa

Space Weather describes the conditions in space that affect Earth and its technological systems.  It is a consequence of the behavior of the Sun, the nature of the Earth’s magnetic field and our location in the solar system.  The Sun is the major source of space weather on Earth.  The features on the Sun reveal the active and turbulent nature of this highly magnetic and hot gaseous star. 

Space Weather events such as solar flares, coronal mass ejections and solar radio bursts can have major effects on space-based technological systems including GPS and WAAS.  Solar flares and CMEs induce geomagnetic storms that make the ionosphere unstable, resulting in rapid changes and strong spatial gradients.  Solar flares travel at the speed of light; however, their effects on GPS receivers have been noted to begin approximately one hour later.  CMEs travel a lot slower, typically reaching Earth in one to five days after the eruption from the Sun.  CMEs are the most threatening of solar events.

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 Space Weather Effects on GPS

Space weather events, such as those described above, can affect the performance of groundand space based technological systems, with effects ranging from minor digital upsets, to severe power grid disruptions that can cause loss of service to millions.

For GPS users, the impact of space weather can usually be characterized by disturbances in the ionosphere.  The ionosphere is that part of the atmosphere that is ionized by solar radiation.  It has practical importance because it influences radio wave propagation between space and earth.  The ionosphere over the CONtiguous United States (CONUS) is normally well ordered with slow and predictable changes in the ionosphere.  During a geomagnetic storm, however, rapid changes and strong gradients over distance can ultimately degrade GPS ranging measurements. 

Very severe magnetic storms can also introduce ionospheric scintillation activity in the mid-latitude CONUS region.  Scintillation is a rapid variation in the intensity of the satellite signal that can cause a GPS receiver to lose lock on the signal.  If many satellite signals are affected by scintillation, it can cause GPS to lose the capability to provide positioning information.

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 Scintillation and the presence of large gradients, as illustrated in Figure 2, are a major concern for wide area ionospheric modeling systems such as the FAA’s Wide Area Augmentation System (WAAS).

Space Weather Effects on WAAS

WAAS is a satellite based navigation system.  It was designed by the FAA to become the future primary means of civil aircraft navigation. It was commissioned for approaches with vertical guidance (APV) in July 2003.  What that means is that aircraft can be guided to 200ft above a runway without the help of an ILS (Instrument Landing System) – even in poor visibility conditions.

In the WAAS system, the standard GPS service is augmented with corrections for time, the GPS satellite orbits and the ionosphere.  These augmentations enable the WAAS system to meet the very stringent aviation requirements for accuracy, availability and integrity.  Quarterly performance reports have shown that the WAAS system generally meets or exceeds these requirements [http://www.nstb.tc.faa.gov].  The performance reports also reveal that one of the greatest challenges for WAAS is maintaining continuous APV availability during extreme geomagnetic storms.  

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Since WAAS was deemed operational, ISR has assisted the FAA in monitoring space weather effects on the WAAS system.  The result of that analysis is primarily captured by Figure 3where availability of APV service is limited under severe magnetic storm activity.  The effects of less intense magnetic storms vary, with some inducing shorter regional losses of availability and others having little effect on availability.

 

Point of Contact for this project is Patricia H. Doherty.