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Abstract. Geometric Form Factor(GFF) or overlap function in transition range is an important factor of a lidar system, especially in a SO 2 and NO 2 DIAL system. An unstable overlap function will seriously affect measurements of gas concentration and degrade its retrieval accuracy in transition range. In technique of SO 2 and NO 2 DIAL, retrieval of gas concentration is directly related with ratio between two return signals of emitting laser beam. Construct stability of overlap between two laser beam and receiver can be estimated using the ratio between two return signals. In this paper data acquired from SO 2 /NO 2 DIAL measurements was analyzed. Relative standard deviation of ratio between two return signals in transition range is less than 1%. It shows that Construct stability of overlap between two laser beam and receiver, and quality of laser beam are very good.