The exact measurement of the sea surface by satellite altimetry determines about 2/3 of the figure of the earth and it supplies high resolution gravity field information. In addition, the multi-observed long time series of almost 20 years permits to describe the changes in sea level on almost all space and time scales. In order to use contemporaneous as well as consecutive altimeter missions together (e.g. in COTAGA, GEOTOP, and RegGRAV) their data sets must be consistent, i.e. they have to be constantly updated, harmonized and calibrated.

The harmonization is performed by applying uniform correction models for all missions whenever possible (e.g. for ocean tides or atmospheric corrections). In addition, the calibration requires the determination of radial errors. These reveal systematic differences in the realization for the origin, range errors, as well as instrumental corrections and drifts. For the calibration the high redundancy is used, which is reached by the double measurements of the sea surface in all crossing tracks. Thus, radial errors can be estimated within an adjustment process by a global multi-mission crossover analysis (MMXO). It includes a variance component estimation in order to estimate the relative weighting of the different altimeter systems. The crossover analysis supplies complete time series of radial errors for each mission, whose statistic characteristics can be described by empirical autocovariance functions. Moreover, geographically correlated error pattern can be computed.

The task covers individual projects for the calibration of ENVISAT, Jason-1/-2 (MuMiCCAS), CryoSat-2 (AO 4415), and SARAL/Altika (CASA4OT2) and contains also investigations regarding past missions TOPEX, ERS-1/2, GFO and ICESat.