GRACE 2002: A Scientific Geodesy
Gravity is the invisible force that pulls two masses together. The branch of science dealing with obtaining precise measurements of the Earth, mapping points on the surface, and studying its gravitational field is known as geodesy. Producing a precise model of the fluctuations in gravity over the Earth's surface has proven to be a formidable task. Currently, data from several dozen satellites must be combined to produce a model of Earth's gravitational field. These models do a good job at simulating the large-scale features of Earth's gravitational field but cannot resolve finer-scale features or accurately describe the small month-to-month variations in the gravity field associated with the hydrologic cycle. The unique design of the GRACE mission (twin-satellites flying in formation) is expected to lead to an improvement of several orders of magnitude in these gravity measurements and allow much improved resolution of the broad to finer-scale features of Earth's gravitational field over both land and sea.

The distribution of mass over the Earth is non-uniform. GRACE will determine this uneven mass distribution by measuring changes in Earth's gravity field. The term mass is a way to talk about the amount of a substance in a given space, and is directly correlated to the density of that substance. For example, a container filled with a more dense material, like granite rock, has more mass than that same container filled with, water. Because mass and density are directly related, there is also a direct relationship between density and gravity. An increase in density results in an increase in mass, and an increase in mass results in an increase in the gravitational force exerted by an object. Density fluctuations on the surface of the Earth and in the underlying mantle are thus reflected in variations in the gravity field.

As the GRACE-twins fly in formation over the Earth the precise speed of each satellite and the distance between them is constantly communicated via a microwave K-band ranging instrument. The uniquely designed Superstar Accelerometer on board each spacecraft is used to separate out the effects of non-gravitational forces. As the gravitational field changes beneath the satellites—correlating to changes in the density of the surface beneath—the orbital motion of each satellite is changed. This change in orbital motion causes the distance between the satellites to change infinitesimally and the K-band can detect these changes, with a resolution of 10 µm—the width of a human hair! These data can then be combined with GPS data to produce monthly maps of Earth's gravitational field.

GRACE satellites orbiting over the surface of Earth. A K-band microwave link "connects" the two satellites and makes extremely precise (down to the width of a human hair) measurements of the changes in distance between the two satellites. These fluctuations in distance are caused by changes in the orbital motion of the twin spacecraft as they respond to changes in the density of the surface they are passing over. Density changes correspond directly to changes in the force exerted by gravity and can thus be used to produce a map of the gravitational field.

A summary of the flight configuration and ground support for the GRACE mission is illustrated here. Fluctuations in density of the Earth's surface result in very small changes in the distance between the two satellites, which are measured with very high precision by the K-band ranging system. The S-band relay (shown protruding from the bottom of each satellite) allows for communication with surface tracking stations. The GPS satellites are used as references to determine the precise location of the two satellites in orbit. The precise positioning information they supply will allow for the creation of gravity maps—approximately once per month.

GRACE will do more than just produce a more accurate gravitational field plot, however. The measurements from GRACE have important implications for improving the accuracy of many scientific measurements related to climate change. Improvements to the accuracy of satellite altimetry, synthetic aperture radar interferometry, and digital terrain models covering large land and ice areas—used in remote sensing applications and cartography—are expected to result from the improved gravitational field measurements provided by GRACE. These techniques provide critical input to many scientific models used in oceanography, hydrology, geology and related disciplines and, for this reason the Earth Science community eagerly anticipates the GRACE launch. Among the expected applications:

• tracking water movement on and beneath Earth's surface;
• tracking the movement and changes in ice sheets and changes in global sea level;
• studying ocean currents both near the surface and far beneath the waves; and
• tracking changes in the structure of the solid Earth.

In addition to the primary gravity measurement, the two Global Positioning Satellites (GPS) receivers on GRACE will be used to scan the Earth's limb and determine how much error is introduced into GPS measurements as the GPS signal passes through the Earth's atmosphere. These is done known as occultation, where the GPS receivers on the GRACE satellite track refracted signals from the GPS satellites as they rise or set through the Earth's atmosphere and compare them to a nonocculting GPS satellite. Improvements to the accuracy of GPS measurements expected to result from these measurements will in turn improve the accuracy of soundings of key atmospheric parameters that serve as input into numerical weather prediction models.

The occultation process. The GRACE satellite tracks a GPS satellite as it "rises" and "sets" through the limb of the atmosphere, while a second GPS serves as a reference point.

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Remote Sensing
Introduction
A Scientific Geodesy
Launch, Components, and Systems
Management and Future
Spacecraft Diagrams