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Friday, June 7, 2013

How Space weather can affect GPS satellite performance

Space weather is the study of changing environmental conditions in near-Earth space, or the space from the Sun's atmosphere to the Earth's atmosphere.  It is a separate concept from weather within the Earth's atmosphere.

One of the events of most concern regarding space weather are coronal mass ejections (CMEs) from the Sun, which are considered separate events from solar flares - though the two appear to be related.  CME's typically bring a shock wave of solar energetic particles to the Earth's atmosphere, causing a visual effect near the poles commonly called the 'aurora borealis'.  Other effects include geomagnetic storms, radio blackouts, and electrical line overloads which can lead to cascading power outages, as well as satellite outages. Geomagnetic storms are categorized from G1 (minor) to G5 (extreme). 

The ionosphere bends radio waves in the same manner that water in a swimming pool bends visible light.  The Global Positioning System (GPS) uses signals at 1575.42MHz (L1) and 1227.6 MHz (L2) which can be distorted by a disturbed ionosphere and a receiver computes an erroneous position or fails to compute any position. Because the GPS signals are used by a wide range of applications, any space weather event which makes the GPS signal unreliable can have a significant impact on society. For example the Wide Area Augmentation System (WAAS) is used as a precision navigation and tracking tool for commercial aviation, personal tracking and commercial fleets in North America. It is disabled by most major space weather events. In some cases WAAS is disabled for minutes, but it can be disabled for days depending on the severity of the storm. Major space weather events can push the disturbed polar ionosphere 10° to 30° of latitude toward the equator and can cause large ionospheric gradients (changes in density over distance of 100's of km) at mid and low latitude. All of these factors can significantly distort GPS signals.