# The Azimuth Project Gravity Recovery and Climate Experiment

## Idea

GRACE is a dedicated satellite mission whose objective is to map the global gravity field every 30 days for 5 years with unprecedented accuracy and with a spatial resolution of 400 km.

Jointly implemented by NASA and DLR (Deutsches Zentrum für Luft- und Raumfahrt) under the NASA Earth System SCience Pathfinder Program in 1997, GRACE was launched on 17 march 2002.

The GRACE mission consists of two identical satellites in near-circular orbits at $\sim 500$ km altitude, seperated from each other by $\sim 220$ km alongtrack and linked by a highly accurate intersatellite $K$-band microwave ranging system. Each satellite carries GPS receivers and attitude sensors and high-precision accelerometers to measure the surface forces. Through the dynamical evolution of the orbits, the intersatellite distance change (which is extracted from phase measurements of the $K$-band signal transmitted between the two satellites) contains implicitly the influence of the globally integrated mass distribution and its movements within the Earth system.

GRACE results of temporal gravity variations are an important additional constraint on the output of hydrological models, as they represent the total vertically integrated effect of mass.

## References

category: earth science