The influence of a horizontally inhomogeneous ionosphere on time delay of satellite-emitted transionospheric radio signals is considered. Theoretical and numerical time delay calculations are performed using a single parabolic ionospheric density model along the signal radio path. The comparison of this method of calculation of signal time delay in the ionosphere with an approximation one, which does not consider horizontal density gradients is given. It is shown that during sunrise, in the regions of equatorial anomaly and midlatitude trough, the errors of the approximate method can be of the order 20%--40%.