GRACE and GRACE-FO

GRACE chiefly detected changes in the distribution of water across the planet. Scientists use GRACE data to estimate ocean bottom pressure (the combined weight of the ocean waters and atmosphere), which is as important to oceanographers as atmospheric pressure is to meteorologists.[15] For example, measuring ocean pressure gradients allows scientists to estimate monthly changes in deep ocean currents.[16] The limited resolution of GRACE is acceptable in this research because large ocean currents can also be estimated and verified by an ocean buoy network.[15] Scientists have also detailed improved methods for using GRACE data to describe Earth's gravity field.[17] GRACE data are critical in helping to determine the cause of sea level rise, whether it is the result of mass being added to the ocean – from melting glaciers, for example – or from thermal expansion of warming water or changes in salinity.[18] High-resolution static gravity fields estimated from GRACE data have helped improve the understanding of global ocean circulation. The hills and valleys in the ocean's surface (ocean surface topography) are due to currents and variations in Earth's gravity field. GRACE enables separation of those two effects to better measure ocean currents and their effect on climate.[19]

GRACE data have provided a record of mass loss within the ice sheets of Greenland and Antarctica. Greenland has been found to lose 280±58 Gt of ice per year between 2003 and 2013, while Antarctica has lost 67±44 Gt per year in the same period.[20] These equate to a total of 0.9 mm/yr of sea level rise. GRACE data have also provided insights into regional hydrology inaccessible to other forms of remote sensing: for example, groundwater depletion in India[21] and California.[22] The annual hydrology of the Amazon basin provides an especially strong signal when viewed by GRACE.[23]

A University of California, Irvine-led study published in Water Resources Research on 16 June 2015 used GRACE data between 2003 and 2013 to conclude that 21 of the world's 37 largest aquifers "have exceeded sustainability tipping points and are being depleted" and thirteen of them are "considered significantly distressed." The most over-stressed is the Arabian Aquifer System, upon which more than 60 million people depend for water.[24]

Change in mass of the Greenland and Antarctic ice sheets as measured by GRACE

GRACE also detects changes in the gravity field due to geophysical processes. Glacial isostatic adjustment—the slow rise of land masses once depressed by the weight of ice sheets from the last ice age—is chief among these signals. GIA signals appear as secular trends in gravity field measurements and must be removed to accurately estimate changes in water and ice mass in a region.[25] GRACE is also sensitive to permanent changes in the gravity field due to earthquakes. For instance, GRACE data have been used to analyze the shifts in the Earth's crust caused by the earthquake that created the 2004 Indian Ocean tsunami.[26]

In 2006, a team of researchers led by Ralph von Frese and Laramie Potts used GRACE data to discover the 480-kilometer-wide (300 mi) Wilkes Land crater in Antarctica, which was probably formed about 250 million years ago.[27]

Data from GRACE has improved the current Earth gravitational field model, leading to improvements in the field of geodesy. This improved model has allowed for corrections in the equipotential surface which land elevations are referenced from. This more accurate reference surface allows for more accurate coordinates of latitude and longitude and for less error in the calculation of geodetic satellite orbits.[28]

GRACE is sensitive to regional variations in the mass of the atmosphere and high-frequency variation in ocean bottom pressure. These variations are well understood and are removed from monthly gravity estimates using forecast models to prevent aliasing.[29] Nonetheless, errors in these models do influence GRACE solutions.[30]

GRACE data also contribute to fundamental physics. They have been used to re-analyze data obtained from the LAGEOS experiment to try to measure the relativistic frame-dragging effect.[31][32]

GRACE's key measurement, satellite gravimetry, is not derived from electromagnetic waves. Instead, the mission uses a microwave ranging system to accurately measure changes in the speed and distance between two identical spacecraft flying in a polar orbit about 220 kilometers (140 mi) apart, 500 kilometers (310 mi) above Earth. The ranging system is sensitive enough to detect separation changes as small as 10 micrometers (approximately one-tenth the width of a human hair) over a distance of 220 kilometers.[4] As the twin GRACE satellites circle the globe 15 times a day, they sense minute variations in Earth's gravitational pull. When the first satellite passes over a region of slightly stronger gravity, a gravity anomaly, it is pulled slightly ahead of the trailing satellite. This causes the distance between the satellites to increase. The first spacecraft then passes the anomaly, and slows down again; meanwhile the following spacecraft accelerates, then decelerates over the same point. By measuring the constantly changing distance between the two satellites and combining that data with precise positioning measurements from Global Positioning System (GPS) instruments, scientists can construct a detailed map of Earth's gravity anomalies.

The two satellites (nicknamed "Tom" and "Jerry") constantly maintain a two-way, K-band microwave-ranging link between them. Fine distance measurements are made by comparing frequency shifts of the link. This is made possible due to the onboard Ultra Stable Oscillator (USO) which produces the frequencies for the K-band ranging system.[33] The micrometer-sensitivity of this measurement requires accordingly precise measurements of each spacecraft's position, motion, and orientation to be useful. To remove the effect of external, non-gravitational forces (e.g., drag, solar radiation pressure), the vehicles use sensitive Super STAR electrostatic accelerometers located near their respective centers of mass. GPS receivers are used to establish the precise positions of each satellite along the baseline between the satellites. The satellites use star cameras and magnetometers to establish attitude. The GRACE vehicles also have optical corner reflectors to enable laser ranging from ground stations using the Center of Mass Trim Assembly (MTA) which ensures the center of mass is modified throughout the flight accordingly.[33]

CSR, GFZ, and JPL process observations and ancillary data downloaded from GRACE to produce monthly geopotential models of Earth.[34] These models are distributed as spherical harmonic coefficients with a maximum degree of 60. Degree 90 products are also available. These products have a typical latency of 1–2 months. These geopotential coefficients may be used to compute geoid height, gravity anomalies, and changes in the distribution of mass on Earth's surface.[35] Gridded products estimating changes in mass in units of liquid water equivalent thickness are available at JPL's GRACE Tellus website.