GOCE (Gravity field and steady-state Ocean Circulation Explorer mission) is one of the candidates of ESA’s planned explorer programme. The aim of GOCE is to provide a global model of the Earth’s gravity field with high spatial resolution and high accuracy. A combination of different sensors is used to determine the gravity field. The simulation of the gradiometer measurements and the propagation of error sources onto the end-products is performed by the SID consortium which consists of three organisations: The Istitut für Astronomische und Physikalische Geodäsie (IAPG), the Delft Institute for Earth-Oriented Space Research (DEOS) and the Space Research Organization Netherlands (SRON). The goal is to provide a reliable error budget for the GOCE system, identify its weaknesses and limitations and make a complete error analysis of the mission. By a spectral analysis of simulated time series of GPS and Gradiometer data, it is possible to estimate the accuracy and resolution (up to degree and order 250) of the gravity field to be determined by GOCE.