Quadric difference frequency (QDF) has high range resolution, but it needs a pair of antennae to transmit two channels of frequencies, which complexes the hardware design and subsequent data processing. Based on the principle of phase comparison, a modified ranging algorithm-once difference frequency (ODF) is proposed. Under the same condition, the performance of ODF is slightly poorer than that of QDF, but ODF only needs to transmit one channel of frequencies, so it lowers the hardware design complexity and production cost, and it can improve the range performance by increasing transmitted frequencies. Finally, simulation results verify the validity of the method