Rummler's three-ray model is used for multipath propagation in modified spherical stratified air. The time delays of the three rays are obtained by using the ray tracing method. Both the time delays and the amplitudes are related to the modified refractive index gradients inside and outside the layer. Because of the random variations of the modified refractive index gradients, the computer simulation method of multipath fading is studied by using the Monte Carlo method. The experiments of frequency selective fading are performed in China on a 48.4 km terrestrial microwave link. The statistical distributions of flat fading, in-band dispersion and the effect of diversity reception and their correlations are studied both by computer simulations and by experiment. On the basis of data measured in China and Japan, and computer simulation results, the correlation between the equal probability values of the in-band linear amplitude dispersion and flat-fading depth due to multipath propagation is analyzed. An excellent linear relationship is found between them. ¿ 2001 American Geophysical Union