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) 3)}

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

• To measure changes in the sea-floor pressure

• To study ocean mass changes

• To measure the mass balance of ice sheets and glaciers

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

Figure 1: Top view of the GRACE spacecraft (image credit: GFZ Potsdam)

The mission concept makes use of measurements of the inter-satellite range changes and its derivatives between two co-planar satellites (in low-altitude and polar orbits), using a microwave tracking system. The orbits of the two separately flying S/C are perturbed differently in the Earth's gravity field, leading to inter-satellite range variations. In addition, each S/C carries a GPS receiver of geodetic quality and high-accuracy accelerometers to enable accurate orbit determination, spatial registration of gravity data and the estimation of gravity field models. The fluctuations in the strength of the Earth's gravity field reflect in turn changes in the distribution of mass in the ocean, atmosphere, and solid Earth, and in the storage of water, snow, and ice on land. Since ocean bottom pressure represents a column integral of the mass of the atmosphere plus ocean, this measurement technique permits the deduction of ocean bottom pressure changes from space.

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

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.

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) 5)}

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

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

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

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

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

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

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

Figure 4: Block diagram of the GRACE instruments and flight systems (image credit: GFZ)

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

Orbit: Circular polar co-planar orbit (non-repeat ground track); the initial altitude is 485 km at launch (near a solar maximum), decaying to about 300 km (near a solar minimum) after five years; inclination = 89º. The two satellites in tandem formation are loosely controlled, they are separated at distances between 170 to 270 km apart. GRACE-1 is leading GRACE-2. The onboard cold-gas propulsion system is being used to maintain the separation between 270 km and 170 km. Since mission launch, orbit maneuvers have been needed about every 50 days to do this. - The rather low orbital altitude is selected to obtain the best possible gravity measurements (note that the gravity signal of any central body is decaying with the square of the orbital distance from the center of mass) taking into account all decaying (drag) effects.

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.

GRACE mission status: The GRACE tandem constellation is operating nominally as of 2008 (> 5 years in orbit). The satellite resources, including fuel, battery life, altitude decay, and thruster actuations portend a total mission life of 9 years from launch. The lifetime of the GRACE mission is predicted through 2011. ⁶⁾

• 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. ⁷⁾

• 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. ⁸⁾

• 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. ⁹⁾

• 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. ^{10) 11)}

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 5 and 6 provide graphical illustrations of how the range between the two satellites changed during the switch.

Figure 5: History of relative distance between the GRACE satellites during the switch (image credit: UTA/CSR)

Figure 6: Scalar distance between GRACE-1 and GRACE-2 around the CA event on Dec. 10, 2005 (image credit: UTA/CSR)

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.

The SIS (Science Instrument System) 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. ¹²⁾

KBR (K/Ka-Band Ranging) instrument assembly of 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. 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. ^{13) 14)}

Figure 7: Conceptual view of the KBR ranging principle (image credit: GFZ Potsdam)

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

SuperSTAR (Super Space Three-axis Accelerometer for Research mission), an accelerometer developed by ONERA/CNES, France (of STAR heritage on CHAMP, with a resolution a factor 10 higher than that on CHAMP). ¹⁵⁾ 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.

Figure 8: SuperSTAR accelerometer (image credit: ONERA/CNES)

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

Figure 9: Illustration of the SCA sensor heads and DPU (image credit: DTU)

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

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

• Calibration of the onboard GPS space receiver (BlackJack)

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

Figure 10: Illustration of the LRR (image credit: GFZ Potsdam)

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

Figure 11: View of the Blackjack GPS receiver during integration (image credit: JPL)

2) http://www.gfz-potsdam.de/pb1/op/grace/

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

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) http://www.gfz-potsdam.de/pb1/op/grace/satellite/satellite.html#AOCS

6) 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

7) 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

8) 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, URL: http://www.jason.oceanobs.com/documents/swt/posters2006/ries.pdf

9) http://www.gfz-potsdam.de/pb1/op/grace/index_GRACE.html

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

11) 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

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

13) C. Dunn, W. Bertiger, G. Franklin, I. Harris, G. Kruizinga, T. Meehan, et al., "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

14) 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

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