GRACE

Weighing the Earth

The Earth's gravity varies slightly from place to place, and changes with time as mass moves: ice melts, groundwater is pumped, water moves through the oceans. The Gravity Recovery and Climate Experiment, GRACE, was launched on 17 March 2002 from Russia on a Rockot, by NASA and the German aerospace centre DLR, with the University of Texas Center for Space Research and the GFZ German research centre. Byron Tapley of the University of Texas was the principal investigator, and Christoph Reigber of GFZ the co-investigator.

The two satellites flew about 220 kilometres apart in the same orbit at an altitude of about 500 kilometres. As the leading satellite passes over a region with extra mass, it is pulled slightly ahead, then the trailing satellite, and the changing separation is measured by a microwave ranging system.

What it found

GRACE measured the loss of mass from the Greenland and Antarctic ice sheets, the decline of aquifers in India, the Middle East and California's Central Valley, and the seasonal shifts of water in the Amazon basin. It gave the first direct measure of how much of sea-level rise comes from land ice and water. The mission was designed for five years and ended in October 2017 after fifteen.

GRACE Follow-On, launched on 22 May 2018, continues the record with a laser ranging interferometer as a technology demonstration.

A time series with a gap

The gap between the two missions, of about a year, is a break in an essential climate record that scientists bridge with models and other data. GRACE also showed that satellite gravity is a unique observation of water on land, and that the record needs to be continued to be useful for water management and climate monitoring.

A single-purpose instrument with many uses

GRACE was designed to measure the gravity field, and its ability to weigh water was a consequence that its designers hoped for but did not fully foresee. The technique has been applied to droughts, to aquifers and to the rates of ice loss in a way that could not have been done with surface measurements. The mission's data must be combined with other information, because gravity cannot say whether a change in mass is ice, water or rock. Understanding the limitation of a single measurement is the basis of sensor fusion.

Instruments and technologies

• K-band microwave ranging A radio link between the satellites measures changes in the distance between them.
• Accelerometer Electrostatic accelerometers on each satellite measure non-gravitational forces, such as air drag, so they can be removed.
• GPS receivers and star cameras Instruments that give each satellite's position and attitude.

The people

• Byron D. Tapley Principal investigator of GRACE at the University of Texas.
• Christoph Reigber Co-investigator at the GFZ German Research Centre for Geosciences.

Timeline

• 2002 Launched on 17 March from Russia on a Rockot.
• 2017 Science mission ends in October after 15 years, far beyond its 5-year design life.
• 2018 GRACE Follow-On launched on 22 May.

Key lessons for observation today

• Measure a change, not a state GRACE measures changes in mass over time. Differential, repeatable measurement can reveal what absolute measurement cannot.
• Bridge the gap between missions The break between GRACE and GRACE-FO is a hole in a record. Continuity planning belongs with the first mission.
• Remove what is not the signal Accelerometers remove drag from the signal. Know your disturbances and measure them.

Sources

• NASA JPL, GRACE mission
• NASA JPL, GRACE mission highlights
• Wikipedia, GRACE and GRACE-FO (secondary)