The parameters of sporadic $D_s$ layer of electric conductivity caused by ultraenergetic relativistic electron (URE) precipitations are determined due to indirect electromagnetic method. Previously we determined the southern boundaries of these precipitations in the frames of the following supposition: the effect of a normal wave reflection and its conversion into other normal waves on the boundary between disturbed and undisturbed parts of a radio path might have been ignored. Now we show by accurate simulation that it was true for strong and moderate disturbances. For the powerful URE disturbances the effect is significant. In order to obtain this result we had to change a traditional problem statement about a normal wave conversion in the terrestrial waveguide and to solve numerically this new problem with a ${}^{\prime \prime}$volume${}^{\prime \prime}$ inhomogeneity.