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GRACE and GRACE-FO

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Gravity Recovery and Climate Experiment
Illustration of the twin GRACE satellites
NamesGRACE-1 and GRACE-2[1][2]
Tom and Jerry[1][2]
ESSP-2A and ESSP-2B[3]
Mission typeGravitational science
OperatorNASA / DLR
COSPAR ID2002-012A and 2002-012B
SATCAT no.27391 and 27392
Websitewww.csr.utexas.edu/grace/
Mission durationPlanned: 5 years
Final: 15 years, 7 months
Spacecraft properties
BusFlexbus[3]
ManufacturerAstrium
Launch mass487 kg (1,074 lb) each[4]
Dimensions1.942 × 3.123 × 0.72 m (6.4 × 10.2 × 2.4 ft)[4]
Start of mission
Launch date17 March 2002, 09:21 (2002-03-17UTC09:21) UTC[5]
RocketRokot-KM #2[3]
Launch sitePlesetsk LC-133/3[3]
ContractorEurockot
End of mission
DisposalEnd of mission
DeactivatedGRACE-1: Early 2018
GRACE-2: mid-October 2017[6]
Decay dateGRACE-1: Early 2018
GRACE-2: Dec 2017-Jan 2018
Orbital parameters
Reference systemGeocentric
RegimeSun-synchronous
Semi-major axis6,873.5 km (4,271.0 mi)
Eccentricity0.00182
Perigee altitude483 km (300 mi)
Apogee altitude508 km (316 mi)
Inclination89.0°
Period94.5 minutes
Epoch17 March 2002, 04:21 UTC[5]

The Gravity Recovery and Climate Experiment (GRACE) was a joint mission of NASA and the German Aerospace Center. Twin satellites took detailed measurements of Earth's gravity field anomalies from its launch in March 2002 to the end of its science mission in mid-October 2017.

By measuring gravity anomalies, GRACE showed how mass is distributed around the planet and how it varies over time. Data from the GRACE satellites is an important tool for studying Earth's ocean, geology, and climate. GRACE was a collaborative endeavor involving the Center for Space Research at the University of Texas at Austin, NASA's Jet Propulsion Laboratory, the German Space Agency and Germany's National Research Center for Geosciences, Potsdam.[7] The Jet Propulsion Laboratory was responsible for the overall mission management under the NASA ESSP program.

The principal investigator is Byron Tapley of the University of Texas Center for Space Research, and the co-principal investigator is Christoph Reigber of the GeoForschungsZentrum (GFZ) Potsdam.[8]

The two GRACE satellites (GRACE-1 and GRACE-2) were launched from Plesetsk Cosmodrome, Russia, on a Rockot (SS-19 + Breeze upper stage) launch vehicle on 17 March 2002. The spacecraft were launched to an initial altitude of approximately 500 km at a near-polar inclination of 89°. During normal operations, the satellites were separated by approximately 200 km along their orbit track. GRACE far exceeded its five-year design lifespan, operating for fifteen years until the decommissioning of GRACE-2 in mid-October 2017.[6] Its successor, GRACE-FO, is expected to launch in early 2018.[9]

Discoveries and applications

Gravity anomaly map from GRACE
Variations in ocean bottom pressure measured by GRACE

The monthly gravity anomalies maps generated by GRACE are up to 1,000 times more accurate than previous maps, substantially improving the accuracy of many techniques used by oceanographers, hydrologists, glaciologists, geologists and other scientists to study phenomena that influence climate.[10]

From the thinning of ice sheets to the flow of water through aquifers and the slow currents of magma inside Earth, mass measurements provided by GRACE help scientists better understand these important natural processes.

Oceanography, hydrology, and ice sheets

GRACE chiefly detected changes in the distribution of water across the planet. Scientists use GRACE data to estimate ocean bottom pressure—as important to oceanographers as atmospheric pressure is to meteorologists.[11] GRACE data are also critical in helping to determine the cause of sea level rise, whether it is the result of mass being added to the ocean, from melting glaciers, for example, or from thermal expansion of warming water or changes in salinity.[12] High-resolution static gravity fields estimated from GRACE data have helped improve the understanding of global ocean circulation. The hills and valleys in the ocean's surface (ocean surface topography) are due to currents and variations in Earth's gravity field. GRACE enables separation of those two effects to better measure ocean currents and their effect on climate.[11]

GRACE data have provided a record of mass loss within the ice sheets of Greenland and Antarctica. Greenland has been found to lose 280±58 Gt of ice per year between 2003 and 2013, while Antarctica has lost 67±44 Gt per year in the same period.[13] These equate to a total of 0.9 mm/yr of sea level rise. GRACE data have also provided insights into regional hydrology inaccessible to other forms of remote sensing: for example, groundwater depletion in India[14] and California.[15] The annual hydrology of the Amazon basin provides an especially strong signal when viewed by GRACE.[16]

A University of California, Irvine-led study published in Water Resources Research on 16 June 2015 used GRACE data between 2003 and 2013 to conclude that 21 of the world's 37 largest aquifers "have exceeded sustainability tipping points and are being depleted" and thirteen of them are "considered significantly distressed." The most over-stressed is the Arabian Aquifer System, upon which more than 60 million people depend for water.[17]

Geophysics

GRACE also detects changes in the gravity field due to geophysical processes. Glacial isostatic adjustment—the slow rise of land masses once depressed by the weight of ice sheets from the last ice age—is chief among these signals. GIA signals appear as secular trends in gravity field measurements and must be removed to accurately estimate changes in water and ice mass in a region.[18] GRACE is also sensitive to permanent changes in the gravity field due to earthquakes. For instance, GRACE data have been used to analyze the shifts in the Earth's crust caused by the earthquake that created the 2004 Indian Ocean tsunami.[19]

In 2006, a team of researchers led by Ralph von Frese and Laramie Potts used GRACE data to discover the 480-kilometer-wide (300 mi) Wilkes Land crater in Antarctica, which were probably formed about 250 million years ago.[20]

Other signals

GRACE is sensitive to regional variations in the mass of the atmosphere (atmospheric pressure) and high-frequency variation in ocean bottom pressure. These variations are well understood and are removed from monthly gravity estimates using forecast models to prevent aliasing.[21] Nonetheless, errors in these models do influence GRACE solutions.[22]

GRACE data also contribute to fundamental physics. They have been used to re-analyze data obtained from the LAGEOS experiment to try to measure the relativistic frame-dragging effect.[23][24]

Spacecraft

Diagrams illustrating the systems and instruments aboard the GRACE spacecraft
Change in mass of the Greenland and Antarctic ice sheets as measured by GRACE
Global gravity anomaly animations over land and oceans by GRACE

The spacecraft were manufactured by Astrium of Germany, using its "Flexbus" platform. The microwave RF systems, and attitude determination and control system algorithms were provided by Space Systems/Loral. The star cameras used to measure the spacecraft attitude were provided by Technical University of Denmark. The instrument computer along with a highly precise BlackJack GPS receiver and digital signal processing system was provided by JPL in Pasadena. The highly precise accelerometer that is needed to separate atmospheric and solar radiation pressure effects from the gravitation data was manufactured by ONERA.

Measurement principle

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

Instruments

The two satellites (nicknamed "Tom" and "Jerry") constantly maintain a two-way, K-band microwave-ranging link between them. Fine distance measurements are made by comparing frequency shifts of the link. The micrometer-sensitivity of this measurement requires accordingly precise measurements of each spacecraft's position, motion, and orientation to be useful. To remove the effect of external, non-gravitational forces (e.g., drag, solar radiation pressure), the vehicles use sensitive Super STAR electrostatic accelerometers located near their respective centers of mass. GPS receivers are used to establish the precise positions of each satellite along the baseline between the satellites. The satellites use star cameras and magnetometers to establish attitude. The GRACE vehicles also have optical corner reflectors to enable laser ranging from ground stations.

Data products

CSR, GFZ, and JPL process observations and ancillary data downloaded from GRACE to produce monthly geopotential models of Earth.[25] These models are distributed as spherical harmonic coefficients with a maximum degree of 60. Degree 90 products are also available. These products have a typical latency of 1–2 months. These geopotential coefficients may be used to compute geoid height, gravity anomalies, and changes in the distribution of mass on Earth's surface.[26] Gridded products estimating changes in mass in units of liquid water equivalent thickness are available at JPL's GRACE Tellus website.

End of mission

Following an age-related battery issue on GRACE-2 in September 2017, it became apparent that GRACE-2's remaining battery capacity would not be sufficient to operate. Therefore, it was decided in mid-October to decommission the GRACE-2 satellite and end GRACE's science mission.[6] Atmospheric reentry of GRACE-2 occurred on 24 December 2017, atmospheric reentry of GRACE-1 is predicted to occur in late February 2018.[6]

GRACE Follow-On

NASA and the GFZ Potsdam have announced a follow-on of the GRACE mission. The GRACE-FO mission will be a collaboration between NASA and GFZ and is scheduled to be launched on 14 April 2018 (UTC) aboard a SpaceX Falcon 9 rocket from Vandenberg AFB, California, sharing the launch with Iridium.[27][28][9]

The orbit and the design of GRACE-FO will be very similar to GRACE. In addition to the proven microwave ranging system used on the GRACE mission, the distance between the two spacecraft of GRACE-FO will also be measured with laser ranging as a technological experiment in preparation for future satellites.[29][30]

See also

References

  1. ^ a b "GRACE 1". National Space Science Data Center. NASA. Retrieved 17 August 2016.
  2. ^ a b "GRACE 2". National Space Science Data Center. NASA. Retrieved 17 August 2016.
  3. ^ a b c d "GRACE (ESSP 2)". Gunter's Space Page. Retrieved 10 December 2017.
  4. ^ a b c "GRACE Launch: Press Kit" (PDF). NASA. March 2002. Retrieved 11 December 2017.
  5. ^ a b "Trajectory Details: GRACE 1". National Space Science Data Center. NASA. Retrieved 10 December 2017.
  6. ^ a b c d NASA (27 October 2017). "Prolific Earth Gravity Satellites End Science Mission". Retrieved 31 October 2017.
  7. ^ "Grace Space Twins Set to Team Up to Track Earth's Water and Gravity". NASA/JPL.
  8. ^ "Mission Overview". University of Texas. 19 November 2008. Archived from the original on 15 May 2009. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  9. ^ a b "GRACE-FO Mission". NASA / JPL. Retrieved 19 November 2017.
  10. ^ "New Gravity Mission on Track to Map Earth's Shifty Mass". NASA/JPL.
  11. ^ a b "Gravity data sheds new light on ocean, climate". NASA/JPL.
  12. ^ "NASA Missions Help Dissect Sea Level Rise". NASA/JPL.
  13. ^ Velicogna, Isabella; Sutterly, T.C.; van den Broeke, M.R. (2014). "Regional acceleration in ice mass loss from Greenland and Antarctica using GRACE time-variable gravity data". J. Geophys. Res. Space Phys. 41 (119): 8130–8137. Bibcode:2014GeoRL..41.8130V. doi:10.1002/2014GL061052.
  14. ^ Tiwari, V.M.; Wahr, J.; Swenson, S. (2009). "Dwindling groundwater resources in northern India, from satellite gravity observations". Geophysical Research Letters. 36.18. Bibcode:2009GeoRL..3618401T. doi:10.1029/2009GL039401. Retrieved 11 June 2015.
  15. ^ Famiglietti, J (2011). "Satellites measure recent rates of groundwater depletion in California's Central Valley". Geophys. Res. Lett. 38. Bibcode:2011GeoRL..38.3403F. doi:10.1029/2010GL046442. Retrieved 11 June 2015.
  16. ^ Tapley, Byron D.; Bettadpur, Srinivas; Ries, John C.; Thompson, Paul F.; Watkins, Michael M. (2004). "GRACE Measurements of Mass Variability in the Earth System". Science. 305 (5683): 503. Bibcode:2004Sci...305..503T. doi:10.1126/science.1099192. PMID 15273390. Retrieved 11 June 2015.
  17. ^ "Study: Third of Big Groundwater Basins in Distress". NASA. 16 June 2015. Retrieved 26 June 2015.
  18. ^ Tregoning; Ramillien; McQueen; Zwartz (2009). "Glacial isostatic adjustment and nonstationary signals observed by GRACE". J. Geophys. Res. 114. Bibcode:2009JGRB..114.6406T. doi:10.1029/2008JB006161. Retrieved 11 June 2015.
  19. ^ Chang, Kenneth (8 August 2006). "Before the '04 Tsunami, an Earthquake So Violent It Even Shook Gravity". The New York Times. Retrieved 4 May 2010.
  20. ^ "Big Bang in Antarctica--Killer Crater Found Under Ice". Ohio State University. Archived from the original on 6 March 2016. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  21. ^ "GRACE AOD1B". gfz-potsdam.de. GFZ German Research Centre for Geosciences. Retrieved 11 June 2015.
  22. ^ Ge, Shengjie (2006). GPS radio occultation and the role of atmospheric pressure on spaceborne gravity estimation over Antarctica. Ohio State University. Retrieved 11 June 2015.
  23. ^ Ciufolini, I.; Pavlis, E.C. (2004). "A confirmation of the general relativistic prediction of the Lense–Thirring effect" (PDF). Nature. 431: 958–960. Bibcode:2004Natur.431..958C. doi:10.1038/nature03007. PMID 15496915. Archived from the original (PDF) on 13 June 2015. {{cite journal}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  24. ^ Ciufolini, I.; Pavlis, E.C.; Peron, R. (2006). "Determination of frame-dragging using Earth gravity models from CHAMP and GRACE". New Astron. 11: 527–550. Bibcode:2006NewA...11..527C. doi:10.1016/j.newast.2006.02.001.
  25. ^ "GRACE PO.DAAC". JPL Physical Oceanography and Distributed Active Archive Center. Retrieved 11 June 2015.
  26. ^ Wahr, John; Molenaar, M.; Bryan, F. (1998). "Time variability of the Earth's gravity field: Hydrological and oceanic effects and their possible detection using GRACE". J. Geophys. Res. 103 (B12): 30205–30229. Bibcode:1998JGR...10330205W. doi:10.1029/98JB02844. Retrieved 11 June 2015.
  27. ^ "Launch Schedule". Spaceflight Now. 19 January 2018. Archived from the original on 21 January 2018. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  28. ^ de Selding, Peter B. (2 February 2017). "Iridium". Space Intel Report. Retrieved 11 December 2017.
  29. ^ "Airbus Defence and Space to build two new research satellites for NASA" (Press release). Airbus Defence and Space. 29 November 2012. Archived from the original on 20 July 2014. {{cite press release}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  30. ^ "Spacecraft: Microwaves and Lasers". GRACE-FO. NASA / JPL. Retrieved 11 December 2017.
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