Compensating differential albedo in topographic lidar through the use of three or more lidar wavelengths

One of the limits to the sensitivity of differential absorption lidar (DIAL) when using a topographic reflector is the spectral variation of the reflectance of the topography (differential albedo). This is especially a problem when DIAL is attempted from a mobile platform that views changing background scenes. Recent advances in technology allow one to generate laser radiation that is tunable over broad spectral ranges. We show that the differential albedo problem can be largely corrected by judicious selection of a set of at least three lidar wavelengths to detect and measure each single species of interest. We show that the degree of correction which can be obtained depends on the joint spectral properties of the reflectance of the background and of the species absorption coefficient. We show that use of a simple polynomial model for the background reflectance provides detection sensitivities at the part per million-meter level for hydrocarbon species in the 3 micron region. We propose that the multi-wavelength technique can also be used to determine changes in background absorption when that background absorption is not small.