The use of single and dual frequency radio navigation systems, like GPS, has grown dramatically in the last decade. GPS devices are now in every cell phone and in many automobiles, trucks of all types, and any equipment that moves and needs precision location measurements. High precision GPS using dual frequencies are used for farming, construction, exploration, surveying, snow removal and many other applications.
There are several ways in which space weather impacts GPS. The GPS signal travels from the satellite to the receiver on the ground, and passes through the ionosphere. The ionosphere bends the path of the GPS signal similar to the way a lens bends the path of light. During quiet times, when there is no space weather activity, the GPS system can compensate for the ionosphere, removing much of its impact on the accuracy of the positioning information. When the ionosphere is disturbed due to space weather, however, the signals are delayed which introduces errors in the calculated position.
Single frequency GPS systems can provide position information with an accuracy of a meter or less. During a severe space weather storm, these errors can increase to tens of meters or more. Dual frequency GPS systems can provide position information with accuracies of a few centimeters. In this case the two different GPS signals are used to better characterize the ionosphere and remove its impact on the position calculation. When the ionosphere becomes highly structured, though, the GPS system loses its lock on the satellite signal and then loses the ability to provide position information at all.
Geomagnetic storms create large scale features in the ionosphere. The currents and energy introduced by a geomagnetic storm enhances the ionosphere and increases the total height-integrated number of ionospheric electrons, or Total Electron Count (TEC). The GPS system cannot correct for this enhancement and so errors are introduced into the position calculations. This usually occurs at high latitudes, though major storms can produce large TEC enhancements and gradients at mid-latitudes as well.
Near the magnetic equator there are current systems and electric fields that create instabilities in the ionosphere. The instabilities are most severe just after sunset. These smaller scale (tens of kilometers) instabilities, or bubbles, can have a scintillating affect on the GPS signal, much like waves on the surface of a body of water will disrupt the path of light as it passes through them. Near the equator, dual frequency GPS systems often lose their lock on the GPS signal on a regular basis. These ionospheric scintillations are not associated with any sort of space weather storm, but are simply part of the natural equatorial ionosphere.