The recovery of gravity field parameters using various heterogeneous data is performed according to the input/output system theory (IOST) method. The combination of different data sets is carried out by the application of a multiple input - multiple output system. The theory of the algorithm is presented and some conclusions on the assumptions made for the data properties are drawn. Comparisons between a combined system and individual uncorrelated systems are made and the proper use of the data sets in each case is discussed. Finally, an application is presented, where input data, such as shipborne gravity anomalies and sea surface heights (SSHs) derived from different satellite missions, are optimally combined in order to estimate marine geoid heights and sea surface topography (SST).