The main objectives of this study are the definition of a global vertical reference system and the outline of a strategy for its precise realization. Together with the usual physical component (geoid and physical heights), a geometrical component (level ellipsoid and ellipsoidal heights) is also considered. This shall enable the new vertical reference system to support geometric and physical heights as well as their combination world-wide with high-precision. For this puerpose, geometrical and physical reference levels (zero-height surfaces) must be globally realized and the long-term stability of the vertical station positions, including their variations with time, has to be guaranteed. According to this, efforts are concentrating on:

• Determination of a global reference frame (in the same way as the ITRF) and the description of the needed station characteristics to reach the required reliability in the realization of the reference system;
• Estimation (definition and realization) of the geometric and physical reference levels;
• Accurate transformation (connection) of the existing height datums into the global reference level;
• Methodology for the computation of station coordinates (positions and velocities) in agreement with the definitions and conventions outlined within the physical and geometrical components.

Empirical analyses are carried out in the frame of SIRGAS, TIGA, the IAG Inter-Commission Project 1.2 “Vertical Reference Frames”, and GGOS.