[1] The radio occultation technique is used to study plasma irregularities in the F2 layer ionosphere and below. The data, acquired by the GPS receiver onboard the geophysical research satellite CHAMP for more than three years, allow to study ionospheric irregularities under different geophysical conditions on a global scale. The dual-frequency GPS signals provide a possibility to trace irregularities with a spatial size ranging from about 20 to 250 km. Whereas the irregularities characterized by longer wavelengths dominate at low latitudes, the shorter scale perturbations are very intense at high latitudes. It is found that the high latitude irregularities occur mostly at night time when the ionization level is low.