Challenging Minisatellite Payload for Geophysical Research and Application (CHAMP) radio occultation data are analyzed to determine causes for signal tracking loss at altitudes above the planetary boundary layer (PBL). This frequently happens within the Intertropical and South Pacific Convergence zones. We focus on events with a sudden full spectrum inversion (FSI) amplitude drop over a narrow atmospheric layer, as caused, for example, by strong atmospheric refractivity gradients. Events show a clear distinction between land- and sea-based observations. At high latitudes and mountainous regions, sudden FSI amplitude drops are caused by GPS satellite settings. Low-latitude (midlatitude) sea-based events also show maxima around 7 km (5 km) and 2.5 km. Events near 2.5 km are found at the upper range of the PBL, possibly involving ducting. Wave optics simulations applying CHAMP receiver characteristics and simulated refractivity profiles show that such events can readily be caused by strong refractivity gradients below the ducting conditions. Hence, around 7 km, events are caused by the relatively stable temperature lapse rate leading to enhanced water vapor layers and radiative cooling, associated with cumulus congestus clouds. The required signal carrier-to-noise density ratio sufficient to avoid tracking losses is 55 dB Hz for low-latitude observations. In the lower atmosphere, near the PBL top, 60 dB Hz is required in the presence of strong vertical gradients in refractivity. CHAMP operates with about 50 dB Hz at low latitudes.