Vertical resolution is a fundamental aspect of any atmospheric remote sensing technique. Occultations using the Global Positioning System (GPS) signals provide inherently high vertical resolution. The phase coherence of the GPS signals allows improvement beyond the resolution limit set by Fresnel diffraction and the viewing geometry. We have characterized this improvement by simulating the forward propagation of GPS signals through the atmosphere to the GPS receiver. The signals are then back-propagated to a position closer to the atmosphere, thereby reducing the effects of diffraction. Comparison between the direct and the back-propagated results indicates that the resolution is improved by a factor of several, down to ~100 m. ¿ 1999 American Geophysical Union