The Earth Clouds, Aerosol and Radiation Explorer (EarthCARE) is a combined ESA/JAXA mission to be flow in 2013. EarthCARE will the study of the spatial (3D) distribution of clouds and aerosols and their impact on their radiative balance, in the Earth's atmosphere. To do this the EarthCARE platform will carry a combination of active and passive sensors namely:

-A Cloud Profiling (Doppler) Radar CPR (94 GHz, 1 km footprint).

-A High-Spectral-Resolution (HSRL) ATmospheric LIDar ATLID (0.355 nm, 10 m footprint).

-A Multi-Spectral Imager MSI (visible, near-infrared and infrared channels, 0.5 km ground pixel size).

-A three-view Broad Band Radiometer BBR (shortwave and longwave channels, 10 km ground pixel size).

The aims of the EarthCARE mission will be pursued exploiting various synergies from combining two or more of the instruments (lidar+radar, lidar+msi or lidar+radar+msi). Next to the standard synergetic algorithms (level L2b) there will be single instrument algorithms (level L2a) which either produce high level input for the L2b algorithms (e.g. the Lidar Target classification and Radar target classification can be combined to a Lidar+Radar target classification), or produce backup retrievals in the case of an instrument malfunction. In this work, two potential ATLID only algorithms are discussed which have been tested in an ESA sponsored study. The two algorithms in question are the feature mask and the three most important measurements from the lidar, namely, the extinction, backscatter and depolarization ratio algorithms.

In order to be able to derive reliable extinction and backscatter profiles, as well as a target classification an accurate feature mask is essential. A feature mask identifies 'significant return' in the lidar signal. Significant in this sense means that the lidar return is higher than some threshold level, which in turn is based on the noise of the signal. The most desirable type of mask is one that is based on probability distribution functions (PDFs). It is most likely that such a PDF will be discretized into a scale from 0 to 10, with the '-1' added for the undetermined case. The feature mask will be processed on two different scales, the single shot mask value and on the EarthCARE standard output (1km horizontal and 100m vertical). The former is intended to define a cloud fraction for the low resolution mask which can be used in higher order algorithms for radiative transfer calculations.

One key component of the EarthCARE processing system are the algorithms which convert the lidar signals into extinction, backscatter and depolarization. Note that the algorithm for a HSLR lidar is notable different from that used to process data from an elastic backscatter lidar (i.e. CALIPSO). In principle the extinction from a HSRL lidar can be calculated by solving the height derivate of the logarithm of Rayleigh signal: However due to the expected noise levels in the ATLID channels it may be advantageous to formulate a retrieval procedure which simultaneously uses both the Rayleigh and Mie signals in order to optimally combine the accurate but les precise (and applicable) Rayleigh information with the less-accurate but more precise(and applicable) Mie channel information. It is envisioned that a variation approach is best suited for this purpose.