Grace & Favour Are You Being Served Again S01e06

GRACE (Gravity Recovery And Climate Experiment)

GRACE is an international cooperative United states-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 fourth dimension-variable components of the Globe's gravity field (a new model of the Globe'southward gravity field every thirty days for five years). GRACE is also office of NASA'southward ESSP (Earth Organisation Scientific discipline Pathfinder) program. Some science objectives are: ^{ane) 2)}

• To enable a ameliorate agreement of body of water surface currents and ocean heat transport
• To measure changes in the sea-floor pressure
• To written report ocean mass changes
• To measure out the mass balance of ice sheets and glaciers
• To monitor changes in the storage of water and snow on the continents

Effigy 1: Summit view of the GRACE spacecraft (image credit: GFZ Potsdam)

The mission concept makes utilize 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 flight Due south/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 loftier-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 World'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 sea bottom pressure represents a cavalcade integral of the mass of the temper plus ocean, this measurement technique permits the deduction of ocean bottom force per unit area changes from space.

GRACE is a collaborative endeavour involving the Center for Space Enquiry (CSR) at the University of Texas, Austin; NASA's Jet Propulsion Laboratory, Pasadena, CA; the German Space Bureau (DLR) and Deutschland'south National Inquiry Center for Geosciences (GFZ), Potsdam.

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

The GRACE mission is led past B. Tapley (PI) of the Academy of Texas at Austin and past 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 2 flight satellites based on the existing Champ S/C charabanc. SS/L provides the attitude control system, microwave musical 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/Centre for Space Research) in the United states of america and GFZ in Germany. Federal republic of germany provides also the Eurockot launch vehicle.

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

GRACE spacecraft:

Both S/C structures are of identical blueprint. The shape of each satellite is trapezoidal in cross section, based on the FLEXBUS design of Astrium (length = 3122 mm, peak = 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 2 spacecraft.

Each Earth-pointing Southward/C is three-axis stabilized by AOCS (Attitude and Orbit Command 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 up-by modes of the satellite. One CESS sensor is mounted on each each of the vi sides of the satellite. The resulting Earth vector has an accurateness of ~5-10o, the lord's day vector ~iii-6o (in that location 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 fashion and for the commanding of the torque rods in fine pointing mode.
• The high precision sensors are SCA (Star Camera Associates) 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 charge per unit data 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 3 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 height 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. Most 80% of the spacecraft's on-board electronics parts are COTS (Commercial Off-the-Shelf) products.

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

Figure 4: Cake diagram of the GRACE instruments and flight systems (prototype credit: GFZ)

Launch: A dual-launch on an Eurockot vehicle took identify on March 17, 2002 from Plesetsk, Russia. The re-ignitable third stage, BREEZE-KM, was used to identify both satellites in the aforementioned nominal orbit. Post-obit separation, the leading GRACE satellite began pulling away from the trailing satellite at a relative speed of nigh 0.5 one thousand/southward to assume its nominal position of 220 km alee of the abaft satellite. At launch, the twin pair of both GRACE spacecraft was immediately nicknamed "Tom and Jerry."

Orbit: Circular polar co-planar orbit (not-repeat ground track); the initial altitude is 485 km at launch (near a solar maximum), decaying to near 300 km (near a solar minimum) after v years; inclination = 89o. The two satellites in tandem formation are loosely controlled, they are separated at distances betwixt 170 to 270 km autonomously. GRACE-1 is leading GRACE-2. The onboard cold-gas propulsion system is being used to maintain the separation betwixt 270 km and 170 km. Since mission launch, orbit maneuvers accept been needed about every 50 days to do this. - The rather depression orbital altitude is selected to obtain the all-time possible gravity measurements (notation that the gravity point of whatsoever cardinal body is decaying with the foursquare of the orbital distance from the center of mass) taking into account all decaying (drag) effects.

The spacecraft orbits accept a 30 mean solar day echo cycle, and a new gravity field is determined each month. The GRACE system accuracy is sufficient to determine a alter in mass equivalent to a volume of water with depth ane 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 nail. 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 information and GPS occultation information. CCSDS protocols are used for all data advice. The S-band frequencies for the two satellite organization are:

• Downlink: 2211.0 MHz for satellite one and 2260.8 MHz for satellite ii. Modulation: BPSK/NRZ is modulated onto the subcarrier which is PM modulated onto the uplink carrier. The information rate is 32 kbit/s for real-time data and 1 Mbit/south 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 conquering stations on Svalbard (Ny Alesund), ane for Champ and one for GRACE, to speed up the data processing and distribution chain for the diverse atmospheric condition services. The polar location of Svalbard makes it possible to accept access to the data on almost all orbits.

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

GRACE mission status:

· June 2013: Effigy six shows water storage maps of the USA acquired by the GRACE mission too as with other satellites and ground-based measurements to model the amount of water stored almost the surface and underground as of June 3, 2013. The maps are experimental products funded by NASA's Practical Sciences Plan and developed past scientists at NASA's Goddard Space Flying Eye and the National Drought Mitigation Eye. They correspond changes in h2o storage related to atmospheric condition, climate, and seasonal patterns. ^{vi) vii)}

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

The superlative map of Figure half-dozen shows the "wetness" or wet content in the "root zone"?the top meter of soil. The bottom map of Figure 6 shows water storage in shallow aquifers. The electric current water content is compared to a long-term average for early June betwixt 1948 and 2009. The darkest scarlet regions represent dry atmospheric condition that should occur just ii% 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 curt-term (weeks to months) water situation; for instance, the passage of a tropical storm tin have a distinct touch on on root zone moisture. Compared to the summer of 2012, moisture about the surface in June 2013 is significantly ameliorate in most of the eastern and northern portions of the continental United States, peculiarly 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 lesser 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 Mount states and most of Texas. Undercover storage has improved in much of the southeastern and cardinal U.S., though not in Florida. Southern California has a arrears despite promising signs in the winter and spring.

Figure vi: Water storage maps of the United states of america - the top map was acquired on Aug. 5, 2012, the lesser map was acquired on June 3, 2013 (paradigm credit: NASA)

· Nov. 2012: The GRACE operations condition depends on the health of the battery and the duration within each orbit when the battery is in apply. 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 projection is trying to prolong the mission as long as possible to span the gap for a planned follow-on mission in the timeframe 2016/17. - Hence, a number of special AOCS operations and analyses accept 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. Shut interaction between the science- and functioning- 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 establish to exist in due east.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 Globe's gravity field varies over fourth dimension and space, and over land and bounding main. The information accept 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 projection hoped for. Early on in the design of GRACE, information technology was realized, that the gravity field could be measured well enough to discover the critical indicators of climate change - sea level rise and polar ice cook. ¹⁰⁾

- In June 2010, NASA and DLR signed an agreement to go on 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 adult 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, Globe to accept an irregular shape, which deviates significantly from sphericity. Known equally the "Potsdam Gravity Potato", the geoid has accomplished global notoriety. Merely this spud shape is equally bailiwick to temporal changes. During the terminal Ice Age, a mile-thick ice sheet covered North America and Scandinavia. Since the water 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 furnish. With GRACE, this glacial-isostatic aligning can for the starting time time be accurately detected globally as a change in the geoid height: the ice ages continue to take an issue, which is especially evident in North America and Scandinavia. ¹¹⁾

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

Legend to Effigy 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), One thousand-band ranging (GRACE), satellite gradiometry (GOCE)
• Increased accuracy in the measurement of surface data (airborne gravimetry and satellite altimetry)
• And of form on the long-term information availability of the GRACE mission.

· The GRACE tandem constellation is operating nominally in February 2012 - completing its 10^th year on orbit (March 17. 2012), which represents double the length of its blueprint life. All instruments are providing measurements with regard to the gravity decision and for the profiles of the weather services. - Since 2011, ESA is supporting the GRACE mission inside the context of a TPM (3rd Party Mission) arrangement. ^{13) fourteen)}

- The GRACE operations status depends on the health of the battery and the duration inside each orbit when the battery is in use. ^{fifteen) 16)}

- In June 2011, the NASA World 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-calibration temporal variations of mass distribution within the Earth organisation, resulting in truly unique constraints on climatically important processes such as mass commutation 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 absorption 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 loftier chance?the weakened ability system may neglect, or consequence in significantly degradation of information 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 Deutschland. GFZ besides 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 Arrangement. 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 distance of ~ 455 km.

The GRACE Science Operations concept for the rest of the mission is driven by the intersection of two factors. Beginning is the project decision to operate the spacecrafts in a manner that maximizes the remaining lifetime, so that the longest possible climate data tape is bachelor 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/twenty-four hour period relative to the Sunday, such that the Sun is in the orbit airplane every 161 days. Due to the power organization status and desire for longevity, this event will henceforth define a 161day piece of work cycle for science operations. As long as the ß' angle (angle betwixt 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 bombardment. When ß' is near zero (i.e. Sunday is in the orbit plane), the battery may be used for every bit much a 40 minutes out of 90 minutes in each orbit. Near ß'=0 events, the mission operations status depends on the battery wellness 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 fourth dimension frame 2013-2015, depending on solar action, thruster actuations or battery status. ^{20) 21)}

GRACE's monthly maps are up to 100 times more authentic than existing maps, substantially improving the accuracy of techniques used past 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 afterward launch, far exceeding its mission blueprint and requirement. ^{22) 23)}

The GRACE satellite mission has demonstrated significant technological and new scientific achievements. GRACE provides a unique mensurate of Earth's temporal gravity field, which includes climate-change signals. No other current satellite provides this blazon of measurement. The scientific achievement is truly cross-disciplinary, covering a broad range of NASA's World Science priority areas, including climate change, terrestrial water storage including groundwater variability, cryospheric changes, ocean circulation and body of water level, and geodynamics. ²⁴⁾

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

Figure 8: GRACE mission status equally of December 2008

Figure 9: GRACE-1 decay scenario prediction as of Nov. 2008 (image credit: NASA/JPL,DLR, CSR, GFZ)

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

· After the GSTM (GRACE Science Team Meeting), Oct. 13-fourteen, 2005, Austin, TX, NASA approved a mission extension through 2009. ²⁵⁾

· Mission accomplishments: 2d 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 decision is invigorating mass balance studies in hydrology, oceanography, glaciology, and in the solid Earth sciences. ²⁶⁾

· GRACE data assay showed that the gravity field of the Earth is variable in both space and time, and is an integral constraint on the hateful and time variable mass distribution in the Globe. From the temporal variations geo-scientists have already derived new insight into dynamic processes in the World interior, into water mass transfer processes over land and in the oceans and into the evolution of ice sheets and glaciers on Greenland and Antarctica. With the GRACE mission, for the showtime time a systematic and thorough monitoring of the amounts of water, ice and thing moving effectually is performed and thus a completely new film of the dynamic processes within and on the World emerges.

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

GRACE-ane (trailing satellite) collects setting occultations

Only atmospheric occultation (l Hz) data are beingness collected

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

· At the AGU fall meeting in San Francisco NASA and the United states Department of the Interior (DOI) presented the coveted William T. Pecora Accolade to the GRACE mission team; December xi, 2007.

Switch maneuver of GRACE satellites (Dec. 2005):

Since launch (March 17, 2002), the abaft satellite (GRACE-2) has been flying "forward" with its K-band antenna horn exposed to the impacting diminutive oxygen. There is some take a chance that overexposure to diminutive oxygen could lead to a loss of thermal command over the K-ring horn, which would affect the accuracy of the KBR signal. To ensure compatible 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 effectually the middle of the mission and so that the trailing satellite would go the atomic number 82 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 squad analyzed the relative motility of each satellite and selected December 10, 2005, as an optimum fourth dimension to perform the switch maneuver that would allow for a minimum take chances of a collision at the point of closest approach (CA). The maneuver was carefully planned so that the two satellites could non get any closer together than 300 m -- they really never got whatever closer than 406 thousand at CA.

The switch was achieved 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 ii satellites changed during the switch.

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

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

Appointment	Event	GRACE-ii	GRACE-1	Range (km)
Dec. 3 2005	OTM-1	Yaw 180o (yaw bias=180o) Execute burn down (688 s; 10.88 cm/south) Nigh the south pole: yaw 180o (yaw bias=0)		-203 (29 km/mean solar day)
December. 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. xi			Yaw 180o (yaw bias=0); re-establish KBR link	29
December. 12	OTM-2	Yaw 180o (yaw bias=0o) Execute fire (611 south; +9.82 cm/s) Yaw 180o (yaw bias=180o)		58 (3.3 km/twenty-four hours)
Jan. xi, 2006	OTM-three	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 behave a suite of contained scientific instruments. Instead, the twin GRACE satellites act in unison as the primary scientific discipline instrument. The One thousand-ring ranging system (KBR) tin detect instantaneous extremely small changes in the distance between the 2 satellites and use this data to brand 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 South/C interior, equally are the fuel tanks and the batteries and other satellite subsystems.

SIS (Science Instrument Arrangement):

The Sister includes all elements of the inter-satellite ranging organisation, the GPS receivers required for precision orbit determination and occultation experiments, and associated sensors such equally SCA. SIS likewise coordinates the integration activities of all sensors, assuring their compatibility with each other and the satellite. ²⁹⁾

KBR (G/Ka-Band Ranging) instrument associates of NASA/JPL

KBR is the cardinal science instrument of the GRACE mission [Note: KBR is also referred to equally HAIRS (High Accuracy Intersatellite Ranging Organisation)]. 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 Mode 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 modify between the satellites, while the stage variation due to long-term instability in each clock cancels out. ³⁰⁾

M-band has a radio frequency of near 24 GHz and Ka-band is nigh 32 GHz. The GRACE G- and Ka-band frequencies are in an exact three-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 2 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 not-gravitational effects by an accelerometer chosen SuperSTAR. KBR consists of the following elements: USO (Ultra Stable Oscillator), the MWA (Microwave Assembly), the horn, and IPU (Musical instrument Processing Unit of measurement). The IPU and the SPU (Signal Processing Unit) establish the heart of the instrument system. ^{31) 32)}

Figure 12: A schematic drawing of the GRACE instrument organisation (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 upwardly-converting the reference frequency to 24 and 32 GHz; downward-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 non only the K-Band carrier phase bespeak, 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 first from each other past 0.5 MHz in the 24 GHz channel, and past 0.67 MHz in the 32 GHz aqueduct. This shifts the downwardly-converted signal abroad from DC, enabling more authentic measurements of the stage. 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 alter between the satellites.

Figure xiii: Cake diagram of the dual one-mode ranging system (prototype credit: NASA, Korea Aerospace University) ³³⁾

SuperSTAR (Super Infinite Three-axis Accelerometer for Inquiry 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 level) acting on the GRACE spacecraft. The measurement principle of the SuperSTAR accelerometer is based on the electrostatic interruption of a parallel-epipedic proof mass within 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 every bit actuators to apply electrostatic forces in lodge 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 reward of the CHAMP mission feel. 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 besides of the modification of electronics role parameters as for instance le reduction of the bias reference voltage past a cistron 2, a better adjustment of the measurement conditioning amplifiers and an optimized exploitation of the 24 bit sigma-delta analog to digital converters. ³⁵⁾

Figure 14: SuperSTAR accelerometer with the sensor unit (correct) and the ICU (left), prototype credit: ONERA

SuperSTAR is mounted at the CG (Center of Gravity) of the satellite. SuperSTAR consists of the following elements: SU (Sensor Unit of measurement, EEU (Electromagnetic Heady Unit), ICU (Interface Control Unit of measurement), and a harness. SU consists of a metallic proof mass, suspended within an electrode cage of gilt-coated silica. The proof mass motion is servo-controlled using capacitive sensors, and is a measure of the non-gravitational accelerations interim on the satellite. The mass and electrode cage cadre 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 evangelize a ten mg acceleration, and is used only in example of an SU outset-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 (ii cameras with sensor heads), each with a FOV of 18o x 16o and one DPU (Data Processing Unit of measurement). Both assemblies are rigidly attached to the accelerometer, and view the heaven at a 45o angle with respect to the zenith, on the port and starboard sides. The SCA is used for both: scientific discipline likewise 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 Southward/C mental attitude to an accuracy of < 0.iii mrad (with a goal of 0.1 mrad) by autonomous detection of star constellations using an onboard star catalog.

Figure 15: Illustration of the SCA sensor heads and DPU (epitome credit: DTU)

LRA (Light amplification by stimulated emission of radiation Corner-cube Reflector Assembly):

LRA is provided by GFZ (also referred to equally 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 accurateness of ane-ii cm (depending on the technological status of the measuring basis station). The LRA information are being used for:

• POD (Precise Orbit Determination) in combination with GPS tracking data for gravity field recovery
• Calibration of the onboard GPS infinite receiver (BlackJack)
• Applied science experiments such as two-color ranging (this involves differential ranging to eliminate tropospheric signal effects).

Effigy 16: Illustration of the LRR (epitome credit: GFZ Potsdam)

BlackJack (GPS Flight Receiver):

BlackJack is a new generation instrument of TRSR (TurboRogue Space Receiver) heritage, provided past 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 3 antennas, the main zenith crossed dipole antenna is used to collect the navigation data. In improver, a fill-in crossed dipole antenna and one helix antenna on the aft panel are used for redundancy navigation and atmospheric occultation data drove, respectively. This system is capable of simultaneously tracking up to 24 dual frequency signals. In addition, this arrangement provides digital betoken processing functions for the KBR and SCA instruments too.

Figure 17: View of the Blackjack GPS receiver during integration (paradigm credit: JPL)

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1) C. W. Hughes, C. Wunsch, Five. Zlotnicki, "Satellite Peers through the Oceans from Infinite," EOS Transmissions of AGU, Vol. 81, No. vii, February. 15, 2000, p. 68

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

3) "A new name for the GFZ: Helmholtz Centre Potsdam - GFZ German language 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, "Mental attitude Control for GRACE: The First Low-Flying Satellite Formation," 18th International Symposium on Space Flying Dynamics, Munich, Frg, Oct. 11-15, 2004

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

half-dozen) "Water Storage Maps Show Comeback," 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 Globe Observatory, Sept. 12, 2012, URL: http://earthobservatory.nasa.gov/Features/GRACEGroundwater/

viii) "Mission Operations Status (Updated: 2012-Nov-19)," CSR/UTexas, URL: http://world wide web.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 twelfth International Conference on Space Operations, Stockholm, Sweden, June 11-fifteen, 2012

10) Alan Buis, "At 10, GRACE Continues Defying, and Defining, Gravity," NASA, March xvi, 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 Tater," URL: http://www.gfz-potsdam.de/portal/gfz/Public+Relations/M40-Bildarchiv/Bildergalerie_Kartoffel

thirteen) Information provided by Franz-Heinrich Massmann of GFZ Potsdam, Federal republic of germany

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

15) "Mission Operations Status (Updated: January. xix, 2012)," UTA/CSR, URL: http://world wide web.csr.utexas.edu/grace/operations/mission_status/

16) J. Herman, A. Davis, M. B. Chin, M. Kinzler, S. Scholz, G. Steinhoff, "Life with a weak Middle - 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, Marking Bourassa, Jennifer Comstock, Peter Cornillon, Andrew Dessler, Gary Egbert, Hans-Peter Marshall, Richard Miller, Liz Ritchie, Phil Townsend, Susan Ustin,"NASA Globe 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://world wide web.csr.utexas.edu/grace/operations/

19) "Mission Operations Condition (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 x, 2010, URL: http://www.dlr.de/en/desktopdefault.aspx/tabid-6604/10829_read-24882/

21) "NASA And DLR To Proceed 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-twenty, 2008, Montréal, Canada.

23) South. Bettadpur, B. Tapley, C. Reigber, "GRACE Condition and Future Plans," 3rd International GOCE User Workshop, Nov. half dozen-8, 2006, ESA/ESRIN, Frascati, Italian republic, 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 Coming together," (Oct. 13-14, 2005, Austin, TX), The Earth Observer, November.-Dec. 2005, Vol. 17, Consequence six, pp. 22-23

26) J. Ries, D. Chambers, South. Bettadpur, B. Tapley, "GRACE Mission Condition and Current Results," Ocean Topography Science Team Coming together, 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. eighteen, Issue ii, pp. 4-5

29) http://world wide web.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 Earth, 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'southward 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/ane/02-2484.pdf

32) W. Bertiger, Y. Bar-Sever, S. Desai, C. Dunn, B. Haines, D. Kuang, S. Nandi, 50. Romans, M. Watkins, Due south. Wu, "GRACE: Millimeters and Microns in Orbit," ION-GPS 2002, Portland, OR, Sept. 24-27, 2002

33) Jeongrae Kim, Seung Woo Lee, "Flying functioning assay of GRACE Chiliad-band ranging musical instrument with simulation data," Acta Astronautica, Vol. 65, 2009, pp. 1571-1581

34) Annotation: 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. four-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: by or time to come?," Proceedings of IAC 2011 (62nd International Astronautical Congress), Cape Town, South Africa, October. iii-7, 2011, paper: IAC-eleven-B1.three.four

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The data 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) likewise equally many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.

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Source: https://earth.esa.int/web/eoportal/satellite-missions/g/grace

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