Over its lifetime of about 20 months, GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) will map the global variations in the gravity field with extreme detail and provide a unique model of the Earth's gravity field and its geoid. The geoid is the surface of equal gravitational potential defined by the gravity field and is crucial for improving our understanding of ocean circulation, sea-level change and terrestrial ice dynamics, all of which are affected by climate change. GOCE-derived data is also much needed for use in practical applications such as surveying and levelling.