Spaceborne lidar calibration from cirrus and molecular backscatter returns

In order to make optimal quantitative use of multi-wavelength spaceborne lidar data it is essential that the lidar be well calibrated. Due to system gain/efficiency changes that can be expected to occur during the course of a shuttle or satellite mission, it is essential to employ a calibration approach that can be implemented on-orbit, preferably repeatable at least a few times per orbit. For wavelengths less than about 550 nm, in situ calibration can be accomplished via normalization to high altitude, nearly molecular scattering regions. However, for longer wavelengths beyond about 800 nm, particularly the popular Nd: YAG fundamental wavelength at 1064 nm, the Rayleigh normalization approach becomes questionable due to both an inherently weaker signal and a stronger, variable and somewhat unknown aerosol scattering contribution. For lidars operating at both longer and shorter wavelengths, a viable approach is to retrieve the longer wavelength calibrations rationed to the shorter wavelength calibrations via comparisons of spectral backscatter from known/quantifiable scatterers. Cirrus clouds are good for this purpose because they occur at high altitudes with significant frequency and provide strong, nearly spectrally flat backscatter. This paper presents the cirrus spectral backscatter ratio calibration approach, including results obtained from case studies of lidar data collected during the LITE shuttle mission. Attention is focused on developing a simple, autonomous approach applicable to satellite lidar missions such as PICASO-CENA.