Gravity Recovery and Climate Experiment

The goal of the Gravity Recovery And Climate Experiment (GRACE) space mission is to obtain accurate global and high-resolution determination of both the static and the time-variable components of the Earth's gravity field. GRACE is intended to enable precise measurement of Earth's shifting water masses by detecting their effects on our planet's gravity field, allowing the study of global climatic issues by enabling a better understanding of ocean surface currents and heat transport, measuring changes in sea-floor pressure, watching the mass of the oceans change, and by monitoring changes in the storage of water and snow on the continents. Also data on ocean and deep sea currents as well on tectonics are derived from the data.

Explanation

GRACE maps variations in the Earth's gravity field over its five-year spacetime (extended to eight years in 2005) with its two identical spacecraft flying about 220 kilometers apart in a polar orbit 500 kilometers above the Earth. The twin GRACE satellites were launched from Plesetsk Cosmodrome, Russia on a Rockot (SS-19 + Breeze upper stage) launch vehicle, on March 17, 2002.

As the pair flies over a gravity anomaly, the leading spacecraft approaches and speeds up due to higher angular momentum. This causes the pair to increase separation. The first spacecraft then passes the anomaly, and slows down again; meanwhile the following spacecraft accelerates, then decelerates over the same point. By recording spacecraft separation, gravity is mapped; cycles of growing/shrinking separation indicate the size and strength of anomalies.

Operation

The two satellites (nicknamed "Tom" and "Jerry") constantly maintain a two-way microwave-ranging link between them. Fine distance measurements are made by comparing frequency shifts of the link. As a cross-check, the vehicles measure their own movements using accelerometers. All of this information is then downloaded to ground stations.

To establish baseline positions and fulfil housekeeping functions, the satellites also use star cameras magnetometers, and GPS receivers. The GRACE vehicles also have optical corner reflectors to enable laser ranging from ground stations, bridging the range between spacecraft positions and Doppler ranges.

Spacecraft manufacture

The spacecraft were manufactured by Astrium of Germany, using its "Flexbus" platform. The microwave RF systems, and attitude determination and control system algorithms were provided by Space Systems/Loral. The star cameras used to measure the spacecraft attitude were provided by Technical University of Denmark. The instrument computer along with a highly precise BlackJack GPS receiver and digital signal processing system has been provided by JPL in Pasadena. The highly precise accelerometer that is needed to separate atmospheric and solar wind effects from the gravitation data has been manufactured by ONERA. JPL is responsible for the overall mission management under the NASA ESSP program.

Discoveries

The data so far obtained by GRACE is the most precise gravimetric data yet recorded: it has been used to re-analyse data obtained from the LAGEOS experiment to try to measure the relativistic frame-dragging effect.

In 2006 a team of researchers led by Ralph von Frese and Laramie Potts used GRACE data to discover the 480 km (300 miles) wide Wilkes Land crater in Antarctica, which probably formed about 250 million years ago.cite web| url=http://researchnews.osu.edu/archive/erthboom.htm| title=Big Bang in Antarctica -- Killer Crater Found Under Ice| publisher=Research News| first=Pam Frost| last=Gorder| date=June 1, 2006]

Other deployment

GRACE data has also been used to analyze the shifts in the Earth's crust caused by the earthquake that created the 2004 Indian Ocean tsunami. cite web| url=http://www.nytimes.com/2006/08/08/science/08find.html?_r=1&ex=1312689600&en=86a5d1a41980f607| title=Before the ’04 Tsunami, an Earthquake So Violent It Even Shook Gravity| publisher=New York Times| first=Kenneth| last=Chang| date=August 8, 2006]

References

ee also

* Gravity Field and Steady-State Ocean Circulation Explorer

External links

* [http://www.csr.utexas.edu/grace/ GRACE mission home page (Primary Investigator)]
* [http://www.gfz-potsdam.de/pb1/op/grace/index_GRACE.html GRACE mission home page (co Primary Investigator) (language: German) ]
* [http://solarsystem.nasa.gov/missions/profile.cfm?MCode=GRACE GRACE Mission Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://science.nasa.gov/headlines/y2001/ast30oct_1.htm "Science@NASA" article about GRACE]
* [http://earthobservatory.nasa.gov/Study/WeighingWater "Weighing Earth's Water from Space"] Estimating ground water using GRACE (written for non-scientists)
* [http://news.bbc.co.uk/2/hi/science/nature/3093927.stm Report by BBC showing early results]
* [http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=46378 GPS World Discussion of instrumentation]

"An early version of this article was adapted from public domain text at [http://www.spacelink.msfc.nasa.gov/NASA.Projects/Earth.Science/Atmosphere/GRACE/.index.html] "