GRACE (Gravity Recovery And Climate Experiment)
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
Figure 1: Top view of the GRACE spacecraft (image credit: GFZ Potsdam)
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 ocean, this measurement technique permits the deduction of ocean bottom pressure changes from space.
GRACE is a collaborative endeavor 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.
Figure 2: Bottom view of GRACE (image credit: GFZ Potsdam)
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 omni-directional, coarse attitude measurement in the initial acquisition, survival and stand-by modes of the satellite. One CESS sensor is mounted on each each of the six sides of the satellite. The resulting Earth vector has an accuracy of ~5-10º, 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 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 on-board electronics parts are COTS (Commercial Off-the-Shelf) products.
Figure 3: Internal view of GRACE (image credit: GFZ Potsdam)
Figure 4: Block diagram of the GRACE instruments and flight systems (image credit: GFZ)
Launch: A dual-launch on an 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 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 depth 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 receive 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 system 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.
Figure 5: Illustration of the flight configuration and ground support for the GRACE mission (image credit: NASA, UTA/CSR) 5)
GRACE mission status:
• 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. 6)
- 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. 7)
- 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, 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 on 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 modeling 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 Earth's surface in big quakes such as the 2004 Sumatra event and 2011 Tohuku (Japan) quake, both of which caused devastating tsunamis.
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. 7).
• December 8, 2016: The twin GRACE satellites are labeled 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 functions in the shadow. The current mission operations strategy is intended to maximize the chances of safe passage through such regime. First indications are that the data collected - using this strategy - during November 2016, can be used to deliver credible science data products. 8)
• 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 an overlap with the GRACE-FO mission as a high priority objective. 9)
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.
Figure 6: GRACE-1 mission lifetime predictions and decay scenario as of August 13, 2015 (image credit: GFZ Potsdam, F.-H. Massmann)
Legend to Figure 6: The reentry predictions depend on accuracy of neutral density models and accuracy of predictions for planetary geomagnetic index (Ap) and the solar flux (F10.7 cm).
Figure 7: Over 14.5 Years in Orbit – 155 Global Gravity Solutions (image credit: GFZ, UT/CSR)
Table 1: GRACE Mean Gravity Model 5 (GGM05)
Figure 8: DTU 13 gravity anomalies (image credit: GFZ, UT/CSR)
Figure 9: Satellite-only models (GGM05S and G), image credit: GFZ, UT/CSR
Table 2: Rapid Products from GRACE
Figure 10: National Drought Index (image credit: NASA, Ref. 9)
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 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 science 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. 10)
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 modeling Earth's rotation. 11)
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 11.
Figure 11: The above two figures depict the relationship between continental water mass and the east-west wobble in Earth's spin axis. Losses of water in Eurasia from 2005-2011 correspond to eastward swings in the general direction of the spin axis [top], whereas water gains in Eurasia from 2012-2015 push the spin axis westward [bottom]. The red regions experienced a mass gain while the blue regions experienced mass loss during the corresponding time frames (image credit: Surendra Adhikari, JPL)
• 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. 12) 13) 14)
- 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 on 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 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 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."
Figure 12: Before about 2000, Earth's spin axis was drifting toward Canada (green arrow, left globe). JPL scientists calculated the effect of changes in water mass in different regions (center globe) in pulling the direction of drift eastward and speeding the rate (right globe), image credit: NASA/JPL-Caltech
• 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 the 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. 15)
- 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 percent 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. 16)
- 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, the 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 ini tiative 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 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.
Figure 13: Pakistan water managers used NASA GRACE satellite data to produce this map of monthly groundwater changes in the Indus River Basin. Orange and yellow indicates areas where groundwater might be depleted, while blue and green highlights areas where groundwater is being replenished (image credit: Pakistan Council of Research in Water Resources)
• 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 14 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. 17)
Figure 14: GRACE gravity maps for Brazil over the years 2012-2014 (image credit: Physics Today)
• November 9, 2015: GRACE science instrument status. 18)
- 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. 19) 20)
- 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.
Figure 15: In this artist's rendition, the GRACE satellites are measuring the Atlantic Ocean bottom pressure as an indicator of deep ocean current speed. In 2009, this pattern of above-average (blue) and below-average (red) seafloor pressure revealed a temporary slowing of the deep currents (image credit: NASA/JPL, Caltech)
- 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. 19).
• October 2015: The GRACE observation data provides a range of applications and interpretation on a global scale. 21)
- 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 ground water 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.
Figure 16: Range of terrestrial water storage, 2002-2014. Period maximum minus period minimum TWS observed by GRACE, in cm (image credit: NASA/GSFC)
Table 3: Summary of GRACE data applications with resppect to water cycle extremes (Ref. 21)
Table 4: GRACE mission status in September 2015 (Ref. 22)
Figure 17: GRACE-1 mission lifetime predictions and decay scenario as of August 13, 2015 (image credit: NASA/JPL, GFZ Potsdam, Ref. 23)
Figure 18: GRACE satellite relative distance since mission start (image credit: NASA/JPL, GFZ Potsdam, Ref. 23)
• In June 2015, the NASA Senior Review extended the GRACE mission through 2019. Operations are focused on extending mission life for overlap with GRACE-FO. Since 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 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. 24)
• 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. 25) 26)
- 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 to 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 labeled highly stressed in the first study.
Figure 19: UC Irvine researchers used data from the GRACE satellites to show aquifer depletion worldwide. They are trying to raise awareness about the lack of information about remaining groundwater supplies on Earth (image credit: UC Irvine, NASA)
• 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. 27)
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.
- 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 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 Dec. 16 at the American Geophysical Union meeting in San Francisco. 30) 31)
- 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 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 decision about reservoir filling and water allocation.
Figure 20: The GRACE data reveal the severity of California's drought on water resources across the state. This map shows the trend in water storage between September 2011 and September 2014 (image credit: NASA/JPL)
• 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. 32)
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 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: 33)
- 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 an 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 baro-clinic ocean motions, tides and currents, as well as con-tributions of ocean circulation to polar motion, and ranged in frequency from semidiurnal (tides), to 30-to-60-day oscillations, to decadal time scales. 34)
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 21).
Figure 21: GRACE, in combination with sea surface height estimates from altimetry and ocean heat content from Argo buoys, helps to quantify potential contributions from deep ocean variability to global sea level change (image credit: William Llovel [JPL] et al.)
Legend to Figure 21: 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, gray, and pink, where shown, denotes 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.
Figure 22: GRACE-1 mission lifetime predictions and decay scenario (image credit: NASA/JPL, GFZ Potsdam)
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 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.
Figure 23: GRACE satellite relative distance since mission start (image credit: NASA/JPL, GFZ Potsdam)
Table 5: Satellite status as of fall 2014
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. 32):
- 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.
Figure 24: GRACE-1, over 12 years on orbit - 135 gravity solutions (image credit: GFZ, UTA/CSR, NASA/JPL)
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.
Figure 25: Ten-year (March 2003 to April 2013) combination to degree/order 180, of GRACE monthly estimates (no Kaula constraint), image credit: GFZ, UTA/CSR, NASA/JPL, DLR, Ref. 32)
• 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: 35)
- 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 aging issues, they received baseline funding for extension through 2015. 36)
• June 2013: Figure 26 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. 37) 38)
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 drought has persisted or deepened in others. Underground, the path out of drought is much slower.
The top map of Figure 26 shows the "wetness" or moisture content in the "root zone"—the top meter of soil. The bottom map of Figure 26 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. 37).
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 26 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.
Figure 26: Water storage maps of the USA - the top map was acquired on Aug. 5, 2012, the bottom map was acquired on June 3, 2013 (image credit: NASA)
• 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. 39)
• 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,40) 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 , and glaciers around the world , 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 sea level is changing and why. 41)
• 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 is 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. 42)
• 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 thanthe 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. 43)
- 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. 43).
- The uneven distribution of mass on and within the planet causes, due 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. 44)
Legend to Figure 27: "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. 46)
• The GRACE tandem constellation is operating nominally in February 2012 - completing its 10th year on orbit (March 17. 2012), which represents double the length of its design life. All instruments are providing measurements with regard to the 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. 47) 48)
- In June 2011, the NASA Earth Science Senior Review recommended an extension of the GRACE mission as 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, continuation of the GRACE mission has to be viewed as high risk—the weakened power system may fail, or result in significantly degradation of data quality within the next two years. 51)
• 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 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. 52)
• 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 spacecrafts 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 the 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 161day 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 a 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. 53)
• 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 actuations or battery status. 54) 55)
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. 56) 57)
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. 58)
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.
Figure 28: GRACE mission status as of December 2008
Figure 29: GRACE-1 decay scenario prediction as of Nov. 2008 (image credit: NASA/JPL,DLR, CSR, GFZ)
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, Austin, TX, NASA approved a mission extension through 2009. 59)
• 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 is invigorating mass balance studies in hydrology, oceanography, glaciology, and in the solid Earth sciences. 60)
• 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 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; December 11, 2007.
Switch maneuver of GRACE satellites (Dec. 2005):
Since 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 aging 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. 61) 62)
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 30 and 31 provide graphical illustrations of how the range between the two satellites changed during the switch.
Figure 30: History of relative distance between the GRACE satellites during the switch (image credit: UTA/CSR)
Figure 31: Scalar distance between GRACE-1 and GRACE-2 around the CA event on Dec. 10, 2005 (image credit: UTA/CSR)
Table 6: Highlights of the timeline during switch maneuver
Sensor/payload complement of the co-orbiting mission
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. 63)
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. 64)
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. 65) 66)
Figure 32: A schematic drawing of the GRACE instrument system (image credit: NASA/JPL)
Legend to Figure 32: 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.
Figure 33: Block diagram of the dual one-way ranging system (image credit: NASA, Korea Aerospace University) 67)
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). 68) 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 analog to digital converters. 69)
Figure 34: SuperSTAR accelerometer with the sensor unit (right) and the ICU (left), image credit: ONERA
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.
Figure 35: Illustration of the SCA sensor heads and DPU (image credit: DTU)
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).
Figure 36: Illustration of the LRR (image credit: GFZ Potsdam)
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.
Figure 37: View of the Blackjack GPS receiver during integration (image credit: JPL)
1) C. W. Hughes, C. Wunsch, V. Zlotnicki, "Satellite Peers through the Oceans from Space," EOS Transmissions of AGU, Vol. 81, No. 7, Feb. 15, 2000, p. 68
3) "A new name for the GFZ: Helmholtz Centre Potsdam – GFZ German Research Centre for Geosciences," Helmholtz Association, June 17, 2008, URL: http://www.helmholtz.de/en/news/press_and_news/press
4) J. Herman, D. Presti, A. Codazzi, C. Belle, "Attitude Control for GRACE: The First Low-Flying Satellite Formation," 18th International Symposium on Space Flight Dynamics, Munich, Germany, Oct. 11-15, 2004
5) Byron Tapley, Sandra Zaragoza, Carol Rasmussen,"GRACE — Gravity Recovery and Climate Experiment," NASA, March 2, 2011, URL: http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=186
6) "GRACE Making Plans for Final Science Data Collection,"NASA, Sept. 14, 2017, URL: https://www.nasa.gov/feature/jpl/grace
7) Alan Buis, "GRACE Mission: 15 Years of Watching Water on Earth," NASA/JPL, Feature 2017-070, March 15, 2017, URL: https://grace.jpl.nasa.gov/news/89/grace
9) Byron Tapley, Frank Flechtner, Carmen Boening, Srinivas Bettadpur, "GRACE Mission: Status and Prospects," Proceedings of the GSTM 2016, Potsdam, Germany, October 5-7, 2017, URL: http://www.gfz-potsdam.de/en/section/global-geomonitoring
10) Stephen Clark, "Iridium to share Falcon 9 launch with NASA-German gravity satellites," Spaceflight Now, Feb. 6, 2017, URL: https://spaceflightnow.com/2017/02/06/iridium
11) Carmen Boening," 2016 GRACE Science Team Meeting Summary," The Earth Observer, January - February 2017. Volume 29, Issue 1, pp: 40-42, URL: https://eospso.nasa.gov/sites/default/
12) Alan Buis, Carol Rasmussen, "NASA Study Solves Two Mysteries About Wobbling Earth," NASA/JPL News, April 8, 2016, URL: http://www.jpl.nasa.gov/news/news.php?release=2016-100
13) Surendra Adhikari, Erik R. Ivins, "Climate-driven polar motion: 2003-2015," Science Advances, April 8, 2016, Vol. 2, No 4, e1501693, DOI: 10.1126/sciadv..1501693, URL: http://advances.sciencemag.org/content/2/4/e1501693.full
14) Shannon Hall, "Earth Is Tipping Because of Climate Change - Melting ice and shifting rain patterns are causing the north and south poles to drift," Scientific American, April 8, 2016, URL: http://www.scientificamerican.com/article
15) Steve Platnick, "Editor's Corner," NASA, The Earth Observer. March - April 2016. Volume 28, Issue 2, p. 2, URL: http://eospso.nasa.gov/sites/default
17) Charles Day, "Weighing Brazil's drought from space - A pair of gravity-sensing spacecraft are tracking changes in the country's water," Physics Today, Dec. 3, 2015, URL: http://scitation.aip.org/content/aip/magazine/physicstoday
18) "Mission Operations Status (Updated: 2015-Nov-09)," URL: http://www.csr.utexas.edu/
19) Alan Buis, Carol Rasmussen, "NASA Finds New Way to Track Ocean Currents from Space," NASA/JPL, Nov. 2, 2015, URL: http://www.jpl.nasa.gov/news/news.php?
20) Felix W. Landerer, David N. Wiese, Katrin Bentel, Carmen Boening, Michael M. Watkins, "North Atlantic meridional overturning circulation variations from GRACE ocean bottom pressure anomalies," Geophysical Research Letters, Volume 42, Issue 19, 16 October 2015, pp: 8114–8121, DOI: 10.1002/2015GL065730
21) Matt Rodell, Bailing Li, Sujay Kumar, Hiroko Beaudoing, Ben Zaitchik, James Famiglietti, "Observing Water Cycle Extremes with GRACE," Earth Observation for Water Cycle Science 2015, Frascati, Italy, 20-23 October, 2015, URL: http://congrexprojects.com/custom
22) Byron Tapley, Frank Flechtner, Michael Watkins, Srinivas Bettadpur, "GRACE Mission: Status and Prospects," GRACE Science Team Meeting, Austin Texas, USA, Sept. 21-23,2015, URL: http://www.csr.utexas.edu/grace/GSTM/2015/proceedings.html
23) Mona Witkowski, Franz-Heinrich Massmann, "Status GRACE Mission Operations," GRACE Science Team Meeting, Austin Texas, USA, Sept. 21-23,2015, URL: http://www.csr.utexas.edu/grace/GSTM/2015/proceedings.html
24) "NASA Earth Science Senior Review 2015 submitted to Michael Freilich," June 22, 2015, URL: http://science.nasa.gov/media/medialibrary
25) "A third of the world's biggest groundwater basins are in distress," UCI News, June 16, 2015, URL: http://news.uci.edu/press-releases
26) Alexandra S. Richey, Brian F. Thomas, Min-Hui Lo, James S. Famiglietti, Sean Swenson, Mathhew Rodell, "Uncertainty in global groundwater storage estimates in a total groundwater stress framework," Water Resources Research, June 16, 2015, doi: 10.1002/2015WR017351
27) "Mission Operations Status (Updated: 2015-May-13)," UTA/CRS, May 13, 2015, URL: http://www.csr.utexas.edu
28) "GRACE operational issue: Planned switch-on of Microwave Assemblies," ESA, January 19, 2015, URL: https://earth.esa.int/web/guest/missions/mission-news/-/article/grace-operational-issue-planned-switch-on-of-microwave-assemblies-jan-2015
29) "Mission Operations Status (Updated: 2015-Jan-14)," UTA/CRS, Jan. 15, 2015, URL: http://www.csr.utexas.edu/grace/operations/mission_status/
30) Steve Cole, Alan Buis, "NASA Analysis: 11 Trillion Gallons to Replenish California Drought Losses," NASA, Release 14-333, Dec. 16, 2014, URL: http://www.nasa.gov/press/2014/december/nasa-analysis-11
31) Steve Cole, Alan Buis, Janet Wilson, "Satellite Study Reveals Parched U.S. West Using Up Underground Water," NASA, Release 14- 200, July 24, 2014, URL: http://www.nasa.gov/press/2014/july/satellite-study-reveals-parched
32) Byron Tapley, Frank Flechtner, Michael Watkins, Srinivars Bettapur, "GRACE Mission: Status and Prospects," Proceedings of the GSTM (GRACE Science Team Meeting), Potsdam, Germany, Sept. 29-Oct. 1, 2014, URL: https://media.gfz-potsdam.de/gfz/sec12/GSTM-2014/GSTM2014-A0.zip
33) Mona Witkowski, Franz-Heinrich Massmann, "Status GRACE Mission Operations," Proceedings of the GSTM (GRACE Science Team Meeting), Potsdam, Germany, Sept. 29-Oct. 2, 2014, URL: https://media.gfz-potsdam.de/gfz/sec12/GSTM-2014/GSTM2014-A0.zip
34) Carmen Boeing, Michael M. Watkins, Felix W. Landerer, Isabella Velicogna, Victor Zlotnicki, Margaret Srinivasan, "2014 GRACE Science Team Meeting," The Earth Observer, May-June 2015, Volume 27, Issue 3, pp:24-27, URL: http://eospso.nasa.gov/sites/default/
35) "Mission Operations Status (Updated: 2013-Sept-25)," CSR at the University of Texas, Austin, URL: http://www.csr.utexas.edu/grace/operations/mission_status/
36) Elizabeth Ritchie (Chair), Ana Barros, Robin Bell, Alexander Braun, Richard Houghton, B. Carol Johnson, Guosheng Liu, Johnny Luo, Jeff Morrill, Derek Posselt, Scott Powell, William Randel, Ted Strub, Douglas Vandemark, "NASA Earth Science Senior Review 2013," June 14, 2013, URL: http://science.nasa.gov/media/medialibrary
37) "Water Storage Maps Show Improvement," NASA Earth Observatory, June 18, 2013, URL: http://earthobservatory.nasa.gov/IOTD/view.php?id=81408
38) Holli Riebeek, Robert Simmon, "The Gravity of Water - The GRACE mission offers a novel and much needed view of Earth's water supplies," NASA Earth Observatory, Sept. 12, 2012, URL: http://earthobservatory.nasa.gov
39) "Mission Operations Status (Updated: 2012-Nov-19)," CSR/UTexas, URL: http://www.csr.utexas.edu/grace
40) Don P. Chambers, John Wahr, Mark E. Tamisiea, R. Steven Nerem, "Ocean mass from GRACE and glacial isostatic adjustment," Journal of Geophysical Research-Solid Earth, Vol. 115, 2010, B11415, doi:10.1029/2010JB007530
41) R. S. Nerem, D. P. Chambers, M. Merrifield, G. Mitchum, D. Masters, "A 20 Year Climate Data Record of Sea Level Change: What have we learned?" Proceedings of the Symposium '20 years of Progress in Radar Altimetry', Venice, Italy, Sept. 24-29, 2012, (ESA SP-710, Feb. 2013)
42) Jaap Herman, Michael Steinhoff, "Balancing, Turning, Saving - Special AOCS Operations to extend the GRACE Mission," Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012
43) Alan Buis, "At 10, GRACE Continues Defying, and Defining, Gravity," NASA, March 16, 2012, URL: http://www.nasa.gov/mission_pages/Grace/news/grace20120316.html
44) "Gravity is climate: Ten Years of climate research satellites GRACE," GFZ Press Release, March 17, 2012, URL: http://www.sciencedaily.com/releases/2012/03/120316195351.htm
45) "Geoid: The Potsdam Gravity Potato," URL: http://www.gfz-potsdam.de/en/media-communication/
46) Matt Williams, "The "Potsdam Gravity Potato" Shows Variations in Earth's Gravity," Universe Today, Nov. 29, 2014, URL: http://www.universetoday.com/116801/the-potsdam-gravity-potato-shows-earths-gravity-variations/
47) Information provided by Franz-Heinrich Massmann of GFZ Potsdam, Germany
48) "'Gravity is climate' - 10 years of climate research satellites GRACE," Space Daily, March 21, 2012, URL: http://www.spacedaily.com/reports/Gravity_is_climate_
49) "Mission Operations Status (Updated: Jan. 19, 2012)," UTA/CSR, URL: http://www.csr.utexas.edu/grace/operations/mission_status/
50) J. Herman, A. Davis, K. B. Chin, M. Kinzler, S. Scholz, M. Steinhoff, "Life with a weak Heart - Prolonging the Grace Mission despite degraded Batteries," Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012
51) George Hurtt (Chair), Ana Barros, Richard Bevilacqua, Mark Bourassa, Jennifer Comstock, Peter Cornillon, Andrew Dessler, Gary Egbert, Hans-Peter Marshall, Richard Miller, Liz Ritchie, Phil Townsend, Susan Ustin,"NASA Earth Science Senior Review 2011," June 30, 2011, URL: http://science.nasa.gov/media/medialibrary/2011
53) "Mission Operations Status (Updated: 2011-July-5)," UTA/CSR , URL: http://www.csr.utexas.edu/grace
54) "NASA and DLR Sign Agreement to continue GRACE Mission through 2015," June 10, 2010, URL: http://www.dlr.de/en/desktopdefault.aspx
55) "NASA And DLR To Continue Grace Mission Through 2015," Space Daily, June 11, 2010, URL: http://www.spacedaily.com/reports/NASA_And_DLR_
56) Byron D. Tapley, Markus Rothacher, Srinivas Bettapur, Frank Flechtner, Michael Watkins, "The GRACE Mission: Status and Future Prospects," 37th COSPAR Scientific Assembly, July 13-20, 2008, Montréal, Canada.
57) S. Bettadpur, B. Tapley, C. Reigber, "GRACE Status and Future Plans," 3rd International GOCE User Workshop, Nov. 6-8, 2006, ESA/ESRIN, Frascati, Italy, URL: http://earth.esa.int/workshops/goce06
58) Steven A. Ackerman (chair), Richard Bevilacqua, Bill Brune, Bill Gail, Dennis Hartmann, George Hurtt, Linwood Jones, Barry Gross, John Kimball, Liz Ritchie, CK Shum, Beata Csatho, William Rose, Carlos Del Castillo, Cheryl Yuhas, "NASA Earth Science Senior Review 2009," URL: http://nasascience.nasa.gov/about-us/science-strategy
59) S. Bettadpur, "GRACE Science Team Meeting," (Oct. 13-14, 2005, Austin, TX), The Earth Observer, Nov.-Dec. 2005, Vol. 17, Issue 6, pp. 22-23
60) J. Ries, D. Chambers, S. Bettadpur, B. Tapley, "GRACE Mission Status and Current Results," Ocean Topography Science Team Meeting, Vienna, Austria, April 16-18, 2006
61) "Switch Maneuver Of GRACE Satellites," URL: http://www.csr.utexas.edu/grace/operations/switch_maneuver.html
62) P. A. M. Abusali, S. Bettadpur, "Switch Maneuver of GRACE Satellites," The Earth Observer (NASA/GSFC), March-April 2006, Vol. 18, Issue 2, pp. 4-5
63) "Science Instrument System (SIS)," GFZ, URL: http://www.gfz-potsdam.de/en/section
64) Charles Dunn, Willy Bertiger, Yoaz Bar-Sever, Shailen Desai, Bruce Haines, Da Kuang, Garth Franklin, Ian Harris, Gerhard Kruizinga, Tom Meehan, Sumita Nandi, Don Nguyen, Tim Rogstad, J. Brooks Thomas, Jeff Tien, Larry Romans, Michael Watkins, Sien-Chong Wu, Srinivas Bettadpur, Jeongrae Kim, "Instrument of Grace," GPS World, March 25, 2003, URL: http://www.csr.utexas.edu/GRACE/publications
65) Charles Dunn, Willy Bertiger, Garth Franklin, Ian Harris, Gerhard Kruizinga, Tom Meehan, Sumita Nandi, Don Nguyen, Tim Rogstad, J. Brooks Thomas, Jeff Tien, "The Instrument on NASA's GRACE Mission: Augmentation of GPS to Achieve Unprecedented Gravity Field Measurements," ION-GPS 2002, Portland, OR, Sept. 24-27, 2002, URL of presentation: http://trs-new.jpl.nasa.gov/dspace
66) W. Bertiger, Y. Bar-Sever, S. Desai, C. Dunn, B. Haines, D. Kuang, S. Nandi, L. Romans, M. Watkins, S. Wu, "GRACE: Millimeters and Microns in Orbit," ION-GPS 2002, Portland, OR, Sept. 24-27, 2002
67) Jeongrae Kim, Seung Woo Lee, "Flight performance analysis of GRACE K-band ranging instrument with simulation data," Acta Astronautica, Vol. 65, 2009, pp. 1571-1581
68) 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)
69) Bernard Foulon, Bruno Christophe, Yannick Bidel, "Two Decades of electrostatic accelerometers for space geodesy: past or future?," Proceedings of IAC 2011 (62nd International Astronautical Congress), Cape Town, South Africa, Oct. 3-7, 2011, paper: IAC-11-B1.3.4
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 (email@example.com).