About two thirds of the figure of the Earth can be determined by accurately measuring the sea surface. Meanwhile, temporal changes of the sea surface can be determined for three decades, using the combined observation record of various satellite altimetry missions. Besides the long time span, multi-mission approaches drastically improve the spatial and temporal resolution of the data set. In order to combine contemporaneous as well as consecutive altimeter missions in a consistent manner, their data must constantly be updated, harmonized, and cross-calibrated. This way, systematic differences between the missions, such as inter-mission biases, instrumental drifts, and different orbit realizations can be taken into account. Data harmonization and calibration are prerequisites for many other research topics.

Data harmonization is performed by applying identical reduction and correction models, e.g. for ocean tides or atmospheric signal delay, for all missions whenever possible. Calibration requires the determination of systematic radial errors, e.g. through differences in the realization for the origin, range errors, as well as instrumental drifts and corrections. The calibration is based on the redundant observations of the sea surface in crossing points of satellite tracks. Within a global multi-mission crossover analysis (MMXO), the radial errors are estimated from these measurements in an adjustment process. In tis process, variance component estimation is applied to determine an appropriate relative weighting of the different altimeter systems. The crossover analysis results in time series of radial corrections for each mission, whose statistic characteristics can be described by empirical autocovariance functions. Moreover, geographically correlated error patterns can be computed. The task covers the calibration of individual missions, such as Jason-2 (MuMiCCAS) or SARAL/Altika (CASA4OT2), and comprehends investigations regarding past missions (e.g. TOPEX, ERS-1/2, GFO). New missions are included in the analysis as soon as data become available. Furthermore, the MMXO is used to investigate the accuracy and consistency of satellite orbits and their impact on the accuracy of sea level determination.