Abstract. The wave packets of atmospheric gravity waves were numerically generated, with a given characteristic wave period, horizontal wave length and projection mean wind along the horizontal wave vector. Their projection phase and group velocities along the oblique radar beam (v_pr and v_gr), with different zenith angle θ and azimuth angle φ, were analyzed by the method of phase- and group-velocity tracing. The results were consistent with the theoretical calculations derived by the dispersion relation, reconfirming the accuracy of the method of analysis. The RTI plot of the numerical wave packets were similar to the striation patterns of the QP echoes from the FAI irregularity region. We propose that the striation range rate of the QP echo is equal to the radial phase velocity v_pr, and the slope of the energy line across the neighboring striations is equal to the radial group velocity v_gr of the wave packet; the horizontal distance between two neighboring striations is equal to the characteristic wave period τ. Then, one can inversely calculate all the properties of the gravity wave responsible for the appearance of the QP echoes. We found that the possibility of some QP echoes being generated by the gravity waves originated from lower altitudes cannot be ruled out.