Go outside tonight and behold the stars — especially bright stars low on the horizon. They twinkle as irregularities in Earth's atmosphere pass by.
Unseen to the human eye, the same thing happens to signals from GPS, the Global Positioning System.
Radio signals twinkle in much the same way as bright stars appear to do at optical wavelengths. This can have effects on GPS, causing the signals to brighten and fade, and reach Earth at unpredictable times. All of this could degrade the accuracy of GPS positioning.