We showed that the amplitude of GPS occultation signal is important indicator of the ionospheric activity. Amplitude is more sensitive to small-scale ionospheric disturbances than the phase of the radio occultation (RO) signals. Local mechanism of strong ionospheric influence on the amplitude and phase of RO GPS signals is described. Critical points (tangent points) in the ionosphere, where the gradient of the electron density is perpendicular to the RO ray trajectory, strongly influence on the amplitude and phase of RO signals, and introduce multi-RO ionospheric effect in the experimental RO data. Positions of the critical points depend on the structure of the ionospheric disturbances. Analytical model for description of multi-RO ionospheric effect is introduced. Model accounts for the horizontal gradients in the ionosphere and gives analytical expressions for the phase path excess and refraction attenuation of the radio wave propagating through the disturbed ionosphere. Analytical model and analysis of the CHAMP RO data indicated that the centers of strong ionospheric influence on RO signals can exist, for example, in the sporadic E-layers inclined by 3-6 degrees relative to the local horizontal direction. In this case one can observe simultaneously with atmospheric RO the appending ROs in the ionospheric layers. Multi- RO effect can be a cause of the ionospheric interference in the communication and RO signals. Multi- RO effect can be used to study the structure of the ionospheric disturbances using the amplitude variations in RO signals. Multi- RO effect allows introducing a classification of the ionospheric influence on RO signals using the amplitude data. This indicates a possibility for separating the regular and random parts in the ionospheric contribution in the RO signals.