Differential absorption lidar signal averaging

This paper presents experimental results using an atmospheric backscatter dual CO2 laser DIAL. It is shown that DIAL signals can be averaged to obtain an N exp -1/2 dependence decrease in the standard deviation of the ratio of backscattered returns from two lasers, where N is the number of DIAL signals averaged, and that such a lidar system can make measurements of gas concentrations with a precision of 0.7 percent in absorptance over 75 m in a short measurement time when the signal strength is high. Factors that eventually limit the rate of improvement in the SNR, such as changes in the ratio of the absorption and/or backscatter at the two laser frequencies and background noise, are discussed. In addition, it is noted that DIAL measurements made using hard-target backscatter often show departures from N exp -1/2 dependence improvement in the standard deviation because they are further limited by the combined effects of atmospheric turbulence and speckle (since the relative reproducibility of the speckle pattern on the receiver gives rise to correlations of the lidar signals).