GRACE (Gravity Recovery And Climate Experiment)

GRACE (Gravity Recovery And Climate Experiment)

Spacecraft  Launch  Mission Status  Sensor Complement  References

GRACE is an international cooperative US-German dual-minisatellite SST (Satellite-to-Satellite Tracking) geodetic mission with the overall objective to obtain long-term data with unprecedented accuracy for global (high-resolution) models of the mean and the time-variable components of the Earth's gravity field (a new model of the Earth's gravity field every 30 days for five years). GRACE is also part of NASA's ESSP (Earth System Science Pathfinder) program.

Some science objectives are: ^{1) 2)}

• To enable a better understanding of ocean surface currents and ocean heat transport

• To measure changes in the sea-floor pressure

• To study ocean mass changes

• To measure the mass balance of ice sheets and glaciers

• To monitor changes in the storage of water and snow on the continents

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The mission concept makes use of measurements of the inter-satellite range changes and its derivatives between two co-planar satellites (in low-altitude and polar orbits), using a microwave tracking system. The orbits of the two separately flying S/C are perturbed differently in the Earth's gravity field, leading to inter-satellite range variations. In addition, each S/C carries a GPS receiver of geodetic quality and high-accuracy accelerometers to enable accurate orbit determination, spatial registration of gravity data and the estimation of gravity field models.

The fluctuations in the strength of the Earth's gravity field reflect in turn changes in the distribution of mass in the ocean, atmosphere, and solid Earth, and in the storage of water, snow, and ice on land. Since ocean bottom pressure represents a column integral of the mass of the atmosphere plus the ocean, this measurement technique permits the deduction of ocean bottom pressure changes from space.

GRACE is a collaborative endeavour involving the Center for Space Research (CSR) at the University of Texas, Austin (UTA/CSR); NASA's Jet Propulsion Laboratory, Pasadena, CA; the German Space Agency (DLR) and Germany's National Research Center for Geosciences (GFZ), Potsdam.

Note: A renaming of GFZ took place on June 17, 2008. The new name is: Helmholtz-Zentrum Potsdam GFZ German Research Center for Geosciences. ³⁾

The GRACE mission is led by B. Tapley (PI) of the University of Texas at Austin and by Frank Flechtner (Co-PI) of GFZ (GeoForschungsZentrum), Potsdam. NASA/JPL has led the S/C development in partnership with EADS Astrium GmbH (formerly DASA/DSS, Friedrichshafen) and SS/L (Space Systems/Loral). Astrium has provided major elements of two flight satellites based on the existing CHAMP S/C bus. SS/L provides the attitude control system, microwave instrument electronics and system and environmental testing. DLR/GSOC performs mission operations with tracking stations at Weilheim and Neustrelitz. Science data distribution/processing is managed in a cooperative approach by JPL and UTA/CSR (University of Texas at Austin/Center for Space Research) in the US and GFZ in Germany. Germany provides also the Eurockot launch vehicle.

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Spacecraft

Both S/C structures are of identical design. The shape of each satellite is trapezoidal in cross-section, based on the FLEXBUS design of Astrium (length = 3122 mm, height = 720 mm, bottom width = 1942 mm, top width = 693 mm) The FLEXBUS structure consists of CFRP (Carbon Fiber Reinforced Plastic). This material, with a very low coefficient of thermal expansion, provides the dimensional stability necessary for precise range change measurements between the two spacecraft.

Each Earth-pointing S/C is three-axis stabilized by AOCS (Attitude and Orbit Control System) consisting of sensors, actuators and software.

The sensors include: ⁴⁾

• CESS (Coarse Earth Sun Sensor) for omnidirectional, coarse attitude measurement in the initial acquisition, survival and stand-by modes of the satellite. One CESS sensor is mounted on each of the six sides of the satellite. The resulting Earth vector has an accuracy of ~5-10º, and the sun vector ~3-6º (there is a dependence upon orbit geometry).

• A boom-mounted Förster magnetometer provides additional rate information. Magnetometer measurements of the magnetic field are used in conjunction with the CESS in safe mode and for the commanding of the torque rods in fine-pointing mode.

• The high-precision sensors are SCA (Star Camera Assembly) of ASC heritage (flown on Ørsted), and the BlackJack (GPS Flight Receiver), see description under CHAMP.

• An IMU (Inertial Measurement Unit) an optical gyro providing 3-axis rate information in survival modes.

The actuators include a cold gas system (with 12 attitude control thrusters and two orbit control thrusters, each rated at 40 mN) and three magnetorquers.

Each S/C has a mass of 432 kg (science payload = 40 kg, fuel = 34 kg); the S/C power is 150-210 W (science payload = 75 W). The top and side panels of each S/C are covered with strings of silicon solar cells; NiH batteries with 16 Ah provide power storage. The S/C design life is five years. About 80% of the spacecraft's onboard electronics parts are COTS (Commercial Off-the-Shelf) products.

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Launch

A dual launch on a Eurockot vehicle took place on March 17, 2002, from Plesetsk, Russia. The re-ignitable third stage, BREEZE-KM, was used to place both satellites in the same nominal orbit. Following separation, the leading GRACE satellite began pulling away from the trailing satellite at a relative speed of about 0.5 m/s to assume its nominal position of 220 km ahead of the trailing satellite. At launch, the twin pair of both GRACE spacecraft was immediately nicknamed "Tom and Jerry."

Orbit:

Circular polar co-planar orbit (non-repeat ground track); the initial altitude is 485 km at launch (near a solar maximum), decaying to about 300 km (near a solar minimum) after five years; inclination = 89º. The two satellites in tandem formation are loosely controlled, they are separated at distances between 170 to 270 km apart. GRACE-1 is leading GRACE-2. The onboard cold-gas propulsion system is being used to maintain the separation between 270 km and 170 km. Since the mission launch, orbit maneuvers have been needed about every 50 days to do this.

- The rather low orbital altitude is selected to obtain the best possible gravity measurements (note that the gravity signal of any central body is decaying with the square of the orbital distance from the center of mass) taking into account all decaying (drag) effects.

The spacecraft orbits have a 30-day repeat cycle, and a new gravity field is determined each month. The GRACE system accuracy is sufficient to determine a change in mass equivalent to a volume of water with a depth of 1 cm over a radius of about 400 km.

RF communications:

The TT&C activities are carried out using a pyro-deployed S-band receive and transmit antenna, mounted on a nadir-facing deployable boom. A backup zenith receives antennae and a backup nadir transmit antenna (SZA-Tx), along with the appropriate RF electronics assembly, complete the telemetry and telecommand subsystem. The daily science data volume is about 50 MByte, including gravity data and GPS occultation data. CCSDS protocols are used for all data communication.

The S-band frequencies for the two satellite systems are:

• Downlink: 2211.0 MHz for satellite 1 and 2260.8 MHz for satellite 2. Modulation: BPSK/NRZ is modulated onto the subcarrier which is PM modulated onto the uplink carrier. The data rate is 32 kbit/s for real-time data and 1 Mbit/s for dump data.

• Uplink: 2051.0 MHz for satellite 1 and 2073.5 MHz for satellite 2. Modulation: BPSK/NRZ.

In addition, GFZ installed two automatic payload data acquisition stations on Svalbard (Ny Alesund), one for CHAMP and one for GRACE, to speed up the data processing and distribution chain for the various weather services. The polar location of Svalbard makes it possible to have access to the data on almost all orbits.

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Mission Status

GRACE was launched on 17 March 2002 and completed science observations on 27 Oct. 2017.

• May 13, 2019: When you hear news about ice loss from Greenland or Antarctica, an aquifer in California that is getting depleted, or a new explanation for a wobble in Earth's rotation, you might not realize that all these findings may rely on data from one single mission: the U.S.-German Gravity Recovery and Climate Experiment (GRACE). GRACE data, collected from 2002 to 2017 while the mission was active, are still being used to improve our understanding of water in motion and its sometimes surprising effects on our planet. A new paper brings together newly calculated and existing summaries of the major results GRACE has generated, showcasing the breadth of topics the mission has illuminated over the years. ⁶⁾

- "Water is an important sign of the health of the planet," said Michael Watkins, the original GRACE project scientist and now director of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "But water is hard to track in some forms - for example, polar ice or water stored deep underground. We need to understand those components as well as we understand water in its more easily assessable forms around the globe. That's what GRACE has enabled us to do." Scientists have used this increased knowledge of how water moves and is stored on Earth to understand global climate and how it is changing.

- Byron Tapley, GRACE's original principal investigator and the motivating force behind the mission (now retired from the University of Texas at Austin), is the lead author of the new paper. Titled "Contributions of GRACE to Understanding Climate Change" and published in the journal Nature Climate Change, it summarizes the latest results and new insights GRACE has enabled up to the present (Ref. 8). The review, which covers aspects of the GRACE measurement technique, scientific breakthroughs and the relevance for climate service applications, was written by a distinguished team of GRACE experts. Most authors contributed to the GRACE mission even before it launched and have done groundbreaking work with its data.

How measuring gravity reveals moving water

GRACE and its successor, GRACE Follow-On, were designed to measure changes in gravitational pull that result from changes in mass on Earth. More than 99 per cent of Earth's mean gravitational pull does not change from one month to the next. That's because it comes from the mass of the solid Earth itself - its surface and interior - and that rarely moves, or moves very slowly. Water, on the other hand, moves continually nearly everywhere: Snow falls, ocean currents flow, ice melts and so on. As the twin GRACE satellites orbited Earth, one closely following the other, the changes in mass below changed the distance between the two satellites very slightly. The record of these changes was analyzed to create monthly global maps of changes and redistribution of Earth's mass near the surface.

- "It was a challenge to write a representative eight-page review of GRACE achievements, which have been documented in over 3,000 peer-reviewed publications," said Ingo Sasgen, GRACE scientist at the Alfred Wegener Institute's Helmholtz Center for Polar and Marine Research in Bremerhaven, Germany, who coordinated the new paper. "We wanted to convey how unique the GRACE mission really was and how important its data are for us to understand how climate change affects water stored in the ocean, the ice and on the continents."

• April 16, 2019: On March 17, 2002, the German-US satellite duo GRACE (Gravity Recovery and Climate Experiment) was launched to map the global gravitational field with unprecedented precision. After all, the mission lasted a good 15 years - more than three times as long as expected. When the two satellites burnt up in the Earth's atmosphere at the end of 2017 and the beginning of 2018, respectively, they recorded the Earth's gravitational field and its changes over time in more than 160 months. ⁷⁾

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- This so-called time-resolved satellite gravimetry makes it possible, among other things, to monitor the terrestrial water cycle, the mass balance of ice sheets and glaciers or sea-level change, and thus to better understand the mechanisms of the global climate system, to assess important climatic trends more precisely and to predict possible consequences.

- A review in the journal Nature Climate Change, in which Frank Flechtner, Christoph Reigber, Christoph Dahle and Henryk Dobslaw from the Helmholtz Center Potsdam German Research Center for Geosciences GFZ and Ingo Sasgen from the AWI (Alfred Wegener Institute), Helmholtz Center for Polar and Marine Research participated, now presents highlights in the field of climate research based on GRACE observations. ⁸⁾

Ice Sheets and Glaciers

- GRACE produced the first direct measurement of ice-mass loss from ice sheets and glaciers ever. Previously, it had only been possible to estimate the masses and their changes using indirect methods. Within the first two years of the mission, it was already possible to observe clear signals of ice-mass loss in Greenland and Antarctica. The measured data showed that 60 per cent of the total mass loss is due to enhanced melt production in response to Arctic warming trends, while 40 per cent is due to an increase in ice flow into the ocean. According to GRACE data, between April 2002 and June 2017, Greenland lost about 260 billion tons of ice per year, and Antarctica about 140 billion tons. In addition to long-term trends, the gravity field data also provide evidence of the direct effects of global climate phenomena such as 'El Niño' on ice sheets and glaciers worldwide.

Terrestrial Water Storage

- Among the most impactful contributions of the GRACE mission has been the unveiling of Earth's changing freshwater landscape, which has profound implications for water, food and human security. Global estimates of GRACE trends suggest increasing water storage in high and low latitudes, with decreased storage in mid-latitudes. Though the GRACE record is relatively short, this observation of large-scale changes in the global hydrological cycle has been an important early confirmation of the changes predicted by climate models through the twenty-first century.

- GRACE data also help to analyze and assess the sea level more accurately, as the storage of freshwater on land is linked to the sea level by various mechanisms. Analyses of GRACE data have enabled the first-ever estimates of groundwater storage changes from space. They confirm excessive rates of groundwater depletion from individual aquifers around the world. The data on terrestrial water storage have also contributed to the validation and calibration of various climate models.

Sea-level Change and Ocean Dynamics

- Within this century, sea-level rise could accelerate to 10 mm per year – a rate unprecedented during the past 5000 years and a profound and direct consequence of a warming climate. High-precision sea-level measurements have been available since the early 1990s but they only show the absolute sea-level change. In the 25 years between 1993 and 2017, the sea level rose by an average of 3.1 mm per year. To find out how thermal expansion, melting ice and the continental influx of water each affect sea level, it is necessary to study the water's mass distribution.

- GRACE has shown that 2.5 mm of the average annual sea-level rise of 3.8 mm between 2005 and 2017 is caused by the inflow of water or other mass and 1.1 mm by the thermal expansion of water. Resolving this composition is important for sea-level projections. GRACE data provide a constraint on ocean mass change and thus indirectly on the Earth's energy imbalance, which is a fundamental global metric of climate change. GRACE has shown that most of the warming released by the rise in temperature occurs in the upper 2000 m of the oceans, which are the most important energy sinks of climate change. GRACE also contributes to a better understanding of the dynamics and impact of ocean currents, in particular for the Arctic Ocean.

Climate Service Applications

- The gravity field data of the GRACE satellites help to improve the United States Drought Monitor. This helps US authorities to react to droughts in a timely and sensible manner. With EGSIEM (European Gravity Service for Improved Emergency Management), the European Union has promoted a service designed to identify regional flood risks as early as possible. Between April and June 2017, test runs with historical flood data took place, showing that the wetness indicators for large river basins determined by GRACE can improve forecasts, for example for the Mississippi or the Danube. Current results also show that GRACE data can be used to accurately predict the risk of seasonal wildfires.

- The GFZ operated the GRACE mission together with the German Aerospace Center (DLR) and on the US side with the NASA Jet Propulsion Laboratory (JPL). In May 2018, the follow-up mission GRACE Follow-on (GRACE-FO) was launched. The first monthly gravity field maps should be available to international users by the end of July this year. Unexpected difficulties delayed the submission of the products. "The reason was the failure of a control unit on the second GRACE-FO satellite," explains Frank Flechtner of GFZ. "This made it necessary to switch to the replacement unit installed for such scenarios. But now, with GRACE-FO, a more than two decades long recording of the mass changes in the system Earth is within reach."

• May 16, 2018: In a first-of-its-kind study, scientists have combined an array of NASA satellite observations of Earth with data on human activities to map locations where freshwater is changing around the globe and to determine why. ⁹⁾

- The study, published on 16 May in the journal Nature, finds that Earth's wetland areas are getting wetter and dry areas are getting drier due to a variety of factors, including human water management, climate change and natural cycles. ¹⁰⁾

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- A team led by Matt Rodell of NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, used 14 years of observations from the U.S./German-led GRACE spacecraft mission to track global trends in freshwater in 34 regions around the world (Figure 9). To understand why these trends emerged, they needed to pull in satellite precipitation data from the Global Precipitation Climatology Project, NASA/USGS (U.S. Geological Survey) Landsat imagery, irrigation maps, and published reports of human activities related to agriculture, mining and reservoir operations. Only through analysis of the combined data sets were the scientists able to get a full understanding of the reasons for Earth's freshwater changes as well as the sizes of those trends.

- "This is the first time that we've used observations from multiple satellites in a thorough assessment of how freshwater availability is changing, everywhere on Earth," said Rodell. "A key goal was to distinguish shifts in terrestrial water storage caused by natural variability – wet periods and dry periods associated with El Niño and La Niña, for example – from trends related to climate change or human impacts, like pumping groundwater out of an aquifer faster than it is replenished."

- "What we are witnessing is major hydrologic change," said co-author Jay Famiglietti of NASA/JPL in Pasadena, California. "We see a distinctive pattern of the wet land areas of the world getting wetter – those are the high latitudes and the tropics – and the dry areas in between getting dryer. Embedded within the dry areas we see multiple hotspots resulting from groundwater depletion."

- Famiglietti noted that while water loss in some regions, like the melting ice sheets and alpine glaciers, is clearly driven by a warming climate, it will require more time and data to determine the driving forces behind other patterns of freshwater change. "The pattern of wet-getting-wetter, dry-getting-drier during the rest of the 21st century is predicted by the Intergovernmental Panel on Climate Change models, but we'll need a much longer dataset to be able to definitively say whether climate change is responsible for the emergence of any similar pattern in the GRACE data," he said.

- The twin GRACE satellites, launched in 2002 as a joint mission with DLR (German Aerospace Center), precisely measured the distance between the two spacecraft to detect changes in Earth's gravity field caused by movements of mass on the planet below. Using this method, GRACE tracked monthly variations in terrestrial water storage until its science mission ended in October 2017.

- Groundwater, soil moisture, surface waters, snow and ice are dynamic components of the terrestrial water cycle. Although they are not static on an annual basis (as early water-budget analyses supposed), in the absence of hydroclimatic shifts or substantial anthropogenic stresses they typically remain range-bound. Recent studies have identified locations where TWS (Terrestrial Water Storage) appears to be trending below previous ranges, notably where ice sheets or glaciers are diminishing in response to climate change and where groundwater is being withdrawn at an unsustainable rate.

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- However, the GRACE satellite observations alone couldn't tell Rodell, Famiglietti and their colleagues what was causing the apparent trends. "We examined information on precipitation, agriculture and groundwater pumping to find a possible explanation for the trends estimated from GRACE," said co-author Hiroko Beaudoing of Goddard and the University of Maryland in College Park.

- For instance, although pumping groundwater for agricultural uses is a significant contributor to freshwater depletion throughout the world, groundwater levels are also sensitive to cycles of persistent drought or rainy conditions. Famiglietti noted that such a combination was likely the cause of the significant groundwater depletion observed in California's Central Valley from 2007 to 2015 when decreased groundwater replenishment from rain and snowfall combined with increased pumping for agriculture.

- Southwestern California lost 4 gigatons (equivalent to 4 x 10⁹ m³ or 4 km³) of freshwater per year during the same period. A gigaton of water would fill 400,000 Olympic swimming pools. A majority of California's freshwater comes in the form of rainfall and snow that collect in the Sierra Nevada snowpack and then is managed as it melts into surface waters through a series of reservoirs. When natural cycles led to less precipitation and caused diminished snowpack and surface waters, people relied on groundwater more heavily.

- Downward trends in freshwater seen in Saudi Arabia also reflect agricultural pressures. From 2002 to 2016, the region lost 6.1 gigatons per year of stored groundwater. Imagery from Landsat satellites shows an explosive growth of irrigated farmland in the arid landscape from 1987 to the present, which may explain the increased drawdown.

- The team's analyses also identified large, decade-long trends in terrestrial freshwater storage that do not appear to be directly related to human activities. Natural cycles of high or low rainfall can cause a trend that is unlikely to persist, Rodell said. An example is Africa's western Zambezi basin and Okavango Delta, a vital watering hole for wildlife in northern Botswana. In this region, water storage increased at an average rate of 29 gigatons per year from 2002 to 2016. This wet period during the GRACE mission followed at least two decades of dryness. Rodell believes it is a case of natural variability that occurs over decades in this region of Africa.

- The researchers found that a combination of natural and human pressures can lead to complex scenarios in some regions. Xinjiang province in northwestern China, about the size of Kansas, is bordered by Kazakhstan to the west and the Taklamakan desert to the south and encompasses the central portion of the Tien Shan Mountains. During the first decades of this century, previously undocumented water declines occurred in Xinjiang.

- Rodell and his colleagues pieced together multiple factors to explain the loss of 5.5 gigatons of terrestrial water storage per year in Xinjiang province. Less rainfall was not the culprit. Additions to surface water were also occurring from climate change-induced glacier melt, and the pumping of groundwater out of coal mines. But these additions were more than offset by depletions caused by an increase in water consumption by irrigated cropland and evaporation of river water from the desert floor.

- The successor to GRACE, called GRACE-FO (GRACE Follow-On), a joint mission with the GFZ (German Research Center for Geosciences), currently is at Vandenberg Air Force Base in California undergoing final preparations for launch no earlier than 22 May 2018.

• March 2018: The GRACE-1 satellite reentered the atmosphere on March 10, 2018. ¹¹⁾

• December 21, 2017: NASA scientists conducting research on the connection between fuel moisture and fires have uncovered a paradox: a wet winter corresponds to more small wildfires in the following fire season, not fewer, as is commonly assumed. Large fires behave more "logically," with fewer large fires after a wet winter and more after a dry one. ¹²⁾

- "This is the most surprising result from our study, because we would expect small fires to follow suit with larger fires," said Daniel Jensen, a PhD candidate at UCLA who worked on the project under the direction of scientist J. T. Reager of NASA's Jet Propulsion Laboratory in Pasadena, California. When there is ample moisture for plant growth, Jensen pointed out, "It seems that the buildup of fuel content alone causes there to be more fires — but not necessarily more devastating fires."

- The research is a step toward understanding the role of fuel moisture in wildfires, which could help in determining how severe a fire season may be several months before it arrives. A paper on the research is online in the journal Environmental Research Letters. ¹³⁾

- As anyone who has ever lit a campfire knows, dry fuel catches fire and burns faster than damp fuel. Knowing the moisture of a fuel supply can improve predictions of how fast a wildfire may spread, but measuring it from samples collected in the field is time-consuming and labour-intensive. Remote sensing offers a possible alternative, and earlier studies have shown that soil moisture (the water contained in the soil) correlates well with fuel moisture.

- Jensen and co-authors correlated records of wildfire occurrences across the contiguous United States from 2003 through 2012 with soil moisture measurements from the U.S./German GRACE (Gravity Recovery and Climate Experiment) satellite mission and U.S. Geological Survey data on vegetation and landscape types. They found that although each landscape type varied in average soil moisture and an average number of fires, in every landscape type, the number of small fires increased after a wet pre-season.

- Jensen explained that a wet winter causes grasses and other small plants to grow profusely. These plants dry out and die at the end of the growing season, leaving abundant fuel for a wildfire. Trees and larger shrubs, however, retain more moisture after a wet winter. That might hamper the ability of small fires to grow into large ones in landscapes containing trees.

- To obtain their results, the researchers developed techniques to assimilate GRACE data into a high-resolution U.S. hydrology model called the Catchment Land Surface Model, from NASA's Goddard Space Flight Center in Greenbelt, Maryland, for a product with both accuracy and high resolution. They parcelled each GRACE estimate, which covers a region about 186 miles (300 kilometres) square, into dozens of smaller "boxes" to match the resolution of the model, using data assimilation techniques to refine the "fit" until the results added up correctly to match the GRACE data. Data assimilation, a technique commonly used with weather forecasting models, adds ongoing observational data throughout the course of a simulation to keep a model on track.

- The scientists chose GRACE because of the mission's longevity, said Reager. Other missions such as NASA's SMAP (Soil Moisture Active Passive) satellite offer higher resolution, but none has been in orbit as long as GRACE. "Without that long record, we wouldn't have been able to do the model fitting," Reager said. "Now that we've built the model, we can plug in SMAP data. This methodology will help us get a better look at the ecosystem dynamics of fire activity."

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• October 27, 2017: After more than 15 productive years in orbit, the U.S./German GRACE (Gravity Recovery and Climate Experiment) satellite mission has ended science operations. During their mission, the twin GRACE satellites have provided unprecedented insights into how our planet is changing by tracking the continuous movement of liquid water, ice and the solid Earth. ^{14) 15)}

- GRACE made science measurements by precisely measuring the distance between its twin satellites, GRACE-1 and GRACE-2, which required that both spacecraft and their instruments be fully functional. Following an age-related battery issue on GRACE-2 in September, it became apparent by mid-October that GRACE-2's remaining battery capacity would not be sufficient to operate its science instruments and telemetry transmitter. Consequently, the decision was made to decommission the GRACE-2 satellite and end GRACE's science mission.

- GRACE, a mission led by Principal Investigator Byron Tapley at the University of Texas at Austin, launched in March 2002 on a planned five-year mission to precisely map our planet's ever-changing gravity field. It has revealed how water, ice and solid Earth mass move on or near Earth's surface due to Earth's changing seasons, weather and climate processes, earthquakes and even human activities, such as from the depletion of large aquifers. It did this by sensing minute changes in the gravitational pull caused by local changes in Earth's mass, which are due mostly to changes in how water is constantly being redistributed around our planet.

- "GRACE has provided paradigm-shifting insights into the interactions of our planet's ocean, atmosphere and solid Earth components," said Tapley. "It has advanced our understanding of the contribution of polar ice melt to global sea level rise and the amount of atmospheric heat absorbed by the ocean. Recent applications include monitoring and managing global water resources used for consumption, agriculture and industry; and assessing flood and earthquake hazards."

- GRACE used a microwave ranging system to measure the change in distance between the twin satellites to within a fraction of the diameter of a human hair over 220 km. The ranging data were combined with GPS tracking for timing, star trackers for attitude information, and an accelerometer to account for non-gravitational effects, such as atmospheric drag and solar radiation. From these data, scientists calculated the planet's gravity field monthly and monitored its changes over time.

- "GRACE was an excellent example of a research satellite mission that advanced science and also provided near-term societal benefits," said Michael Freilich, director of NASA's Earth Science Division at the agency's headquarters in Washington. "Using cutting-edge technology to make exquisitely precise distance measurements, GRACE improved our scientific understanding of our complex home planet, while at the same time providing information — such as measurements related to groundwater, drought and aquifer water storage changes worldwide — that was used in the U.S. and internationally to improve the accuracy of environmental monitoring and forecasts."

- GRACE established that measuring the redistribution of mass around Earth is an essential observation for understanding the Earth system. GRACE's monthly maps of regional gravity variations have given scientists new insights into Earth system processes. Among its innovations, GRACE has monitored the loss of ice mass from Earth's ice sheets, improved understanding of the processes responsible for sea level rise and ocean circulation, provided insights into where global groundwater resources may be shrinking or growing and where dry soils are contributing to drought and monitored changes in the solid Earth. Users in more than 100 countries routinely download GRACE data for analyses.

- "GRACE was a pioneering mission that advanced our understanding across the Earth system — land, ocean and ice," said Michael Watkins, director of NASA's Jet Propulsion Laboratory in Pasadena, California, and the mission's original project scientist. "The entire mission team was creative and successful in its truly heroic efforts over the last few years, extending the science return of the mission to help minimize the gap between GRACE and its successor mission, GRACE Follow-On, scheduled to launch in early 2018."

- Despite the loss of one of the twin GRACE satellites, the other satellite, GRACE-1, will continue operating through the end of 2017. "GRACE-1's remaining fuel will be used to complete previously planned maneuvers to calibrate and characterize its accelerometer to improve the final scientific return and insights from the 15-year GRACE record," said GRACE Project Scientist Carmen Boening of JPL.

- Currently, GRACE-2's remaining fuel is being expended and the satellite has begun to slowly deorbit. Atmospheric reentry of GRACE-2 is expected sometime in December or January. Decommissioning and atmospheric reentry of GRACE-1 are expected in early 2018. NASA and the German Space Operations Center will jointly monitor the deorbit and reentry of both satellites.

- GRACE Follow-On, a joint NASA/Helmholtz Center Potsdam German Research Center for Geosciences (GFZ) mission, will continue GRACE's legacy. It will also test a new laser-ranging interferometer developed by a joint German/U.S. collaboration for use in future generations of gravitational research satellites.

- GRACE is a joint NASA/DLR (German Aerospace Center) mission led by Tapley and Co-principal Investigator Frank Flechtner at GFZ. The GRACE ground segment operations are co-funded by GFZ, DLR and the European Space Agency. JPL manages GRACE for NASA's Science Mission Directorate at the agency's headquarters in Washington. GRACE was the first mission launched under NASA's Earth System Science Pathfinder program, designed to develop new measurement technologies for studying the Earth system.

- Responsible for the operation of the two GRACE satellites was DLR/GSOC (German Space Operations Center) in Oberpfaffenhofen with its ground stations in Weilheim and Neustrelitz. "The failure of eight of a total of 20 battery cells on board GRACE-2 has reduced the capacity of the ageing battery to the point where scientific use of the satellite is no longer possible," explained Sebastian Löw. Especially during the eclipse phases when the satellite was without solar radiation, the energy reserves were no longer sufficient to prevent reboots of the onboard computer and associated communications breakdown. The battery is always a critical resource for a mission of this length. GRACE-2's battery started showing the first signs of ageing in 2013, after more than 10 years in orbit.

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• September 22, 2017: From gripping drought and the depletion of reservoirs, to pounding monsoon rains and devastating floods, water woes have become familiar to the people of India in recent years. But new research offers some good news: the water stored naturally underground appears to be rebounding in some parts of India. ¹⁶⁾

- The excessive withdrawal of groundwater from Earth's aquifers is a global issue. But aquifers in some parts of the world are witnessing greater depletion than others. Studies indicate that in 2011, India pumped out 245 billion cubic meters of groundwater for irrigation. This equates to 25 per cent of the total withdrawn globally that year. The present and growing demand for water in India has led groundwater experts to look closely at the region and its water policies.

- In one recent study, scientists used ground and satellite observations to examine decadal trends, which are displayed in the map in Figure 12. It shows groundwater trends across India as observed with data from the GRACE (Gravity Recovery and Climate Experiment) satellites between 2003–2014. Shades of blue indicate areas that have gained water, while browns are where losses were detected.

- The greatest replenishment occurred in the western and southern parts of the country. For some of these areas—such as the Indian states of Gujarat and Andhra Pradesh—the gains mark a significant turnaround from water losses between 1996–2001.

- Interestingly, rainfall does not entirely explain the reversal. In Gujarat, for example, annual rainfall actually decreased during the period of replenishment. Instead, the paper's authors point to policy changes in groundwater management and conservation.

- The researchers, including Soumendra Bhanja of Athabasca University (affiliated with the Indian Institute of Technology in Kharagpur and a visiting scientist at NASA's Goddard Space Flight Center at the time of the study) show that groundwater gains in Gujarat and Andhra Pradesh happened alongside a measured reduction in power use for agriculture. (Traditionally, about 90 per cent of groundwater is removed by wells powered by electricity.) In Andhra Pradesh, farmers have been trained in sustainable groundwater management practices, and there has been a discernible increase in irrigation via surface water. ¹⁷⁾

- Still, not all areas show the same trend. Rapid depletion of groundwater continued throughout the study period in northern and eastern India. "Despite continuing groundwater storage depletion in some parts of the country, sustainable management strategies could help preserve the precious natural resource," Bhanja said.

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• September 14, 2017: With one of its twin satellites almost out of fuel after more than 15 years of chasing each other around our planet to measure Earth's ever-changing gravity field, the operations team for the U.S./German GRACE (Gravity Recovery and Climate Experiment) mission is making plans for an anticipated final science collection. ¹⁸⁾

- On Sept. 3, one of 20 battery cells aboard the GRACE-2 satellite stopped operating due to an age-related issue. It was the eighth battery cell loss on GRACE-2 since the twin satellites that compose the GRACE mission launched in March 2002 on a mission designed to last five years. The following day, contact was lost with GRACE-2.

- On Sept. 8, following numerous attempts, the GRACE mission operations team at NASA/JPL (Jet Propulsion Laboratory) in Pasadena, California; Deutsches Zentrum für Luft- und Raumfahrt (DLR, the German Aerospace Center) in Oberpfaffenhofen, Germany; and the Helmholtz Center GFZ (German Research Center for Geosciences) in Potsdam, Germany, uplinked commands to GRACE-2 to bypass the satellite's flight software system. The procedure restored communications with the spacecraft, allowing the team to regain control. Subsequent analyses revealed that the battery cell lost on Sept. 3 had recovered its full voltage and that GRACE-2 had essentially hibernated during the period of lost contact, consuming no fuel. Following an assessment of the satellite's overall health, the team has determined that GRACE's dual satellite science mission can continue.

- The team has uplinked commands to GRACE-2 to place it in a passive state that will allow it to maintain its current level of fuel. Operational procedures have begun that will extend the GRACE mission to its next science operations phase, which runs from mid-October to early November. During that time, GRACE-2 will be in full Sun, so it will not need to use its batteries.

- The team expects the October/November science data collection to be the mission's last before GRACE-2 runs out of fuel. The additional monthly gravity map produced will help further extend GRACE's data record closer to the launch of GRACE's successor mission, GRACE-Follow-On, scheduled for early 2018.

- As directed by the mission's Joint Steering Group, final decommissioning for both GRACE-1 and GRACE-2 will begin once the dual satellite science phase concludes.

• On March 17, 2017, the GRACE mission is 15 years on orbit - unlocking the mysteries of how water moves around our planet. "Revolutionary" is a word you hear often when people talk about the GRACE mission. Since the twin satellites of the U.S./German Gravity Recovery and Climate Experiment launched on March 17, 2002, their data have transformed scientists' view of how water moves and is stored around the planet. "With GRACE, we effectively created a new field of spaceborne remote sensing: tracking the movement of water via its mass," said Michael Watkins, the original GRACE project scientist and now director of NASA's Jet Propulsion Laboratory, Pasadena, California. ¹⁹⁾

- Like many other transformations, GRACE began with an insight. "The completely new idea about GRACE was the perception that measuring mass gives you a way to probe the Earth system," said Principal Investigator Byron Tapley, director of the Center for Space Research and professor in the Cockrell School of Engineering at UTA (University of Texas at Austin). Measuring changes in mass has been a key to discovering how water and the solid Earth are changing in places humans can't go or can't see.

The Weight of Water

The greater an object's mass, the greater its gravitational pull. For example, the massive Rocky Mountains exert more gravitational pull than the flat plains of the Midwest. Humans don't notice the tiny difference, but satellites do. While orbiting Earth, satellites accelerate very slightly as they approach a massive feature, and slow down as they move away.

- The vast majority of Earth's gravitational pull is due to the mass of Earth's interior. A small part, however, is due to the mass of water on or near Earth's surface. The ocean, rivers, glaciers and underground water change much more rapidly than Earth's interior does, responding to changing seasons, storms, droughts and other weather and climate effects. GRACE grew from the recognition that a specially designed mission could observe these changes in water from space, revealing hidden secrets of the water cycle.

- GRACE measures changes in mass through their effects on twin satellites orbiting one behind the other about 220 km apart. The small accelerations and decelerations caused by changing mass below the spacecraft alter the distance between them very slightly — by a few µm . To measure this ever-changing distance, the spacecraft constantly beam microwave pulses at each other and time the arrival of returning signals. GPS keeps track of where the spacecraft are relative to Earth's surface, and onboard accelerometers record forces on the spacecraft other than gravity, such as atmospheric drag and solar radiation. Scientists process these data to produce monthly maps of regional variations in global gravity, showing how water on or near Earth's surface has moved every month.

- When NASA selected this complex, the high-precision mission for launch under its Earth System Science Pathfinder program, "A lot of people thought it was a bit improbable that we could actually bring it off," Tapley said. He credits the mission's success to a close collaboration between NASA and two German partners, DLR (Deutsches Zentrum für Luft- und Raumfahrt - German Aerospace agency) and the Helmholtz Centre Potsdam German Research Centre for Geosciences (GFZ), with leadership from original co-principal investigator Christoph Reigber and project manager Frank Flechtner of GFZ. The collaboration has run very smoothly, according to Flechtner, who has now taken the role of GRACE's German co-principal investigator. "It's as if we are one family on both sides of the Atlantic."

- GRACE was built in Germany at Airbus Defense and Space. DLR procured a Russian "Rockot" as the launch vehicle. GFZ is involved in the U.S./German Science Data System and mission operations at DLR/GSOC (German Space Operations Center). The GRACE ground segment operations are currently co-funded by GFZ and ESA (European Space Agency). NASA, ESA, GFZ and DLR are supporting the continuation of the measurements of mass redistribution in the Earth system.

What GRACE Has Taught Us

Here are a few highlights of discoveries from GRACE during its 15 years of operation. These discoveries reflect the work of researchers worldwide, who have developed innovative techniques to use the data and combine it with other observations and models for new insights into the Earth system.

Underground water:

Water stored in soil and aquifers below Earth's surface is very sparsely measured worldwide. In describing GRACE's contribution to understanding this life-giving resource, JPL Senior Water Scientist Jay Famiglietti said, "I can't think of another set of measurements that have so revolutionized the science."

- Hydrologist Matt Rodell of NASA/GSFC (Goddard Space Flight Center), Greenbelt, Maryland, did his doctoral research on GRACE's hydrological uses. Rodell said no one guessed before launch that GRACE would reveal unknown groundwater depletion, but over the last decade, he, Famiglietti and other researchers have found more and more locations where humans are pumping out groundwater faster than it is replenished. In 2015, their team published a comprehensive survey showing a third of Earth's largest groundwater basins are being rapidly depleted.

- Dry soils can add to drought risk or increase the length of a drought. Rodell and his team provide GRACE data on deep soil moisture and groundwater to the National Drought Mitigation Center each week, using a hydrology model to calculate how the moisture is changing throughout the month between one map and the next. The data are used in preparing weekly maps of U.S. drought risk.

Melting ice sheets:

Antarctica is one of the world's toughest places to collect data, and Greenland isn't far behind. Yet we need to know how fast these ice sheets are melting to better understand rates and variations of sea level rise around the world. Scientists studying ice sheets and glaciers were among the first to start working with GRACE data to extract the information they needed. In the mid-2000s, Jianli Chen (University of Texas at Austin); Isabella Velicogna (University of California, Irvine); and the late John Wahr showed that ice losses from Greenland and Antarctica were dramatically larger than previously estimated, using estimates of the changing height of the ice sheets and other types of data. Since GRACE launched, its measurements show Greenland has been losing about 280 gigatons of ice per year on average — a bit less than twice the weight of Mt. Everest — and Antarctica has lost slightly under 120 gigatons a year. There are indications that both melt rates are increasing.

Sea level:

The sea level is rising both because melting ice from land is flowing into the ocean and because seawater is expanding as it warms. Scientists have a very precise, continuous measurement of sea level heights worldwide beginning in 1992 with the NASA-French TOPEX-Poseidon mission and continuing through the Jason series of sea level altimetry missions. The altimeter measurements, however, see only the full effect of ocean height changes from all causes — warming, ice melting and runoff from land. To get an in-depth view of the processes responsible for the changes, scientists need to know how much of the full effect is due to each one.

- With GRACE, scientists are able to distinguish between changes in water mass and changes in ocean temperatures. An example of the value of this ability is a study led by GRACE Project Scientist Carmen Boening of JPL, which both documented and explained a significant drop in sea level with the 2011 La Niña event. The study showed that the water that left the ocean, causing the drop in sea level, was rained out over Australia, South America and Asia. The finding gave scientists a new view of the global water cycle.

Solid Earth changes:

The viscous mantle under Earth's crust is also moving ever so slightly in response to mass changes from water near the surface. GRACE has a community of users that calculate these shifts for their research. JPL scientists Surendra Adhikari and Erik Ivins recently used GRACE data to calculate how ice sheet loss and groundwater depletion have actually changed the rotation of Earth as the system adjusts to these movements of mass.

- GRACE's planners didn't have much hope that the mission's measurement could be used to pinpoint the abrupt changes in mass associated with large earthquakes because of the difference in scale: earthquakes are sudden and local, whereas GRACE's monthly maps average over an area the size of Illinois and an entire month of time. However, by devising new data processing and modelling techniques, researchers have found a way to isolate the earthquake effects. "We're able to measure the instantaneous mass shift in an earthquake, and we've found there's a very measurable relaxation that goes on for one or two months after the earthquake," Tapley said. These measurements provide unprecedented insights into what is happening far below the Earth's surface in big quakes such as the 2004 Sumatra event and the 2011 Tohuku (Japan) quake, both of which caused devastating tsunamis.

The Future

At 15 years, GRACE has lasted three times as long as originally planned. Project managers have done everything possible to extend its life, but the spacecraft will run out of fuel soon — probably this summer. NASA and GFZ have been working since 2012 on a second GRACE mission called GRACE-FO (GRACE Follow-On), with Germany again procuring a launch vehicle and the twin satellites built at Airbus DS in Germany. "With GRACE, we have gained new insight into how global and regional water resources are evolving," said Frank Webb, the GRACE-FO project scientist. "Through GRACE-FO, we will extend into the next decade our capacity to gain an accurate picture of the global water cycle."

GRACE-FO is scheduled for launch between December 2017 and February 2018. The new mission focuses on continuing GRACE's successful data record. The new satellites use similar hardware to GRACE and will also carry a technology demonstrator with a new laser-ranging instrument to track the separation distance between the satellites. The laser instrument has the potential to produce an even more accurate measurement. "GRACE-FO allows us to continue the revolutionary legacy of GRACE," said JPL's Watkins. "There are sure to be more unexpected and innovative findings ahead" (Ref. 19).

• December 8, 2016: The twin GRACE satellites are labelled GRACE-1 and GRACE-2 by the Operations Team. The GRACE-1 satellite continues to collect nominal science data as before. The GRACE-2 satellite collects data in reduced circumstances. The accelerometer on GRACE-2 is turned off. The K-band instrument collects inter-satellite ranging data in the sunlight and through partial shadow. All the spacecraft functions are carried out nominally as long as shadows are short. When the shadows grow long, the diminished battery capacity can affect the spacecraft's functions in the shadow. The current mission operations strategy is intended to maximize the chances of safe passage through such a regime. The first indications are that the data collected - using this strategy - during November 2016, can be used to deliver credible science data products. ²⁰⁾

• GRACE mission status in October 2016: The 2016 GRACE Science Team Meeting (GSTM) took place October 5-7, 2016, at the GFZ (GeoForschungszentrum ), in Potsdam, Germany. GRACE with a design mission life of five years has been operating for 14.6 years and has undergone a degradation in operational capability. The 2015 NASA Senior review identified overlap with the GRACE-FO mission as a high-priority objective. ²¹⁾

Mission Lifetime Issues:

- Altitude Decay: Drag estimates predict lifetime to last at least through 2017. Initial altitude: 485 km; current altitude: ~349.3 km (-88 m/d)

- Battery Capacity: Uncertain, but current strategy might allow operation until 2018.

- Propellant for Attitude Control: Available until mid-2018

- Single String Instrument Operations: Degraded science mission options under study.

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Legend to Figure 13: The reentry predictions depend on the accuracy of neutral density models and the accuracy of predictions for planetary geomagnetic index (Ap) and the solar flux (F10.7 cm).

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GRACE Mean Gravity Model 5 (GGM05)

• GGM05S – GRACE-only solution complete to 180x180 (released in December 2013)

- Ten years of RL05 solutions spanning March 2003 to May 2013

- C20 from satellite laser ranging

• GGM05G – GRACE/GOCE combination complete to 240x240 (released June 2015)

- XX, YY, XZ and ZZ processed for the entire mission (Nov 2009 – Oct 2013)

- Polar gap fill = ZZ gradients computed at altitude based on GGM05S

• GGM05C – GRACE/GOCE/DTU13 combination complete to 360x360

- DTU13 anomalies = DTU13 mean sea surface + EGM2008 over land

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Rapid Products from GRACE

Currently, GRACE rapid products are generated using rapid L1B data products. The solution is available within 24 hours of data acquisition.

• Candidate Users

- EGSIEM (European Gravity Service for Improved Emergency Management)

- GSFC North American Drought Monitor

- Regional Water Cycle Studies (e.g. California, Texas, India, etc)

- Flood Assessment

- Earthquake Rapid Assessment.

• Latency

- 24 hours for a Daily Sliding Window solution

- Quick Look Level 2, data products used

- 21-day data batch with epoch at 14 days.

• GPS Occultation Measurements of atmospheric temperature and water vapour

- International Weather Services

- 45-minute latency

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Overlap of GRACE with GRACE-FO is Important

- Primary Objective of the Follow-On Mission is to continue the GRACE Mission's 15-year record of key climate change observations based on high-resolution global models of Earth's gravity field and its variation over time.

- NASA's 2010 Climate Centric Architecture proposed to initiate new space missions to address the continuity of high-priority climate observations.

- Continuation of the time series of GRACE measurements was one of the proposed observation sets and responsibility for extending the GRACE measurement was assigned to GRACE-FO.

- Maintaining climate quality requires that differences in scientific conclusions drawn from the two data sets be resolved and cross-calibration of the data two data sets is a primary requirement.

- Overlap of the two missions enables this calibration process.

- Current GRACE mission activity is focused on maintaining a minimal science measurement capability until the GRACE-FO launch.

The multinational mission operations team—made up of the DLR/GSOC), GFZ, NASA/JPL, and UT/CSR, together with industry support—continues to work to minimize the data gap that may occur before GRACE-FO continues these measurements into the next decade.

The launch of GRACE-FO is expected in early 2018 on a SpaceX mission. Iridium has secured a launch for five more of its next-generation communications craft in a rideshare arrangement with two U.S.-German GRACE-FO research satellites aboard a SpaceX Falcon 9 rocket by early 2018. ²²⁾

A meeting summary report in "The Earth Observer" magazine of NASA entitled "2016 GRACE Science Team Meeting Summary," gave a review of the GSTM in October 2016. In a Multidisciplinary Science Session, a discussion of GRACE-derived interannual hydrospheric and cryospheric mass budgets in modelling Earth's rotation. ²³⁾

The study answered the long-debated topic of a shift in direction of polar motion. While previous work hypothesized that this shift is due to changes in Earth's ice sheet due to increased melt, the current study adds that hydrologic changes play a significant role in explaining the current signature in the data, which exhibits an amplitude of (83 ± 23%) and mean directional shift (within 5.9° ± 7.6°) in polar motion — as illustrated in Figure 18.

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• April 2016: Using satellite data on how water moves around Earth, NASA scientists have solved two mysteries about wobbles in the planet's rotation — one new and one more than a century old. The research may help improve our knowledge of past and future climate. Scientists and navigators have been accurately measuring the true pole and polar motion since 1899 and for almost the entire 20th century they migrated a bit toward Canada. But that has changed with this century and now it's moving toward England, said study lead author Surendra Adhikari at NASA's Jet Propulsion Lab. While scientists say the shift is harmless, it is meaningful. ^{24) 25) 26)}

- The north pole is on the run. Although it can drift as much as 10 meters across a century, sometimes returning to near its origin, it has recently taken a sharp turn to the east. Climate change is the likely culprit, yet scientists are debating how much melting ice or changing rain patterns affect the pole's wanderlust.

- In a paper published in Science Advances, Surendra Adhikari and Erik Ivins of NASA/JPL ( Jet Propulsion Laboratory), Pasadena, California, researched how the movement of water around the world contributes to Earth's rotational wobbles. Earlier studies have pinpointed many connections between processes on Earth's surface or interior and our planet's wandering ways. For example, Earth's mantle is still readjusting to the loss of ice in North America after the last ice age, and the reduced mass beneath that continent pulls the spin axis toward Canada at the rate of a few inches each year. But some motions are still puzzling.

- Around the year 2000, Earth's spin axis took an abrupt turn toward the east and is now drifting almost twice as fast as before, at a rate of almost 17 cm a year. "It's no longer moving toward Hudson Bay, but instead toward the British Isles," said Adhikari. "That's a massive swing." Adhikari and Ivins set out to explain this unexpected change.

- Scientists have suggested that the loss of mass from Greenland and Antarctica's rapidly melting ice sheet could be causing the eastward shift of the spin axis. The JPL scientists assessed this idea using observations from the GRACE (Gravity Recovery and Climate Experiment) satellites — a joint NASA mission with the German Aerospace Center (DLR) and the German Research Center for Geosciences (GFZ), in partnership with the University of Texas at Austin — which provide a monthly record of changes in mass around Earth. Those changes are largely caused by movements of water through everyday processes such as accumulating snowpack and groundwater depletion. They calculated how much mass was involved in water cycling between Earth's land areas and its oceans from 2003 to 2015, and the extent to which the mass losses and gains pulled and pushed on the spin axis.

- Adhikari and Ivins' calculations showed that the changes in Greenland alone do not generate the gigantic amount of energy needed to pull the spin axis as far as it has shifted. In the Southern Hemisphere, ice mass loss from West Antarctica is pulling, and ice mass gain in East Antarctica is pushing, Earth's spin axis in the same direction that Greenland is pulling it from the north, but the combined effect is still not enough to explain the speedup and new direction. Something east of Greenland has to be exerting an additional pull.

- The researchers found the answer in Eurasia. "The bulk of the answer is a deficit of water in Eurasia: the Indian subcontinent and the Caspian Sea area," Adhikari said. The finding was a surprise. This region has lost water mass due to the depletion of aquifers and drought, but the loss is nowhere near as great as the change in the ice sheets.

- So why did the smaller loss have such a strong effect? The researchers say it's because the spin axis is very sensitive to changes occurring around 45 degrees latitude, both north and south. "This is well explained in the theory of rotating objects," Adhikari explained. "That's why changes in the Indian subcontinent, for example, are so important."

- In the process of solving this recent mystery, the researchers unexpectedly came up with a promising new solution to a very old problem, as well. One particular wobble in Earth's rotation has perplexed scientists since observations began in 1899. Every six to 14 years, the spin axis wobbles about 0.5 to 1.5 m either east or west of its general direction of the drift. "Despite tremendous theoretical and modeling efforts, no plausible mechanism has been put forward that could explain this enigmatic oscillation," Adhikari said.

- Lining up a graph of the east-west wobble during the period when GRACE data were available against a graph of changes in continental water storage for the same period, the JPL scientists spotted a startling similarity between the two. Changes in polar ice appeared to have no relationship to the wobble — only changes in water on land. Dry years in Eurasia, for example, corresponded to eastward swings, while wet years corresponded to westward swings.

- When the researchers input the GRACE observations on changes in land water mass from April 2002 to March 2015 into classic physics equations that predict pole positions, they found that the results matched the observed east-west wobble very closely. "This is much more than a simple correlation," coauthor Ivins said. "We have isolated the cause."

- The discovery raises the possibility that the 115-year record of east-west wobbles in Earth's spin axis may, in fact, be a remarkably good record of changes in land water storage. "That could tell us something about past climate — whether the intensity of drought or wetness has amplified over time, and in which locations," said Adhikari.

- "Historical records of polar motion are both globally comprehensive in their sensitivity and extraordinarily accurate," said Ivins. "Our study shows that this legacy data set can be used to leverage vital information about changes in continental water storage and ice sheets over time."

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• As of March 17, 2016, the GRACE mission has been on orbit for 14 years, almost 10 years past the planned mission lifetime- an impressive milestone. The twin satellites have been experiencing degradation in battery performance since 2011. Nevertheless, the twin spacecraft continue to function and collect nominal science data at this time. The GRACE team is carefully managing the satellite propellant and the remaining battery life for both satellites, developing and testing procedures for adjusting loads and for adjusting attitude to minimize solar power production to help facilitate lower battery temperature, should this capability be necessary. Best estimates at present suggest that the effects of atmospheric drag will end the mission sometime between mid-2017 and the first quarter of 2018. It is expected that nominal science data will be collected during the remainder of the mission life. ²⁷⁾

- Current mission operation efforts are focused on extending mission life to allow for overlap with the GRACE Follow-On (GRACE-FO) mission, which is scheduled for launch in late 2017. The multinational mission operations team at GSOC (German Space Operations Center), GFZ, JPL, and UT/CSR, together with industry support, continues to work towards minimizing any data gap that might occur before GRACE-FO continues these measurements into the next decade.

• February 29, 2016: GRACE data from NASA is used to track groundwater in Pakistan. — The farmlands of Pakistan rely on one of the largest continuous irrigation systems in the world. Farmers were once able to depend solely on rivers and man-made canals fed by glaciers and rain. But as population and urbanization boomed in recent decades, the country turned to groundwater to keep up with demand. Today, more than 60 per cent of Pakistan's water is pumped from natural underground reservoirs, with no limits placed on how many wells can be drilled or how much anyone can take. — Now, Pakistan's water managers are looking to NASA satellites to help them more effectively monitor and manage that precious resource, thanks to a partnership with engineers and hydrologists at the University of Washington, Seattle. ²⁸⁾

- After training at the University of Washington, the Pakistan Council of Research in Water Resources in January 2016 began using satellite data from the GRACE ( Gravity Recovery and Climate Experiment) mission to create monthly updates on groundwater storage changes in the Indus River basin. This will allow them to see where groundwater supplies are being depleted and where they are being adequately recharged. Like all NASA satellite data, GRACE data are freely available for download from open NASA data centers (GRACE Tellus and the Physical Oceanography Distributed Active Archive Center) at NASA's Jet Propulsion Laboratory in Pasadena, California.

- "Using data from GRACE, we can indicate the areas that are most threatened by groundwater depletion. We can tell the farmers and water managers and help decision makers formulate better and more sustainable policies," said Naveed Iqbal, an assistant director and hydrogeologist at the Pakistan Council of Research in Water Resources. Iqbal spent six months at the University of Washington learning how to analyze and process the GRACE data to enhance decision-making at his agency.

- Compared to traditional groundwater monitoring efforts, satellite information offers less spatial resolution but huge benefits in terms of cost and efficiency. For example, Pakistani water managers spent eight years building a groundwater monitoring network in the Indus River basin alone, and that network provides readings only twice a year.

- This Pakistan project is a collaboration led by the University of Washington with the University of Houston, Ohio State University, SERVIR and the NASA Applied Sciences Program's Water Resources application area. SERVIR is a joint initiative of NASA and the USAID ( U.S. Agency for International Development) to use the vast amount of data and observations collected by Earth-orbiting satellites for the greater good — for example, to give residents in flood-prone areas early warning before their homes and fields are inundated by floodwaters, predict where mosquito-borne disease outbreaks are likely to occur or monitor soil to grow healthier crops.

- Note:

SERVIR (Regional Visualization and Monitoring System). SERVIR — an acronym meaning "to serve" in Spanish — provides this critical information to help countries assess environmental threats and respond to and assess damage from natural disasters.

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• December 03, 2015: The drought that has been afflicting southeastern Brazil for three years has become the country's worst since the 1920s. Water is rationed in São Paolo and other cities. Crop yields are plummeting. If the drought does not break, energy rationing could follow, as Brazil's hydropower stations struggle to operate. To gain a continent-scale overview of the disaster, Augusto Getirana of NASA/GSFC ( Goddard Space Flight Center) turned to data from the GRACE (Gravity Recovery and Climate Experiment) mission.

The two GRACE spacecraft circle Earth in a low-altitude polar orbit, one trailing the other by ~220 km. Whenever the lead craft flies over, say, a mountain, it feels a slightly increased gravitational tug, which temporarily pulls it farther away from the trailing craft. Measured interferometrically, such fluctuations in separation—positive and negative—are translated into a time-dependent map of Earth's gravity. On seasonal time scales, the fluctuations arise largely from changes in the disposition of the planet's liquid and frozen water.

When Augusto Getirana looked at GRACE maps of Brazil, he could see the seasonal changes in the total amount of water above and below ground. As Figure 21 shows, by 2014 a severe drought had stricken the southeast. Because droughts arise from planet-scale shifts in climate, Getirana's study suggests that gravitational data could help tie those shifts to local shifts in ground and surface water. ²⁹⁾

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• November 9, 2015: GRACE science instrument status. ³⁰⁾

- The accelerometers and the microwave assembly have been turned off since September 27, 2015.

- The GPS and star camera data are being collected.

- beta_prime (β'): -1.6º; altitude: 374 km; separation: 138 km.

• November 2, 2015: A team of NASA and university scientists has developed a new way to use satellite measurements to track changes in Atlantic Ocean currents, which are a driving force in global climate. The finding opens a path to better monitoring and understanding of how ocean circulation is changing and what the changes may mean for future climate. ^{31) 32)}

- In the Atlantic, currents at the ocean surface, such as the Gulf Stream, carry sun-warmed water from the tropics northeastward. As the water moves through colder regions, it sheds its heat. By the time it gets to Greenland, it's so cold and dense that it sinks a couple of miles down into the ocean depths. There it turns and flows back south. This open loop of shallow and deep currents is known to oceanographers as the AMOC (Atlantic Meridional Overturning Circulation) — part of the "conveyor belt" of ocean currents circulating water, heat and nutrients around the globe and affecting climate.

- Because the AMOC moves so much heat, any change in it is likely to be an important indicator of how our planet is responding to warming caused by increasing greenhouse gases. In the last decade, a few isolated measurements have suggested that the AMOC is slowing down and moving less water. Many researchers are expecting the current to weaken as a consequence of global warming, but natural variations may also be involved. To better understand what is going on, scientists would like to have consistent observations over time that cover the entire Atlantic.

- "This [new] satellite approach allows us to improve projections of future changes and — quite literally — get to the bottom of what drives ocean current changes," said Felix Landerer of NASA's Jet Propulsion Laboratory, Pasadena, California, who led the research team.

- Landerer and his colleagues used data from the twin satellites of the GRACE (Gravity Recovery and Climate Experiment) mission. Launched in 2002, GRACE provides a monthly record of tiny changes in Earth's gravitational field, caused by changes in the amount of mass below the satellites. The mass of Earth's land surfaces doesn't change much over the course of a month; but the mass of water on or near Earth's surface does, for example, as ice sheets melt and water is pumped from underground aquifers. GRACE has proven invaluable in tracking these changes.

- At the bottom of the atmosphere — on Earth's surface — changes in air pressure (a measure of the mass of the air) tell us about flowing air or wind. At the bottom of the ocean, changes in pressure tell us about flowing water, or currents. Landerer and his team developed a way to isolate in the GRACE gravity data the signal of tiny pressure differences at the ocean bottom that are caused by changes in the deep ocean currents.

- "We've wanted to observe this phenomenon with GRACE since we launched 13 years ago, but it took us this long to figure out how to squeeze the information out of the data stream," said Michael Watkins, director of the Center for Space Research at the University of Texas at Austin, former GRACE project scientist and a co-author of the study.

- The squeezing process required some very advanced data processing, but not as many data points as one might think. "In principle, you'd think you'd have to measure every 10 yards or so across the ocean to know the whole flow," Landerer explained. "But in fact, if you can measure the farthest eastern and western points very accurately, that's all you need to know how much water is flowing north and south in the entire Atlantic at that section. That theory has long been known and is exploited in buoy networks, but this is the first time we've been able to do it successfully from space."

- The new measurements agreed well with estimates from a network of ocean buoys that span the Atlantic Ocean near 26 degrees north latitude. The agreement gives the researchers confidence that the technique can be expanded to provide estimates throughout the Atlantic. In fact, the GRACE measurements showed that a significant weakening in the overturning circulation, which the buoys recorded in the winter of 2009-10, extended several thousand miles north and south of the buoys' latitude.

- The ocean buoy network, known as RAPID, is operated by the Rapid Climate Change group at the U.K.'s National Oceanography Center, Southampton, together with the University of Miami and the Atlantic Oceanographic and Meteorological Laboratory of the National Oceanic and Atmospheric Administration.

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- Gerard McCarthy, a research scientist in the RAPID group who was not involved with the study, said, "The results highlight synergies between [direct measurements] like [those from] RAPID and remote sensing — all the more important given the rapid and surprising changes occurring in the North Atlantic at the present time." Eric Lindstrom, NASA's Physical Oceanography Program manager at the agency's headquarters in Washington, pointed out, "It's awesome that GRACE can see variations of deep water transport, [but] this signal might never have been detected or verified without the RAPID array. We will continue to need both in situ and spaceborne systems to monitor the subtle but significant variations of the ocean circulation" (Ref. 31).

• October 2015: The GRACE observation data provides a range of applications and interpretations on a global scale. ³³⁾

- Time variations in the gravity field as observed by the GRACE mission provide for the first time quantitative estimates of the terrestrial water storage (TWS) at monthly resolution over more than one decade (2002–2014). The gravity variations that GRACE studies include: changes due to surface and deep currents in the ocean; runoff and groundwater storage on land masses; exchanges between ice sheets or glaciers and the oceans; and variations of mass within the Earth. Another goal of the mission is to create a better profile of the Earth's atmosphere.

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Summary of GRACE data applications with respect to water cycle extremes (Ref. 33)

• GRACE enables an improved understanding of the dynamics and extremes of moisture worldwide.

• Apparent trends in terrestrial water storage during the 13-year GRACE period may reflect the hydroclimatic change, direct human impacts, or natural interannual variability.

• Occurrences of GRACE-period max or min TWS (Terrestrial Water Storage) are consistent with other measures of water cycle extremes (precipitation anomalies; reported droughts and floods; etc.), with few exceptions.

• The value of GRACE data for flood risk and drought monitoring can be enhanced through data assimilation, which improves spatial and temporal resolution, reduces data latency, and enables vertical disaggregation of the TWS observations.

• We now perform gridded GRACE data assimilation within LIS (Land Information System), with 0.125° output resolution over the continental U.S.; 0.25° globally.

• Future directions include drought/flood forecasting and global wetness/drought monitoring.

• Sept. 21-23, 2015: GSTM (GRACE Science Team Meeting) in Austin, Texas. ^{34) 35)}

GRACE mission status in September 2015 (Ref. 34)

Nominal mission: 5 years; 13.5 years in orbit (4936 days); Initial altitude: 500 km; Current altitude: ~382 km (-85 m/d )

Challenge: Operate to 2018 to overlap with GRACE-FO

• Battery Issues:

- After April 2011, active thermal control was stopped to accommodate reduced battery power

- In addition, instruments were shut down for ~40 days during each 160 days β' cycle

- After appropriate processing, there has been no evident degradation in data quality so far

• Mission Lifetime Issues:

- Altitude Decay: Drag estimates predict lifetime to last at least into late 2017.

- Battery Capacity: This is unpredictable, but the current strategy should allow operation until 2018.

- Propellant for Attitude Control: Available until mid-2018

- Single String Instrument Operations: Degraded science mission options under study

• GRACE Gravity Model 5 (GGM05):

- GGM05G – GRACE/GOCE combination complete to 240 x 240 (released June 2015)

GRACE-2 Battery Status:

• Cell failure:

- On 6 July 2015 a third cell in the GRACE-2 battery failed

- Attempts to recover the cell were not successful

- Charge control parameters and low voltage fault protection thresholds were modified for operations at the lower system voltage.

• Current Status:

- The battery and satellite are stable

- The MWA was powered continuously between July 31 and Sept. 9, 2015.

- Full orbit science data was collected through the full sun period or until Sept. 4, 2015.

- Strategies for maximizing the science data collected during the occultation period are being evaluated.

• Expected Impact:

- Science data collection will vary under separate regimes depending on Beta' angle.

- The Beta' angle values that define the proposed regimes will be refined in ongoing analysis.

Summary:

• With improvements in fuel usage, the likelihood that GRACE will achieve continuity with GRACE-FO has improved.

• Aging batteries continue to be a focus for the operations team

• Quality of Science Data Products has been maintained but future solutions may require more effort

• Science and Application User communities continue to grow.

​

​

• In June 2015, the NASA Senior Review extended the GRACE mission through 2019. Operations are focused on extending mission life to overlap with GRACE-FO. Since its launch in 2002, the GRACE mission has produced a series of over 140 global gravity models, providing an unprecedented view of mass redistribution within the Earth system on monthly to inter-annual time scales. These gravity variations result primarily from the transport of water between the oceans, land, cryosphere and atmosphere, making GRACE a unique and important component of NASA's climate measurement capability; it was designated a Climate Mission in the 2010 ESD Climate Initiative. ³⁶⁾

• June 16, 2015: Two new studies led by UCI (University of California, Irvine), using data from the US/German GRACE (Gravity Recovery and Climate Experiment) satellites, show that human consumption is rapidly draining some of its largest groundwater basins, yet there is little to no accurate data about how much water remains in them. The result is that significant segments of Earth's population are consuming groundwater quickly without knowing when it might run out, the researchers conclude. ^{37) 38)}

- The studies are the first to characterize groundwater losses via data from space, using readings generated by the twin GRACE satellites that measure dips and bumps in Earth's gravity, which is affected by the weight of water.

- Groundwater is a finite resource under continuous external pressures. Current unsustainable groundwater use threatens the resilience of aquifer systems and their ability to provide a long-term water source. Groundwater storage is considered to be a factor of groundwater resilience, although the extent to which resilience can be maintained has yet to be explored in depth. In this study, we assess the limit of groundwater resilience in the world's largest groundwater systems with remote sensing observations.

The Total Groundwater Stress (TGS) ratio, defined as the ratio of total storage to the groundwater depletion rate, is used to explore the timescales of depletion in the world's largest aquifer systems and associated groundwater buffer capacity. We find that the current state of knowledge of large-scale groundwater storage has uncertainty ranges across orders of magnitude that severely limit the characterization of resilience in the study aquifers.

- For the first paper, researchers examined the planet's 37 largest aquifers between 2003 and 2013. The eight worst off were classified as overstressed, with nearly no natural replenishment to offset usage. Another five aquifers were found, in descending order, to be extremely or highly stressed, depending upon the level of replenishment in each – still in trouble but with some water flowing back into them.

- The most overburdened are in the world's driest areas, which draw heavily on underground water. Climate change and population growth are expected to intensify the problem.

- The research team – which included co-authors from NASA, the National Center for Atmospheric Research (NCAR), National Taiwan University and UC Santa Barbara – found that the Arabian Aquifer System, an important water source for more than 60 million people, is the most overstressed in the world.

- The Indus Basin aquifer of northwestern India and Pakistan is the second-most overstressed, and the Murzuk-Djado Basin in northern Africa is third. California's Central Valley, utilized heavily for agriculture and suffering rapid depletion, was slightly better off but still labelled highly stressed in the first study.

​

• May 13, 2015: The GRACE operations status depends on the health of the battery and the duration within each orbit when the battery is in use. ³⁹⁾

Science instrument status:

- The accelerometers on both satellites were turned off at UTC 06:36, May 13, 2015.

- The MWAs were turned off at UTC 07:27 (GRACE-1) and 07:05 (GRACE-2) on May 11, 2015.

- The GPS and star camera data are being collected continuously.

• On March 17, 2015, the GRACE mission celebrated 13 years on orbit. Both satellites continue to operate nominally with the exception of the batteries.

• January 19, 2015: The GRACE-A and -B MWA (Microwave Assemblies) and ICU (Accelerometer Instrument Control Units) have been switched on again on: ^{40) 41)}

- 07 January 2015, 12:03 UTC: GRACE-A and GRACE-B ICU

- 13 January 2015, 5:32 UTC: GRACE-A MWA (continuously on since 23:40, before ca. 7-12 minutes off each orbit)

- 13 January 2015, 5:32 UTC: GRACE-B MWA (continuously on since 06:42)

- The GRACE operations status depends on the health of the battery and the duration within each orbit when the battery is in use.

Additionally on 15 December 2014, an orbit maneuver was executed on GRACE-B in order to accelerate the drift towards GRACE-A as the initiation of a satellite switch. The satellite swap intends to save fuel on GRACE-B by staying on the better-performing GRACE-B star camera for the first half of 2015. - During the maneuver the two satellites had a minimum distance of 251 m on 28 December 2014. A drift stop maneuver followed by a 180-degree yaw turn of both satellites on 15 January finished the swap.

• Dec. 16, 2014: Groundwater shortage in California. It will take around 1.5 times the maximum volume of the largest U.S. reservoir — to recover from California's continuing drought, according to a new analysis of satellite data from the GRACE mission. The finding was part of a sobering update on the state's drought made possible by space and airborne measurements and presented by NASA scientists on Dec. 16 at the American Geophysical Union meeting in San Francisco. ^{42) 43)}

- A team of scientists led by Jay Famiglietti of NASA/JPL used data from the GRACE satellites to develop the first-ever calculation of this kind — the volume of water required to end an episode of drought. Earlier this year, at the peak of California's current three-year drought, the team found that water storage in the state's Sacramento and San Joaquin river basins was 11 trillion gallons below normal seasonal levels. Data collected since the launch of GRACE in 2002 shows this deficit has increased steadily.

- GRACE data reveal that, since 2011, the Sacramento and San Joaquin river basins decreased in volume by four trillion gallons of water each year (15 km³). That's more water than California's 38 million residents use each year for domestic and municipal purposes. About two-thirds of the loss is due to the depletion of groundwater beneath California's Central Valley.

- The observatory is providing the first-ever high-resolution observations of snow water volume in the Tuolumne River, Merced, Kings and Lakes basins of the Sierra Nevada and Uncompahgre watershed in the Upper Colorado River Basin. To develop these calculations, the observatory measures how much water is in the snowpack and how much sunlight the snow absorbs, which influences how fast the snow melts. These data enable accurate estimates of how much water will flow out of a basin when the snow melts, which helps guide decisions about reservoir filling and water allocation.

​

• October 2014: The GRACE mission is over 12.5 years on orbit (nominal mission life of 5 years). A total of 137 monthly gravity solutions have been released. GRACE measurements have improved the understanding of the climate system's secular, seasonal and inter-annual signals (Recognized as a Climate Mission) and have contributed to the development of an accurate mean gravity model. ⁴⁴⁾

Mission lifetime issues:

- Altitude Decay: Drag estimates predict lifetime until ~ 2020

- Propellant for Attitude Control is available until late-2016/early-2017

- Battery Capacity: This is unpredictable, but the current strategy looks to allow operation until 2018.

- Single String Instrument Operations: Degraded science mission options under study

- Battery Issues: After April 11, 2011 stopped active thermal control. Instrument shut down during each160 day beta prime cycle: ~40 days lost during each cycle. Requires more complicated data analysis to account for the effects of thermal variations. After appropriate processing, there are no evident degradation in science outcomes.

Orbit status as of Sept. 15, 2014: ⁴⁵⁾

- Initial altitude: 500 km, current altitude ~ 410 km

- Semi-major axis: 6788 km, 410 km above 6378 km

- Altitude decrease: ~ 49 m/day

- Inter-satellite Distance: 220 km (± 50 km)

- Last Satellite Swap Maneuver: 30 June – 28 July 2014

- GRACE is 4,565 days on orbit, 70,000 revolutions completed

- Cold gas resources: GR-1: 9.25 kg (i.e. ~ 3.0 years), GR-2: 10.18 kg (i.e. ~ 4.6 years)

- End of Life (prediction): GR-1 2017/2019 (gas/decay, based on MSFC Nom) GR-2 2017/2019

An overall objective is to increase the chances of overlap with the GRACE Follow-On mission.

Oceanography: 

OceanographyThe oceanography session brought forth major advances due to improvements in data quality enabled by Release-05 gravity field solutions. Locations discussed ranged from the Bay of Bengal to the global oceans, from the Arctic Ocean to the Antarctic Bellingshausen Basin. The topics included both barotropic and baroclinic ocean motions, tides and currents, as well as contributions of ocean circulation to polar motion, and ranged in frequency from semidiurnal (tides), to 30-to-60-day oscillations, to decadal time scales. ⁴⁶⁾

From a global perspective, the important topic of Earth's surface temperature "hiatus" was discussed, showing that surface temperatures over land and ocean have, on average, not increased over the past decade, while energy input to the Earth and greenhouse gases have not changed significantly—a discrepancy of 0.64 W/m2 in the energy balance calculation. Various explanations have been offered for this puzzling observation, most notably that the deep ocean has absorbed this excess heat. This work — using GRACE, altimetry, and Argo float data — demonstrated that within the uncertainties, the upper 2000 m of the ocean has absorbed this "missing" heat (Figure 28).

​

Legend to Figure 28: The estimates are observed variations by satellite altimetry, ocean mass contributions based on GRACE data, and steric sea level based on in situ observations. The dashed black curve shows the indirect steric mean sea-level estimate inferred by removing ocean mass contributions from the observed sea-level time series. Seasonal signals have been removed from all curves, and the curves are offset for clarity. Shaded blue, grey, and pink, where shown, denote one standard deviation of uncertainty in the respective estimates. The agreement between the red (in situ) line and the dashed black (steric mean sea level) curve indicates that the heat absorbed by the ocean is stored in the upper 2000 m of the ocean.

​

The satellite swap (June 30-July 28, 2014) did not affect life expectancy or average fuel consumption for GR-1; slightly better performance by staying on a better SCA (Star Camera Assembly) head.

Fuel expenditure after satellite swap is ~3-4 gr/day on GR-2; this was 12-15 gr/day with the poorer performing SCA head.

​

Overall, the satellite health continues to be excellent, with the exception of the batteries. There have been no failures of loss of redundancy since 2012!

GRACE Science Status (Ref. 44):

- Science Contributions: Sea Level Change; Ocean Heat Storage; Polar Ice Melt and Sea Level; Earth System Mass Transport; Drought and Flooding; Water Availability; Modeling and Assimilation.

GRACE Science Data Status: Status of the RL05 (Release 05 products) Monthly Solutions

- Solutions from January 2003 – June 2014 have been released; 150 Monthly Repeats; 12 Outages through September 2014; 137 Monthly Solutions Released.

​

GGM05 (GRACE Gravity Model 05):

- GGM05S – GRACE-only solution complete to 180 x 180 (released Dec. 2013). Ten years of RL05 solutions spanning March 2003 through May 2013 C20 from satellite laser ranging.

- GGM05G – GRACE/GOCE combination complete to 240x240; >900 days of ZZ, XX, YY and XZ (11/2/2009 – 10/20/2013); Polar gap fill = ZZ gradients computed at altitude based on GGM05S.

- GGM05C – GRACE/GOCE/DTU10 combination complete to 360 x 360 DTU10 anomalies = DTU10 mean sea surface + EGM2008 over land.

​

• September 2013: The GRACE operations status depends on the health of the battery and the duration within each orbit when the battery is in use.

Science instrument status: ⁴⁷⁾

- The mission has resumed full science data collection since UTC 07:50, Sept. 24 2013, after approximately 49 days of outage.

- The K-Band Ranging system was turned-on on both satellites at UTC 07:50, Sep 24 2013.

- The Accelerometers were turned on at UTC 10:35 Sept. 13 2013 (GRACE-1), and at UTC 08:33 Sept. 16, 2013 (GRACE-2).

- The GPS and star camera data has been collected continuously throughout.

• June 2013: The 2013 Senior Review evaluated 13 NASA satellite missions in extended operations: ACRIMSAT, Aqua, Aura, CALIPSO, CloudSat, EO-1, GRACE, Jason-1, OSTM, QuikSCAT, SORCE, Terra, and TRMM. The Senior Review was tasked with reviewing proposals submitted by each mission team for extended operations and funding for FY14-FY15, and FY16-FY17. Since CloudSat, GRACE, QuikSCAT and SORCE have shown evidence of ageing issues, they received baseline funding for extension through 2015. ⁴⁸⁾

• June 2013: Figure 33 shows water storage maps of the USA acquired by the GRACE mission as well as with other satellites and ground-based measurements to model the amount of water stored near the surface and underground as of June 3, 2013. The maps are experimental products funded by NASA's Applied Sciences Program and developed by scientists at NASA's Goddard Space Flight Center and the National Drought Mitigation Center. They represent changes in water storage related to weather, climate, and seasonal patterns. ^{49) 50)}

In 2012, the continental United States suffered through one of its worst droughts in decades. Nearly 80% of the nation's farm, orchard, and grazing land was affected in some way, and 28% experienced extreme to exceptional drought. As another summer arrives in North America, surface water conditions have improved in many places, but the drought has persisted or deepened in others. Underground, the path out of drought is much slower.

The top map of Figure 33 shows the "wetness" or moisture content in the "root zone"—the top meter of soil. The bottom map of Figure 33 shows water storage in shallow aquifers. The current water content is compared to a long-term average for early June between 1948 and 2009. The darkest red regions represent dry conditions that should occur only 2% of the time (about once every 50 years). To see the monthly changes from August 2002 through May 2013, download the animation of Ref. 49).

The root zone map offers perspective on the short-term (weeks to months) water situation; for instance, the passage of a tropical storm can have a distinct impact on root zone moisture. Compared to the summer of 2012, moisture near the surface in June 2013 is significantly better in most of the eastern and northern portions of the continental United States, particularly the Midwestern areas around the Mississippi River. Flooding has instead become the problem in Montana and North Dakota. Portions of Arizona, Nevada, and southeastern California are extremely dry, even by desert standards.

The bottom map of Figure 33 tells more of a long-range story. Groundwater takes months to seep down and recharge aquifers, and that clearly has not happened in the Rocky Mountain states and most of Texas. Underground storage has improved in much of the southeastern and central U.S., though not in Florida. Southern California has a deficit despite promising signs in the winter and spring.

​

• Nov. 2012: The GRACE operations status depends on the health of the battery and the duration within each orbit when the battery is in use. The GRACE mission has experienced battery degradation that requires careful electrical load and battery charging management. ⁵¹⁾

• The data of the GRACE mission represents a great advance for sea level change studies. GRACE has provided the ability to directly observe changes in global ocean mass,⁵²⁾ as well as provide a means of observing water storage changes on land that contribute to sea level changes over a broad range of time scales. Over the last decade, GRACE has provided estimates of ice mass loss in Greenland and Antarctica [10], and glaciers around the world [4], but it has also provided a way for the altimetry community to study more generally how changes in land-water storage affect changes in sea level. In many ways, GRACE is the perfect complement to satellite altimetry, and it is equally important for understanding how much the sea level is changing and why. ⁵³⁾

• Summer 2012: The GRACE mission is extremely successful from a scientific point of view and the originally envisaged duration of 5 years has more than doubled by now. The project is trying to prolong the mission as long as possible to bridge the gap for a planned follow-on mission in the timeframe 2016/17. - Hence, a number of special AOCS operations and analyses have evolved over the years to extend the mission life. This encompasses such obvious measures as the minimization of fuel usage and thruster cycles, but also the continuous optimization of parameter settings and the balancing of several consumables. Close interaction between the science- and operation- teams are required throughout because the satellites themselves are part of the experiment.

The resources on both GRACE satellites are still sufficient to prolong the mission until at least 2016. Extensive parameter adjustments and dedicated operational efforts are used to mitigate the effects of some imbalances that were found to exist in e.g. fuel expenditure or thruster firings. ⁵⁴⁾

• On March 17, 2012, the GRACE twin satellites completed 10 years on orbit. The GRACE measurements are used to produce monthly gravity maps that are more than 100 times more precise than previous models, providing the resolution necessary to characterize how Earth's gravity field varies over time and space, and over land and sea. The data have substantially improved the accuracy of techniques used by oceanographers, hydrologists, glaciologists, geologists and climate scientists.

- GRACE essentially demonstrated a new form of remote sensing for climate research that has turned out even better than the project hoped for. Early on in the design of GRACE, it was realized, that the gravity field could be measured well enough to observe the critical indicators of climate change – sea level rise and polar ice melt. ⁵⁵⁾

- In June 2010, NASA and DLR signed an agreement to continue GRACE through 2015—a full 10 years past the planned mission duration. Recognizing the importance of extending this long-term dataset, NASA has approved the development and launch of the GRACE Follow-On mission, also developed jointly with Germany, and planned for launch in 2017 (Ref. 55).

- The uneven distribution of mass on and within the planet causes, due to the resulting variability of gravity, Earth to have an irregular shape, which deviates significantly from sphericity. Known as the "Potsdam Gravity Potato", the geoid has achieved global notoriety. But this potato shape is equally subject to temporal changes. During the last Ice Age, a mile-thick ice sheet covered North America and Scandinavia. Since the ice melted, the crust, now liberated from its load, continues to rise to this day. This causes material flow in Earth's interior, in the mantle, to replenish. With GRACE, this glacial-isostatic adjustment can for the first time be accurately detected globally as a change in the geoid height: the ice ages continue to have an effect, which is especially evident in North America and Scandinavia. ⁵⁶⁾

​

Legend to Figure 34: "The Geoid 2011" (created on June 28, 2011), the data is based on satellite LAGEOS, GRACE and GOCE and surface data (airborne gravimetry and satellite altimetry).

The improved resolution is partly due to:

- Improved and new methods of satellite measurements SLR (LAGEOS, ERS), GPS (CHAMP), K-band ranging (GRACE), satellite gradiometry (GOCE)

- Increased accuracy in the measurement of surface data (airborne gravimetry and satellite altimetry)

- And of course on the long-term data availability of the GRACE mission.

This new gravity field model is designated EIGEN-6C. Compared to the previous model obtained in 2005, EIGEN-6C has a fourfold increase in spatial resolution. Of particular importance is the inclusion of measurements from the satellite GOCE. Long-term measurement data from the GFZ's twin-satellite mission GRACE were also included in the model. By monitoring climate-based variables like the melting of large glaciers in the polar regions and the amount of seasonal water stored in large river systems, GRACE was able to determine the influence of large-scale temporal changes on the gravitational field.

In total, some 800 million observations went into the computation of the final model which is composed of more than 75,000 parameters representing the global gravitational field. The GOCE satellite alone made 27,000 orbits during its period of service (between March 2009 and November 2013) in order to collect data on the variations in the Earth's gravitational field. ⁵⁸⁾

• The GRACE tandem constellation is operating nominally in February 2012 - completing its 10^th year on orbit (March 17. 2012), which represents double the length of its design life. All instruments are providing measurements with regard to gravity determination and for the profiles of the weather services. — Since 2011, ESA is supporting the GRACE mission within the context of a TPM (Third Party Mission) arrangement. ^{59) 60)}

- The GRACE operations status depends on the health of the battery and the duration within each orbit when the battery is in use. ^{61) 62)}

- In June 2011, the NASA Earth Science Senior Review recommended an extension of the GRACE mission as an augmentation to 2013, and another augmentation to 2015. - Within its mission life, the GRACE mission has provided a synoptic view of large-scale temporal variations of mass distribution within the Earth system, resulting in truly unique constraints on climatically important processes such as mass exchange between ice sheets and the oceans, mass redistribution within the oceans, and large scale variability in precipitation and water availability. The mission is also of operational use, especially through the "aeronomy co-experiment", which is providing radio occultation data for assimilation into atmospheric models, and unique and very valuable data on atmospheric neutral density and thermospheric winds. However, a continuation of the GRACE mission has to be viewed as high risk—the weakened power system may fail, or result in significant degradation of data quality within the next two years. ⁶³⁾

• GRACE flight operations:

GRACE Flight Operations are carried out by a multi-national team from US and Germany. The German Space Operations Center (GSOC), with funding support from DLR and GFZ, operates the satellites from its facilities in Oberpfaffenhofen (near Munich) in Germany. GFZ also uses its antenna at Ny Alesund for satellite monitoring and real-time radio occultation analysis and supports the Deputy Operations Mission Manager. Starting in 2011, ESA is also supporting the ground segment operations at GSOC, in its support of the continuation of measurement of mass redistribution in the Earth System. The operations mission management is from JPL; science operations management is at UTCSR; both of which are funded by NASA. Operations Team members come from JPL, Space Systems/Loral, UTCSR, Astrium and GSOC. ⁶⁴⁾

• The GRACE tandem constellation is operating nominally in 2011 at an orbital altitude of ~ 455 km.

The GRACE Science Operations concept for the remainder of the mission is driven by the intersection of two factors. First is the project decision to operate the spacecraft in a manner that maximizes the remaining lifetime, so that the longest possible climate data record is available from GRACE. The second is the degraded battery capacity that limits the availability of power in certain orbital configurations.

The GRACE orbit plane precesses at -1.117º/day relative to the Sun, such that the Sun is in the orbit plane every 161 days. Due to the power system status and desire for longevity, this event will henceforth define a 161-day work cycle for science operations. As long as the β' angle (angle between the orbit plane and the Earth-Sun line) is greater than 69º, the satellite operates using power only from its solar array. For smaller β' angles, the satellites operate partly using the arrays, and partly using the battery. When β' is near zero (i.e. Sun is in the orbit plane), the battery may be used for as much as 40 minutes out of 90 minutes in each orbit. Near β'=0 events, the mission operations status depends on the battery health and operating environment. ⁶⁵⁾

• In June 2010, NASA and DLR signed an agreement during a bilateral meeting in Berlin, to extend the GRACE mission through the end of its on-orbit life, which is expected in the time frame 2013-2015, depending on solar activity, thruster actuation or battery status. ^{66) 67)}

GRACE's monthly maps are up to 100 times more accurate than existing maps, substantially improving the accuracy of techniques used by oceanographers, hydrologists, glaciologists, geologists and climate scientists.

• The GRACE tandem constellation is operating nominally in February 2010 (> 7 years in orbit). The lifetime of the GRACE mission is predicted through 2013. This would represent a total mission span of 11 years after launch, far exceeding its mission design and requirement. ^{68) 69)}

The GRACE satellite mission has demonstrated significant technological and new scientific achievements. GRACE provides a unique measure of Earth's temporal gravity field, which includes climate-change signals. No other current satellite provides this type of measurement. The scientific achievement is truly cross-disciplinary, covering a broad range of NASA's Earth Science priority areas, including climate change, terrestrial water storage including groundwater variability, cryospheric changes, ocean circulation and sea level, and geodynamics. ⁷⁰⁾

There is also synergy with other missions, including altimetry missions (ICESat, Envisat, Jason-1/-2, CryoSat), ESA's SMOS and NASA's Aquarius and SMAP, and ESA's GOCE missions.

​

​

After launch (March 17, 2002), the S/C commissioning phase was completed on May 14, 2003.

• After the GSTM (GRACE Science Team Meeting), Oct. 13-14, 2005, in Austin, TX, NASA approved a mission extension through 2009. ⁷¹⁾

• Mission accomplishments: Second-generation gravity models are available for the mean-field (GGM02, and EIGEN-CG03C), representing over 40 months of solutions. The orders of magnitude improvement in gravity field determination are invigorating mass balance studies in hydrology, oceanography, glaciology, and in solid Earth sciences. ⁷²⁾

• GRACE data analysis showed that the gravity field of the Earth is variable in both space and time, and is an integral constraint on the mean and time-variable mass distribution in the Earth. From the temporal variations, geo-scientists have already derived new insight into dynamic processes in the Earth's interior, into water mass transfer processes over land and in the oceans and into the development of ice sheets and glaciers on Greenland and Antarctica. With the GRACE mission, for the first time, a systematic and thorough monitoring of the amounts of water, ice and matter moving around is performed and thus a completely new picture of the dynamic processes within and on the Earth emerges.

• The GRACE mission activated routine collection of GPS atmospheric radio occultation data on May 22, 2006

- GRACE-1 (trailing satellite) collects setting occultations

- Only atmospheric occultation (50 Hz) data are being collected

- Software is not able to collect ionospheric occultation (1 Hz) data.

• At the AGU fall meeting in San Francisco NASA and the US Department of the Interior (DOI) presented the coveted William T. Pecora Award to the GRACE mission team; on December 11, 2007.

Switch Maneuver of GRACE Satellites (Dec. 2005)

Since its launch (March 17, 2002), the trailing satellite (GRACE-2) has been flying "forward" with its K-band antenna horn exposed to the impacting atomic oxygen. There is some risk that overexposure to atomic oxygen could lead to a loss of thermal control over the K-band horn, which would affect the accuracy of the KBR signal. To ensure uniform ageing and exposure for the K-band antennas on each of the satellites, the GRACE team has been planning a switch of the two satellites around the middle of the mission so that the trailing satellite would become the lead satellite. During this maneuver the trailing satellite had to cross the path of the leading satellite and take over the lead position. ^{73) 74)}

The GRACE team analyzed the relative motion of each satellite and selected December 10, 2005, as an optimum time to perform the switch maneuver that would allow for a minimum risk of a collision at the point of closest approach (CA). The maneuver was carefully planned so that the two satellites could not get any closer together than 300 m -- they actually never got any closer than 406 m at CA.

The switch was accomplished with only three OTMs (Orbit Thrust Maneuvers). OTM1 took place on December 3, 2005, and the two subsequent maneuvers (OTM2 and OTM3) occurred respectively on December 12, 2005, and January 11, 2006. The maneuver was a success and GRACE-2 is now the leading satellite (Jan. 2006). Figures 37 and 38 provide graphical illustrations of how the range between the two satellites changed during the switch.

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​

Sensor Complement

GRACE does not carry a suite of independent scientific instruments. Instead, the twin GRACE satellites act in unison as the primary science instrument. The K-band ranging system (KBR) can detect instantaneous extremely small changes in the distance between the two satellites and use this information to make gravitational measurements with a level of precision never before possible.

The "science instruments" are mounted on a CFRP (Carbon Fiber Reinforced Plastic) bench in the S/C interior, as are the fuel tanks and the batteries and other satellite subsystems.

SIS (Science Instrument System)

The SIS includes all elements of the inter-satellite ranging system, the GPS receivers required for precision orbit determination and occultation experiments, and associated sensors such as SCA. SIS also coordinates the integration activities of all sensors, assuring their compatibility with each other and the satellite. ⁷⁵⁾

KBR (K/Ka-Band Ranging) Instrument Assembly of NASA/JPL

KBR is the key science instrument of the GRACE mission [Note: KBR is also referred to as HAIRS (High Accuracy Intersatellite Ranging System)]. The objective is ultra-precise satellite-to-satellite tracking (SST) in low-low orbit. The measurement method employed is referred to as DOWR (Dual One Way Ranging). In this approach, each of the two satellites transmits a carrier signal and measures the phase of the carrier generated by the other satellite relative to the signal it is transmitting. The sum of the phases generated is proportional to the range change between the satellites, while the phase variation due to long-term instability in each clock cancels out. ⁷⁶⁾

K-band has a radio frequency of about 24 GHz and Ka-band is near 32 GHz. The GRACE K- and Ka-band frequencies are in an exact 3-to-4 ratio on each satellite. The KBR system can measure the range (with a bias) to the µm level.

Variations in the gravity field cause the range between the two satellites to vary. The relative range is measured by KBR (a microwave link which is integrated with a GPS receiver). The measured range variations are corrected for non-gravitational effects by an accelerometer called SuperSTAR. KBR consists of the following elements: USO (Ultra Stable Oscillator), the MWA (Microwave Assembly), the horn, and IPU (Instrument Processing Unit). The IPU and the SPU (Signal Processing Unit) constitute the heart of the instrument system. ^{77) 78)}

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Legend to Figure 39: The IPU, SPU, KBR and ACC are internally redundant, and the ultra-stable oscillator (USO) is redundant.

USO (of JHU/APL) serves as the frequency reference. The microwave assembly, or sampler, is used for up-converting the reference frequency to 24 and 32 GHz; down-converting the received phase from the other satellite; and for amplifying and mixing the received and the reference carrier phase. The horn is used to transmit and receive the carrier phase between the satellites.

- The IPU is used for sampling and digital signal processing of not only the K-Band carrier phase signal, but also the signals received by the GPS antenna and the star cameras. Each satellite transmits carrier phase to the other at two frequencies, allowing for ionospheric corrections. The transmit and receive frequencies are offset from each other by 0.5 MHz in the 24 GHz channel, and by 0.67 MHz in the 32 GHz channel. This shifts the down-converted signal away from DC, enabling more accurate measurements of the phase. The 10 Hz samples of phase change at the two frequencies are downlinked from each satellite, where the appropriately decimated linear combination of the sum of the phase measurements at each frequency gives an ionosphere-corrected measurement of the range change between the satellites.

​

SuperSTAR (Super Space Three-axis Accelerometer for Research mission)

SuperSTAR is an accelerometer developed by ONERA/CNES, France (of STAR heritage on CHAMP, with a resolution a factor 10 higher than that on CHAMP). ⁸⁰⁾ The objective of SuperSTAR is the measurement of all non-gravitational accelerations (drag, solar and Earth radiation pressure) acting on the GRACE spacecraft. The measurement principle of the SuperSTAR accelerometer is based on the electrostatic suspension of a parallel-epipedic proof mass inside a cage. The cage walls are equipped with control electrodes which serve both as capacitive sensors to derive the instantaneous proof mass (PM) position and as actuators to apply electrostatic forces in order to keep the PM motionless in the center of the cage.

The configuration of the two SuperSTAR accelerometers is quasi-identical to STAR and takes advantage of the CHAMP mission experience.

The improvement of the performances with respect to STAR comes mainly from the increased gap between the proof-mass and the sensitive axes electrodes: 175 µm instead of 75 µm in the CHAMP model and also of the modification of electronics function parameters as for example le reduction of the bias reference voltage by a factor 2, a better adjustment of the measurement conditioning amplifiers and an optimized exploitation of the 24-bit sigma-delta analogue to digital converters. ⁸¹⁾

​

SuperSTAR is mounted at the CG (Center of Gravity) of the satellite. SuperSTAR consists of the following elements: SU (Sensor Unit, EEU (Electromagnetic Exciting Unit), ICU (Interface Control Unit), and a harness. SU consists of a metallic proof mass, suspended inside an electrode cage of gold-coated silica. The proof mass motion is servo-controlled using capacitive sensors, and is a measure of the non-gravitational accelerations acting on the satellite. The mass and electrode cage core is enclosed by a sole plate and a housing in which vacuum is maintained using a getter. The SU vacuum unit is surrounded by analog electronics. The EEU is used to deliver a 10 mg acceleration, and is used only in case of an SU start-up problem. The ICU supplies power to the SU and EEU, and operates the accelerometer through a micro-controller board.

SCA (Star Camera Assembly)

SCA is of CHAMP heritage. The objective is the precise measurement of satellite attitude. SCA consists actually of two DTU (Technical University of Denmark) star camera assemblies (2 cameras with sensor heads), each with a FOV of 18º x 16º and one DPU (Data Processing Unit). Both assemblies are rigidly attached to the accelerometer, and view the sky at a 45º angle with respect to the zenith, on the port and starboard sides. The SCA is used for both: science as well as AOCS; the two assemblies provide the primary precise attitude determination for each satellite. The baffles are used to avoid the degradation due to solar heating. SCA measures the S/C attitude to an accuracy of < 0.3 mrad (with a goal of 0.1 mrad) by autonomous detection of star constellations using an onboard star catalog.

​

LRA (Laser Corner-cube Reflector Assembly)

LRA is provided by GFZ (also referred to as LRR (Laser Retro-Reflector). LRA is mounted on the underside of the spacecraft to permit orbit verification from terrestrial laser tracking networks. The direct distance can be measured with an accuracy of 1-2 cm (depending on the technological status of the measuring ground station). The LRA data are being used for:

• POD (Precise Orbit Determination) in combination with GPS tracking data for gravity field recovery

• Calibration of the onboard GPS space receiver (BlackJack)

• Technology experiments such as two-color ranging (this involves differential ranging to eliminate tropospheric signal effects).

​

BlackJack (GPS Flight Receiver)

BlackJack is a new generation instrument of TRSR (TurboRogue Space Receiver) heritage, provided by JPL (see description under CHAMP). The objective is to use the GPS instrument for navigation (precise orbit determination) and radio-occultation (refractive occultation monitoring) applications. BlackJack features three antennas, the main zenith crossed dipole antenna is used to collect the navigation data. In addition, a backup crossed dipole antenna and one helix antenna on the aft panel are used for back-up navigation and atmospheric occultation data collection, respectively. This system is capable of simultaneously tracking up to 24 dual frequency signals. In addition, this system provides digital signal processing functions for the KBR and SCA instruments as well.

​

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80) Note: STAR and SuperSTAR are of ASTRE (Accéléromètre Spatial Triaxial Electrostatique) heritage, built by ONERA. ASTRE was part of the ESA Microgravity Measurement Assembly (MMA), and flown on STS-55 (Apr. 26 - May 6, 1993), STS-83 (Apr. 4-8, 1997) and on STS-94 (Jul. 1-17, 1997)

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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (eoportal@symbios.space).

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