Introduction

This chapter reviews three sets of missions that do not easily fit into the previous chapters. The first set includes three gravity missions, the Challenging Minisatellite Payload mission (CHAMP) launched in July 2000, the US/German Gravity Recovery and Climate Experiment (GRACE) mission launched in March 2002 and the European Gravity field and steady-state Ocean Circulation Explorer (GOCE) launched in March 2009. The second set includes three altimeter missions used for studies of sea and glacier ice. These are the two NASA laser altimeter missions, the Ice, Cloud, and land Elevation Satellite-1 (ICESat-1) that operated from 2003 to 2009, the ICESat-2 satellite planned for launch in 2017, and the dual-beam radar altimeter on the ESA CryoSat-2 satellite. The third set includes the ESA Soil Moisture and Ocean Salinity (SMOS) and the NASA/Argentinian Aquarius/SAC-D missions used to measure sea surface salinity (SSS), respectively launched in November 2009 and in June 2011.

Gravity missions

There are two reasons for the importance of a detailed knowledge of the Earth's gravity field. First, in the steady state, altimetric retrieval of sea surface height depends on the shape of the ocean geoid and its accompanying gravity field. Second, understanding the time variability of the Earth's gravity field contributes to our knowledge of the global water cycle. This variability includes the loss or gain of mass in the polar ice caps, changes in the mass distribution associated with the oceanic general circulation, redistribution of mass caused by shifts in bottom currents and the effects of runoff, precipitation and evaporation. Before gravity satellites, many different kinds of measurements contributed to the modeling of the geoid. These included combinations of land- and ship-based gravity measurements and data from the altimeter satellites. The launch of gravity satellites presented the first opportunity to derive the geoid from a single set of measurements (GRACE, 2013).