For various purposes, including the calibration of observations, the evaluation of models, data assimilation, or the preparation of inversion algorithms, we need to simulate Level 1 observations collected by the lidar, the radar or the radiometers being part of the A-train and EarthCare. For this purpose, we use forward radiative transfer models that require development and adaptation to the specificities of the instrument (wavelength, resolution, sensitivity etc..), and that need to be designed for specific application (i.e. computation speed for assimilation). In the context of the EECLAT project, we will develop different tools for dedicated application using the A-train or EarthCare. In most cases, these tools are made available to the community when their maturity is sufficient.