Minimize GRACE

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

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Figure 2: Bottom view of GRACE (image credit: GFZ Potsdam)

GRACE spacecraft:

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

Each Earth-pointing S/C is three-axis stabilized by AOCS (Attitude and Orbit Control System) consisting of sensors, actuators and software. The sensors include: 4)

  • CESS (Coarse Earth Sun Sensor) for 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 Orsted), 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.

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Figure 3: Internal view of GRACE (image credit: GFZ Potsdam)

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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.

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Figure 5: Illustration of the flight configuration and ground support for the GRACE mission (image credit: NASA, CSR/UTexas) 5)

 

GRACE mission status:

· June 2013: Figure 6 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. 6) 7)

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 6 shows the "wetness" or moisture content in the "root zone"?the top meter of soil. The bottom map of Figure 6 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. 6).

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 6 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.

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Figure 6: 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. 8)

· 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. 9)

· On March 17, 2012, the GRACE twin satellites completed 10 years on orbit. The GRACE measurements are used to produce monthly gravity maps that are more than 100 times more precise than previous models, providing the resolution necessary to characterize how Earth's gravity field varies over time and space, and over land and sea. The data have substantially improved the accuracy of techniques used by oceanographers, hydrologists, glaciologists, geologists and climate scientists. - GRACE essentially demonstrated a new form of remote sensing for climate research that has turned out even better than the project hoped for. Early on in the design of GRACE, it was realized, that the gravity field could be measured well enough to observe the critical indicators of climate change - sea level rise and polar ice melt. 10)

- 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. 10).

- 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. 11)

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Figure 7: The Earth's gravity field (vertically enhanced), also known as the "Potsdam Gravity Potato" (image credit: GFZ) 12)

Legend to Figure 7: "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.

· 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. 13) 14)

- The GRACE operations status depends on the health of the battery and the duration within each orbit when the battery is in use. 15) 16)

- 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. 17)

· 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. 18)

· 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 ß' 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 ß' angles, the satellites operate partly using the arrays, and partly using the battery. When ß' 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 ß'=0 events, the mission operations status depends on the battery health and operating environment. 19)

· 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. 20) 21)

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. 22) 23)

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. 24)

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.

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Figure 8: GRACE mission status as of December 2008

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Figure 9: 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. 25)

· 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. 26)

· 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. 27) 28)

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 10 and 11 provide graphical illustrations of how the range between the two satellites changed during the switch.

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Figure 10: History of relative distance between the GRACE satellites during the switch (image credit: UTA/CSR)

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Figure 11: Scalar distance between GRACE-1 and GRACE-2 around the CA event on Dec. 10, 2005 (image credit: UTA/CSR)

Date

Event

GRACE-2

GRACE-1

Range (km)

Dec. 3 2005

OTM-1

Yaw 180o (yaw bias=180o)
Execute burn (688 s; 10.88 cm/s)
Near the south pole: yaw 180o (yaw bias=0)

 

-203
(29 km/day)

Dec. 9

 

Yaw 180o (yaw bias=180o) for KBR, receiver safety (link breaks)

 

-29

Dec. 10

Closest approach (CA)

CA at ~04:00 UTC; GRACE-2 passes GRACE-1 and becomes the leader

 

0

Dec. 11

 

 

Yaw 180o (yaw bias=0); re-establish KBR link

29

Dec. 12

OTM-2

Yaw 180o (yaw bias=0o)
Execute burn (611 s; +9.82 cm/s)
Yaw 180o (yaw bias=180o)

 

58 (3.3 km/day)

Jan. 11, 2006

OTM-3

Yaw 180o (yaw bias=0o)
Execute burn;
Yaw 180o (yaw bias=180o)

 

170 (0.5 km/day)

Table 1: 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. 29)

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. 30)

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. 31) 32)

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Figure 12: A schematic drawing of the GRACE instrument system (image credit: NASA/JPL)

Legend to Figure 12: 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.

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Figure 13: Block diagram of the dual one-way ranging system (image credit: NASA, Korea Aerospace University) 33)

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). 34) 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. 35)

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Figure 14: 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.

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Figure 15: 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).
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Figure 16: 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.

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Figure 17: 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

2) http://www.csr.utexas.edu/grace/

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_archive/artikel/artikeldetail/
a_new_name_for_the_gfz_helmholtz_centre_potsdam_gfz_german_research_centre_for_geosciences/

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, "GRACE - Gravity Recovery and Climate Experiment," NASA, March 2, 2011, URL: http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=186

6) "Water Storage Maps Show Improvement," NASA Earth Observatory, June 18, 2013, URL: http://earthobservatory.nasa.gov/IOTD/view.php?id=81408

7) 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/Features/GRACEGroundwater/

8) "Mission Operations Status (Updated: 2012-Nov-19)," CSR/UTexas, URL: http://www.csr.utexas.edu/grace/operations/mission_status/

9) 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

10) 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

11) "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

12) "Geoid: The Potsdam Gravity Potato," URL: http://www.gfz-potsdam.de/portal/gfz/Public+Relations/M40-Bildarchiv/Bildergalerie_Kartoffel

13) Information provided by Franz-Heinrich Massmann of GFZ Potsdam, Germany

14) "'Gravity is climate' - 10 years of climate research satellites GRACE," Space Daily, March 21, 2012, URL: http://www.spacedaily.com/.../Gravity_is_climate_10_years_of_climate_research_satellites_GRACE

15) "Mission Operations Status (Updated: Jan. 19, 2012)," UTA/CSR, URL: http://www.csr.utexas.edu/grace/operations/mission_status/

16) 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

17) 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/2011/07/22/2011-NASA-ESSR-v3-CY-CleanCopy_3x.pdf

18) "Operations," URL: http://www.csr.utexas.edu/grace/operations/

19) "Mission Operations Status (Updated: 2011-July-5)," UTA/CSR , URL: http://www.csr.utexas.edu/grace/operations/mission_status/

20) "NASA and DLR Sign Agreement to continue GRACE Mission through 2015," June 10, 2010, URL: http://www.dlr.de/en/desktopdefault.aspx/tabid-6604/10829_read-24882/

21) "NASA And DLR To Continue Grace Mission Through 2015," Space Daily, June 11, 2010, URL: http://www.spacedaily.com/reports/NASA_And_DLR_To_Continue_Grace_Mission_Through_2015_999.html

22) 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.

23) 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/participants/315/pres_tapley_315.pdf

24) 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/senior-reviews/2009SeniorReviewSciencePanelReportFINAL.pdf

25) 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

26) 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

27) "Switch Maneuver Of GRACE Satellites," URL: http://www.csr.utexas.edu/grace/operations/switch_maneuver.html

28) 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

29) http://www.gfz-potsdam.de/grace/payload/payload.html#ACC

30) 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/press/03-02-01-GRACE_gpsworld.pdf

31) 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/bitstream/2014/10486/1/02-2484.pdf

32) 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

33) 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

34) Note: STAR and SuperSTAR are of ASTRE (Accélérometre 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)

35) 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.