In the scope of the QA4EO Project undertaken by ESA, a work package “QA4EO WP 2185 - Support to cloud mask validation for CMIX” had been executed. The objective of this work package had been defined to prepare for establishing and for operating a network of cloud mask validation sites. Based on the experience in Europe (Rome site – La Sapienza University) and the United States of America (Goddard site – Goddard Space Flight Center) a concept for instrumentation and operations had been initially developed and tested. The goal of this work package was to prototype algorithms and methods to process data and compare them with satellite algorithms for cloud masking. A critical requirement is the cloud base. There are two approaches that have been planned to be compared: using stereo sky camera data (US) and a ceilometer (Rome). The Rome site was upgraded and received 2 sky cameras identical to the US site. This WP was designed to deliver a proof-of-concept and a plan for global upscaling. The work package included 4 tasks and delivered a report on the experiences of this experiment and the results of the cloud screening validation at the example of Sentinel 3 OLCI and Sentinel 2 MSI. Additionally, a roadmap towards a global network was developed which is this document. The scope of this document is to document the test operation of the sky camera setup and to provide some initial validation numbers.