ICARUS project proposes an innovative solution to the challenge of the Common Altitude Reference inside VLL airspaces with the definition of a new U-space service and its validation in a real operational environment. In manned aviation, the methods of determining the altitude of an aircraft are based on pressure altitude difference measurements (e.g. QFE, QNH and FL) referred to a common datum. The UA flights superimpose a new challenge, since a small drone may take off and land almost from everywhere, hence reducing the original significance of QFE settings, introduced on behalf of manned pilots to display on the altimeter the 0-height at touchdown on the local runway. In fact, the possibility for n drones to take off at n different places would generate a series of n different QFE corresponding to different heights of ground pressures referred to the take-off “Home points”. Therefore for a large number drones, new methodologies and procedures shall be put in place. The ICARUS defines a new U-space U3 service tightly coupled with the interface of the existing U-space services (e.g. Tracking, and Flight Planning services). The users of ICARUS service shall be remote pilots competent to fly in BVLOS in the specific category of UAS operations and ultralight GA pilots potentially sharing the same VLL airspace. The ICARUS proposed approach foresees the realization of DTM service embedded in an Application Program Interface (API) that can be queried by UAS pilot/operator (or by drone itself) based on the actual positioning of the UA along its trajectory, computed by the (E)GNSS receiver. The output of the DTM service would provide information on distance from ground/obstacles in combination with the common altitude reference. Accuracy, continuity, integrity and availability requirements for GNSS-based altimetry together with accuracy and resolution requirements of the DTM to be provided by ICARUS service are key topics of the study.