Abstract: Special grouppath Doppler spectra with characteristic shape called a “smilergram” colloquially were obtained in ionospheric oblique sounding experiment. On the basis of simple geometry, the theoretical model of radio wave reflection/dispersion from moving ionospheric irregularities was formed. Based on this model, the grouppath Doppler spectrum was simulated. The simulation results accord with the experiment results very well by changing the model parameters, which shows that the interpretation is reasonable, that is, “smilergram” arises from moving ionospheric irregularities. Then the velocity of moving ionospheric irregularities can be estimated in accordance with the model parameters.