A scientist at the National Oceanographic and Aeronautics Administration (NOAA) said recently increased solar storm activity could pose serious problems for GPS signals. The satellite transmissions that power the GPS system are relatively weak in strength and vulnerable to interference from instability on the surface of the sun. NOAA scientist Joe Kunches and other scientists at NASA's heliophysics division told Internet aviation news service AvWeb that solar storms have shut down GPS in the past, and can do it again — for days at a time. And predicting when and where that might happen is considered a serious challenge by solar experts. With increased reliance on GPS for critical phases of flight — and the coming transition to ADS-B for air traffic control functions, the prospects of a GPS shutdown are severe.