Algorithm Theoretical Basis Document (ATBD) for Lidar Aerosol inversion

The objective of this algorithm is to retrieve the aerosols optical properties (the extinction coefficient α, the backscatter coefficient β and depolarization ratio δ) from the ATLID instrument on board the EarthCARE satellite. Unlike the 1-km scale Cloud and Aerosol extinction, backscatter and depolarization ratio, the algorithm uses the lidar signals directly and does not rely on a priori assumptions on the backscatter-to-extinction ratio. The output of this algorithm will serve as an input to the 1-km scale algorithm.

Retrieving the value of the extinction coefficient is a relatively simple procedure for signals with a sufficient SNR (Signal-to-Noise ratio). This procedure involves the determination of the vertical derivation of the logarithm of the range corrected Rayleigh signal. However, this direct procedure can not be applied directly in the case of the EarthCARE signals. Due to the relatively low SNR associated with the EarthCARE Rayleigh channel, the direct derivation will lead to an amplification of the extinction variance and to an unreliable retrieval of the extinction. It is therefore important to take into account the low expected SNR of the EarthCARE signals. It should be noted that, similar arguments apply to the derivation of the backscatter, however, the extinction retrieval is more sensitive to the SNR of the input lidar signals. Different methods will be used to deal with the noisy nature of the signals.