The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission is a partnership between NASA and the German Research Centre for Geosciences (GFZ).
GRACE-FO is a successor to the original GRACE mission, which orbited Earth from 2002-2017. GRACE-FO will carry on the extremely successful work of its predecessor while testing a new technology designed to dramatically improve the already remarkable precision of its measurement system.
GRACE-FO, which launched May 22, 2018, will continue the work of tracking Earth’s water movement to monitor changes in underground water storage, the amount of water in large lakes and rivers, soil moisture, ice sheets and glaciers, and sea level caused by the addition of water to the ocean.
How GRACE-FO works?
By measuring gravity anomalies, GRACE showed how mass is distributed around the planet and how it varies over time.
- GRACE-FO’s raw data will be a series of measurements showing how far apart two satellites are from each other. The twin satellites follow each other in orbit around the Earth, separated by about 137 miles (220 km). They will constantly send microwave signals to each other to measure the distance between them.
- As the pair circles the Earth, areas of slightly stronger gravity (greater mass concentration) affect the lead satellite first, pulling it away from the trailiing satellite. As the satellites continue, the trailing satellite is pulled toward the lead satellite as it passes over the gravity anomaly. The change in distance would certainly be imperceptible to our eyes, but the extremely precise microwave ranging system on GRACE-FO is designed to detect minuscule changes in the distance between the satellites.
- Satellite Global Positioning System (GPS) receivers determine the exact position of the satellite over the Earth to within a centimeter or less.
- All this information from the satellites will be used to construct monthly maps of the Earth’s average gravity field, offering details of how mass, in most cases water, is moving around the planet.
Applications of GRACE-FO:
1.Ground water Measurement:
NASA’s GRACE mission provides the first opportunity to directly measure groundwater changes from space. By observing changes in the Earth’s gravity field, scientists can estimate changes in the amount of water stored in a region, which cause changes in gravity. GRACE provides a more than 10 year-long data record for scientific analysis. This makes a huge difference for scientists and water managers who want to understand trends in how our resources are being consumed over the long term.
GRACE has returned data on some of the world’s biggest aquifers and how their water storage is changing Using estimates of changes in snow and surface soil moisture, scientists can calculate an exact change in groundwater in volume over a given time period.
A study by Rodell et al.  in northwest India used terrestrial water storage-change observations from GRACE and simulated soil-water variations from a data-integrating hydrological modeling system to show that groundwater is being depleted at a mean rate of 4.0 cm yr-1 equivalent height of water over the Indian states of Rajasthan, Punjab and Haryana (including Delhi).
During the study period of August 2002 to October 2008, groundwater depletion was equivalent to a net loss of 109 km3 of water, which is double the capacity of India’s largest surface-water reservoir.
The GRACE satellite mission provides a means to observe monthly variations in total water storage within large (>200,000 km2) river basins based on measurements of changes in Earth’s gravity field: when the amount of water stored in a region increases, the gravity signal in that region increases proportionately, and is detected by the GRACE mission with tremendous accuracy. The terrestrial water storage signal defines the time-variable ability of the land to absorb and process water, and accounts for the water beneath the surface.
The weekly availability of soil moisture and groundwater data help built complete picture of drought. The elements of drought characterization typically include drought type, frequency, duration, magnitude (including peak magnitude), severity, and areal extent of drought occurrence. Determining one definition of drought that can be considered comprehensive is complex. Still, there is a need for the development of more accurate identification methods that are able to describe the evolution of drought conditions in space and time.
NASA’s Gravity Recovery and Climate Experiment (GRACE) mission provides monthly, integrated information about water storage variations throughout all components of the surface and subsurface water balance that was previously unobtainable.
Because GRACE measures total water storage variability, a regional GRACE time series can be use to characterize “typical’ variability, and also deviations from typical behavior. These deviations provide a quantitative estimate of essentially how much water would be needed to be added to storage in order to return to “normal” conditions and recover from a drought event