GPS radio occultation measurements from CHAMP, GRACE-A and FORMOSAT-3/COSMIC are used to derive global information on small-scale ionospheric irregularities such as sporadic E layers between January 2002 and December 2007. The investigations are based on the analysis of amplitude variations of the GPS radio occultation signals. The global distribution of ionospheric irregularities shows strong seasonal variations with highest occurrence rates during summer in the middle latitudes. The long-term data set of CHAMP allows for first climatological studies, while the data coverage increases significantly with the combination of CHAMP, GRACE and FORMOSAT-3/COSMIC measurements. This allows for global maps of sporadic E occurrence rates of very high spatial resolution where the influence of the Earth's magnetic field becomes visible in global sporadic E maps for the first time.