GRACE Gravity Measurement
GRACE will obtain a gravity field map by looking at how the Earth's mass varies from place to place on the surface as the twin satellites pass over. Mass and gravity are positively correlated - that is to say an increase in mass relates to an increase in the gravitational force exerted. Mass is also related to the density and amount of materials located in any one place. For example rock is more dense (i.e., more mass per unit of volume) than water, and water in its liquid form is generally more dense than in its solid form. Also, a very large amount of rock, such as would be found in a large mountain, has more mass than a small amount of rock, such as might be found in a coastal area . Since the Earth has varied topographic features such as mountains, valleys, and underground caverns, the mass is not evenly distributed around the globe and different physical features can be distinguished. The lumps observed in the Earth's gravity field result from this uneven distribution of mass on the Earth's surface and GRACE will map these perturbations with unprecedented accuracy.
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. As the gravitational field changes beneath the satellites - correlating to changes in mass (topography) of the surface beneath - the orbital motion of each satellite is changed. This change in orbital motion causes the distance between the satellites to expand or contract and can be measured using the K-band instrument. From this, the fluctuations in the Earth's gravitational field can be determined.
Here's an example of how it works. The two GRACE satellites are traveling in space, both 500 kilometers above the earth. As the front satellite approaches an area of higher gravity, it will be pulled toward the area of higher gravity and speed up. This increases the distance betweenthe two satellites. As the satellites straddle the area of higher gravity, the front satellite will slow down and the trailing satellite will speed up. As the trailing satellite passes the area of higher gravity, it will slow down and the lead satellite will not be affected. As the satellites move around the Earth, the speeding up and slowing down of the satellites will allow scientists to measure the distance between the two satellites, and, therefore, map the earth's gravity field.