The Gravity Recovery and Climate Experiment measures changes of the mass distribution on Earth. The twin satellites travel in the same polar orbit 500 km above the Earth, with one satellite leading the other by approximately 220 km (Figure). When the lead satellite passes over a region of relatively high mass, it will accelerate because of increased gravitational attraction and will increase the distance between the satellites. On the other side of the region of high mass, it will slow again. The same effect applies to the trailing satellite. By monitoring the changing distances between the satellites, and knowing their positions in space accurately via GPS and star cameras, the distribution of mass below the satellites can be determined. Mass redistributions of the Earth are manifested in temporal gravity signals with a monthly sampling and spatial resolution longer than 300–400 km (half-wavelength; Tapley et al., 2004). GRACE data can be used to measure changes in mass of the ocean and its land reservoirs (e.g., land ice and groundwater; see Chapter 3). Launched in 2002, the mission is expected to end in 2015.
FIGURE An artist’s concept of GRACE satellites with ranging link between the two craft. SOURCE: National Aeronautics and Space Administration.
ment Report, with long-term (50–100 years) rates of about 1.8 mm yr-1 estimated from tide gages, and recent (post-1990) rates of about 3.2 mm yr-1 estimated from satellite altimetry and tide gages. The higher rates of recent sea-level rise may reflect interannual and longer variations due to ENSO and other climate patterns. Increases of 3–4 times the current rate would be required to realize scenarios of 1 m sea-level rise by 2100. Such an acceleration has not yet been detected.