The Gravity Recovery and Climate Experiment-Follow-On (GRACE-FO) Mission, successfully launched on 22 May 2018, is a NASA directed mission to continue the goals of the original GRACE mission and provide continuity for the GRACE data set.
The primary goal of the GRACE-FO Mission is to obtain accurate global and high-resolution models for both the static and the time variable components of the Earth's gravity field. As in the original GRACE mission, this goal is achieved by making accurate measurements of the inter-satellite range between two co-planar, low altitude polar orbiting twin satellites using a K/Ka-Band microwave tracking system. Additionally, each satellite carries geodetic quality Global Navigation Satellite System (GNSS) receivers, a Laser Retro-Reflector (LRR) for ground station ranging, and high accuracy accelerometers to precisely measure the non-gravitational accelerations acting on the satellite.
As a secondary goal, GRACE-FO will carry a Laser Ranging Interferometer (LRI) as a technology demonstration. It will provide laser interferometry measurements of inter-satellite range changes in orbit to demonstrate laser-ranging technology in support of future GRACE-like missions. Another secondary objective is the continuation of GRACE radio occultation measurements.
The GRACE-FO project will be executed in the US under the direction of the NASA Earth Science Division (ESD) within the NASA Science Mission Directorate (SMD) and the Earth Systematic Missions Program Office at Goddard Space Flight Center (GSFC). The Jet Propulsion Laboratory (JPL) is assigned responsibility for the GRACE-FO project.
The GRACE-FO mission has significant German participation managed by the German Research Centre for Geosciences (GFZ). Funding of the German contributions is jointly secured by the Federal Ministry of Education and Research (BMBF), the Federal Ministry for Economic Affairs and Energy (BMWi), the Helmholtz Association (HGF), the German Aerospace Center (DLR) (LRI in kind contributions) and the German Research Centre for Geosciences (GFZ).
The overall agreement between NASA and GFZ will be codified in a Memorandum of Understanding (MOU), the roles and responsibilities are described in a Cooperative Project Plan (CPP). According to that GFZ will be responsible for
Details on the various mission elements, recent news or related publications are described in the corresponding links beside.
In the following table the most important events during the realization of GRACE-FO are chronologically listed.
29.05.2018 | K-band ranging between both satellites has been started.
28.05.2018 | On both satellites the accelerometers have been successfully switched on.
26.05.2018 | The Launch and Early Operations Phase has been successfully and untimely completed. The mean altitude (above 6378.1 km) of the satellites was 491.5 km, the distance between the satellites 216.7 km with a drift of 0.4 km/d. On both satellites the USO (Ultra Stable Oscillator), GPS and MWA (Microwave Assembly) have been switched on.
22.05.2018 | Launch of GRACE-FO at 19:47 UTC which will be a rideshare with 5 Iridium-Next satellites on a SpaceX/Falcon-9 from Vandenberg Air Force Base in California.
27.03.2018 | Successful Key Decision Point E Review to transit in phase E (KDP-E).
13.03.2018 | The Mission Readiness Review (MRR) was successfully passed at NASA headquarter in Washington.
18./19.01.2018 | The Operations Readiness Review (ORR) was successfully passed at DLR/GSOC in Oberpfaffenhofen.
12.12.2017 | Transport of the satellites, the MSD and further equipment from IABG in Ottobrunn to Vandenberg Air Force Base in California. Further details can be found here.
10.11.2017 | Press conference on the successfully completion of the environmental qualification tests of the twin GRACE-FO satellites at IABG in Ottobrunn. Further details can be found here
07./08.11.2017 | The Pre-ship Review (PSR) has been successfully performed at Airbus Defence and Space in Friedrichshafen.
12.10.2017 | The Science Data System Readiness Review has been successfully performed at UTCSR in Austin/Texas.
19.09.2017 | The separation test between the MSD and the spacecraft dummies has been successfully performed.
11.-15.09.2007 | The EGSIEM Autumn School on Satellite Gravimetry Applications took place in Potsdam, Germany at GFZ, the German Research Centre for Geosciences. Around 45 participants from 16 different countries (plus 12 lecturers) enjoyed a very interesting week in Potsdam. In various lectures and practical (computer) exercises they learned how to handle the GRACE and future GRACE-FO data and how to use the information for various geophysical applications. Further information is available at www.egsiem.eu/139-potsdam-autumn-school
18.07.2017 | The mechanical fit check between the SpaceX Payload Adapter Ring and the MSD was successfully performed.
17.07.2017 | The mechanical fit check between the MSD and the two GRACE-FO spacecraft has been successfully performed.
07.07.2017 | The MSD was delivered to IABG.
07.02.2017 | The Multi Satellite Dispenser (MSD) Critical Design Review (CDR) has been successfully performed at Airbus Defence and Space S.A. (CASA Espacio premises), Madrid, Spain.
14.11.2016 | As the Russian/Ukraine Dnepr and corresponding launch services can no longer be provided by the International Space Company Kosmotras (ISCK), the joint NASA-GFZ Joint Steering Group has decided to exchange the GRACE-FO launcher. The corresponding contract was signed today by the Board of GFZ and Iridium Satellite LLC. This includes a "Rideshare" between GRACE-FO and 5 Iridium-Next satellites on a Space-X Falcon-9 from Vandenberg Air Force Base in California within the launch period December 2017 till February 2018. This also includes a contract with Airbus D&S who will build the necessary Multi Satellite Dispenser and will perform the Launch Service Management under contract of GFZ. Further information can be found here.
01.11.2016 | The Environmental Test Campaign for FM1 has begun at IABG.
28.10.2016 | Airbus D&S has completed the first of the twin GRACE-FO satellites. More infos can be found here.
21./22.09.2016 | The Environmental Test Readiness Review was successfully passed at at the IABG (Industrieanlagen-Betriebsgesellschaft mbH) in Ottobrun.
08/2015-10/2016 | Integration of the instruments and satellites at Airbus D&S
21.-22.07.2015 | The System Integration Review (SIR) was sucessfully passed at Airbus in Immenstaad.
26.05.2015 | Airbus Defence and Space begins building GRACE-FO climate satellites. Further information can be found here.
29./30.04.2015 | The Mission Operations System (MOS) Critical Design Review (CDR) has been successfully accomplished at GFZ/GSOC in Oberpfaffenhofen.
26.03.2015 | The joint US/D Science Data System (SDS) Critical Design Review (CDR) has been successfully accomplished at JPL, Pasadena.
25.-26.02.2015 | The 5. Launcher Progress Meeting was held at STI.
02.-05.02.2015 | The Project Critical Design Review (CDR) was sucessfully passed at Airbus in Immenstaad.
19.-20.11.2014 | The 4. Launcher Progress Meeting was held at STI.
29.-31.07.2014 | The Launch System Preliminary Design Review (PDR) has been successfully accomplished at STI in Immenstaad.
16.-17.07.2014 | The Mission Operations System (MOS) Preliminary Design Review (PDR) has been successfully accomplished at DLR/GSOC in Oberpfaffenhofen.
26.06.2014 | The joint US/D Science Data System (SDS) Preliminary Design Review (PDR) has been successfully accomplished at JPL, Pasadena.
05.06.2014 | The joint US/D Laser Ranging Interferometer (LRI) Critical Design Review (CDR) has been successfully accomplished at JPL, Pasadena.
15.05.2014 | The Microwave Instrument (MWI) Critical Design Review (CDR) has been successfully accomplished at JPL, Pasadena.
02.05.2014 | A European GRACE-FO Science Team Meeting took place during the EGU conference in Vienna.
28.-30.04.2014 | The 3. Launcher Progress Meeting was held at STI.
19.02.2014 | The 2. Launcher Progress Meeting took place at STI.
11.02.2014 | The Board of the German Research Centre for Geosciences has signed the Memorandum of Understanding (MOU) with NASA Administrator Charles Bolden Jr. to mutually realize the follow-on mission of GRACE (Gravity Recovery and Climate Experiment). The corresponding press release can be found here.
21.-24.01.2014 | The Project Preliminary Design Review (PDR) was sucessfully passed at Airbus in Immenstaad. The transition into phase C happened on March 3, 2014.
19.12.2013 | The 1. Launcher Progress Meeting was accomplished at STI.
16.12.2013 | The LRI Maturity Assessment Review took place at JPL.
06.12.2013 | The contract to provide the DNEPR launcher and corresponding launch services has been signed by GFZ and the International Space Company Kosmotras (ISCK).
18.-19.11.2013 | The Accelerometer Preliminary Design Review was held at ONERA, France.
28.10.2013 | The Flight System Star Tracker Mission Readiness Review took place at the Danish Technical University (DTU), Copenhagen.
22.10.2013 | A GRACE-FO Science Data System Meeting took place in Austin, Texas, in forefront of the GSTM2013.
21.-22.10.2013 | The Launch Vehicle Kick-off Meeting was held at STI in Immenstaad.
18.10.2013 | The On-Board Computer (OBC) Mission Readiness Review was held at Thales-Alenia, Italy.
15.-16.10.2013 | The Power and Thermal Preliminary Design Review was held at Astrium GmbH in Immenstaad.
02.-03.10.2013 | The Flight Software Preliminary Design Review was held in Bristol, England.
03.09.2013 | The European GRACE-FO Science Team Kick-off Meeting took place during the IAG2013 conference in Potsdam.
16.-17.07.2013 | The Accelerometer Electronics Preliminary Design Review was held at Onera in Paris.
20.06.2013 | GFZ and NASA have jointly selected the DNEPR rocket to launch GRACE-FO in August 2017.
17.06.2013 | The funds for the German participation in GRACE-FO have now been approved by BMBF. Further information can be found here.
30.-31.05 2013 | The joint US/D Laser Ranging Interferometer (LRI) Preliminary Design Review (PDR) has been successfully accomplished at JPL, Pasadena.
16.05.2013 | Firm agreement by BMBF to GFZ to provide funds for the realization of the German mission elements within time frame 01.10.2012 and 31.08.2017.
06.-07.05.2013 | The Laser Ranging Interferometer (LRI) Tripple Mirror Assembly (TMA) Subsystem Preliminary Design Review (PDR) has been successfully accomplished at Cassidian, Oberkochen, and SpaceTech, Immenstaad.
29.04.2013 | The Microwave Instrument (MWI) Preliminary Design Review (PDR) has been successfully accomplished at JPL, Pasadena.
23.-26.04.2013 | The Laser Ranging Interferometer (LRI) Optical Bench System (OBS) Subsystem Preliminary Design Review (PDR) has been successfully accomplished at DLR, Berlin, Astrium, Friedrichshafen, and SpaceTech, Immenstaad.
22.04.2013 | The Microwave Instrument (MWI) Preliminary Design Review was held at JPL.
15.03.2013 | GRACE-FO Science Data System (SDS) Kick-off Meeting, JPL, Pasadena
14.03.2013 | The Laser Ranging Interferometer (LRI) Laser Stabilization Cavity Subsystem Critical Design Review (CDR) has been successfully accomplished at Ball Aerospace.
07.-08.03.2013 | The K-Band Ranging (KBR) Subsystem Mechanical/Thermal Preliminary Design Review (PDR) has been successfully accomplished at Astrium, Friedrichshafen.
28.02.2013 | The Ultra-Stable Oscillator (USO) Critical Design Review (CDR) has been successfully accomplished at the John Hopkins University Applied Physics Laboratory in Maryland/USA.
26.-28.02.2013 | The Attitude and Orbit Control System (AOCS) Preliminary Design Review (PDR) has been successfully accomplished at Astrium, Friedrichshafen.
09.01.2013 | The US Laser Ranging Interferometer (LRI) Preliminary Design Review (PDR) has been successfully accomplished at JPL, Pasadena.
29.11.2012 | Astrium will build, as for GRACE, under contract of JPL the GRACE-FO satellites. The corresponding agreement has been signed today in Friedrichshafen. Further informationen you can found at GFZ or at Astrium.
14.11.2012 | The Laser Preliminary Design Review (PDR) has been successfully accomplished at TESAT.
01.11.2012 | The LRI Laser Cavity Preliminary Design Review (PDR) has been successfully accomplished at JPL, Pasadena.
01.10.2012 | The BMBF assured GFZ of funding of the German project contributions within a Letter of Intent (LOI).
27.09.2012 | The Satellite Structure Inheritance Review has been successfully accomplished at Astrium, Friedrichshafen.
25.09.2012 | The Mass Trim Mechanism (MTE) and Mass Trim Equipment (MTE) Inheritance Review (PDR) have been successfully accomplished at Astrium, Friedrichshafen.
03.09.2012 | Begin of Phase-B
01.-02.08.2012 | German Kick-off Meeting of the Laser Ranging Interferometer (LRI) at DLR, Bremen
11.07.2012 | The Mission Definition (MDR) and System Requirements Review (SRR) has been successfully accomplished at JPL/NASA.
21.06.2012 | The star camera Preliminary Design Review (PDR) has been successfully accomplished at DTU in Denmark.
19.06.2012 | The S/W Coding Preliminary Design Review (PDR) has been successfully accomplished at Astrium, Friedrichshafen.
15.-16.05.2012 | A delegation of NASA/JPL (Jet Propulsion Laboratory) has visited GFZ to discuss the Memorundum of Understanding between NASA and GFZ and the Cooperative Project Plan between JPL and GFZ in the frame of the GRACE-FO satellite mission. These to contracts and documents describe the roles and responsibilities within the mission. GFZ will be responsible for the launcher and launch services, mission operations, provision of Laser Retro-Reflectors (LRR) for each satellite and contributions to the Laser Ranging Interferometer (LRI) and the Science Data System developments.
22.03.2012 | The onboard computer (OBC) Preliminary Design Review (PDR) has been successfully accomplished at TAS in Milano, Italy.
19.03.2012 | The Inertial Measurement Unit (IMU) Preliminary Design Review (PDR) has been successfully accomplished at Astrium, Toulouse.
01.01.2012 | Begin of Phase-A
28.10.2011 | The Mission Concept Review (MCR) has been successfully accomplished at JPL, Pasadena.
25.10.2011 | State Secretary Schütte (BMBF) assured NASA Administrator Charles F. Bolden Jr. the joint funding of German GRACE-FO contributions „Provision of a launcher and launch services, mission operations (with US support), scientific analysis and dissemination, contribution to a joint development of a experimental Laser Ranging Interferometer (LRI) as well as provision of Laser Retro-Reflectors (LRR) for each satellite“ by BMBF, BMWi, HGF, GFZ and DLR.
20.07.2011 | NASA Administrator Charles F. Bolden Jr. invited BMBF, BMWi, HGF, GFZ and DLR to build a GRACE Follow-on mission „GRACE-FO“ again in German-US partnership.
06.10.2009 | Meeting at GFZ in Potsdam of the German-US GRACE partners on invitation of Prof. Hüttl. JPL, UTCSR, Astrium, STI and GFZ decided to jointly work on the realization of GRACE continuation mission.
The main function of the SDS is the processing of the GRACE-FO science data. This includes the generation of GRACE-FO Level-0 to Level-3 products as well as their distribution and archiving. Just as the whole mission, the GRACE-FO SDS is a joint US/German cooperation consisting of the Jet Propulsion Laboratory (JPL), the University of Texas Center for Space Research (UTCSR) and the German Research Centre for Geosciences (GFZ). Thereby, internal validation and comparison of the science products are assured and in addition, backup capabilities concerning data processing and archiving are present.
A detailed description of the GRACE-FO SDS including the roles and responsibilities of the SDS institutions, the data flow and the products and documents to be provided will be written down in a Science Data System Development Plan. In the following, the key aspects of the SDS are described very briefly.
Starting from Level-0 data, i.e. the raw telemetry data received from the two GRACE-FO satellites, the Level-1 processing comprises two steps. During the first step, the binary encoded Level-0 measurements are converted to engineering units. Subsequently, the resulting Level-1A data are correctly time tagged and their sampling rate is reduced to obtain the Level-1B products. The essential Level-1B data are the K/Ka-Band Microwave Link and the Laser Ranging Interferometer measurements and the GPS, accelerometer and star camera observations. The GRACE-FO Level-1 processing will be performed by JPL with GFZ as backup capability.
Level-1 processing also includes the generation of atmosphere and ocean de-aliasing products (AOD1B) which will be done by GFZ.
Using Level-1B data, spherical harmonic coefficients representing the Earth’s gravitational potential are estimated. These coefficients are typically based on monthly batches of Level-1B data, so that the resulting Level-2 products build a time-series of monthly global gravity field models. The standard Level-2 products are due 60 days after Level-0 data acquisition; besides, operations of low latency NRT (“Near Real Time”) products will also be investigated. GRACE-FO Level-2 products will be generated by all three SDS partners.
As additional Level-2 products, monthly means of the AOD1B de-aliasing products (so-called GAx-products) are generated and provided in order to offer users to restore the corresponding atmospheric and/or oceanic signals.
In addition to the Level-2 products, more user-friendly Level-3 products are generated by transforming the spherical harmonic coefficients to gridded geopotential functionals (e.g. water equivalent mass change). Level-3 products also comprise ancillary data like time-series of geocenter motion or SLR derived C20 (describing the Earth’s oblateness) which need to be applied for correct geophysical interpretation of the Level-2 products. A limited number of official GRACE-FO Level-3 products will be generated by JPL; in order to expand the variety of offered products and to appeal to as much users as possible, GFZ is planning to provide additional own Level-3 products.
All GRACE-FO SDS Level-0 to Level-3 products as well as the corresponding documents will be archived at JPL/NASA's Physical Oceanography Distributed Active Data Center (PODAAC) and at GFZ’s Information System and Data Center (ISDC). To guarantee that one archive is a mirror of the other, both archives will be harmonized regularly.
As the Russian/Ukraine Dnepr and corresponding launch services can no longer be provided by the International Space Company Kosmotras (ISCK), the joint NASA-GFZ Joint Steering Group has decided to exchange the GRACE-FO launcher. The corresponding contract was signed on 14. November 2016 by the Board of GFZ and Iridium Satellite LLC. It stipulates a "Rideshare" between GRACE-FO and 5 Iridium-Next satellites on a Space-X Falcon-9 from Vandenberg Air Force Base in California within the launch period December 2017 till February 2018. This also includes a contract with Airbus D&S who will build the necessary Multi Satellite Dispenser and will perform the Launch Service Management under contract of GFZ. The GRACE-FO satellites will be launched into a co-planar orbit and following (GRACE-like) orbital parameters:
Further information can be found here.
JPL leads the development of the Satellite System (SAT) in partnership (contract) with Astrium GmbH. Astrium provides major elements of two flight satellites based on an existing small satellite design for the CHAMP, GRACE and SWARM missions. The GRACE-FO satellites are shown in the following figure.
The Satellite System (SAT) consists of the following sub-systems where most are available with main and redundant units.
The Telemetry, Tracking & Control (TT&C) activities are carried out using a pyro-deployed S-Band receive and transmit antenna, mounted on a nadir-facing deployable boom. Two back-up zenith antennae, one each for transmitting and receiving, along with the appropriate RF electronics assembly, complete the telemetry and telecommand sub-system.
The telecommand function of the satellite is designed according to the ESA CCSDS (Consultative Committee for Space Data Systems) Packet Telecommand Standard tailored for GRACE-FO Packet Utilization Standard (G-PUS) with adaptations mutually agreed with the German Space Operation Center (GSOC). The satellites support the command and control capabilities of the MOS by means of:
After reception of the uplinked command stream via the S-band antenna and the receiver of the RF Electronic Assembly, the telecommands are decoded in above two command categories. The High Priority Commands of level 1 (HPC 1) are directly executed within the telecommand module of the OBC; i.e. corresponding bi-stable relays are set. The normal telecommands are read from the telecommand handler of the on-board flight S/W via system calls. The telecommand handler further validates and converts the telecommand packets into on-board command packets (OCP's). The on-board command packets are further distributed according to their indicated functionality.
The Power System is responsible for generation, storage, conditioning and distribution of electrical power in accordance with instrument and satellite bus users needs. Electrical energy is generated using solar arrays of triple junction Gallium Arsenid (GaAs) cells, placed on the top and side exterior surface of the satellites. Excess energy is stored in a battery of Li-Ion cells with a capacity of 66 Ah at mission start. The power bus delivers unregulated power to all users at the respective user interface.
The Thermal Control System consists of 96 independent heater circuits, 128 YSI-type thermistors and 36 PT-type thermistors for in-flight temperature housekeeping, monitoring and heater control, as well as for on-ground verification testing.
The On-Board Computer (OBC) System provides processor and software resources, as well as necessary I/O capabilities for AOCS (Attitude and Orbit Control System), Power and Thermal Systems operations, including necessary fault detection, isolation and recovery operations.
Attitude and Orbit Control System (AOCS) The Attitude and Orbit Control System (AOCS) consists of sensors, actuators and software to:
The sensors include a Coarse Earth Sun Sensor (CESS), an Inertial Measurement Unit (IMU) and a fluxgate magnetometer, as well as the Star Tracker Assembly (STR) and GNSS receiver described in the Satellite Instrument System (SIS) page.
The CESS provides for omni-directional, coarse attitude measurement in the initial acquisition, survival and stand-by modes of the satellite. It comprises of six thermistors orthogonally mounted on the satellite. By assuming that the Sun is the hottest object in the field of view and the earth is the second hottest object in the field of view, the CESS provides the Sun and Earth vectors relative to the body frame at a rate of 1 Hz and accuracy of 5-degrees [TBC].
The IMU is used in survival modes and provides 4-axis rate information. The unit comprises of three solid-state fiber optic gyros, and three solid-state silicon accelerometers that measures velocity and angle changes in a coordinate system fixed relative to its case.
A fluxgate magnetometer provides additional rate information.
The actuators for the AOCS include a Cold Gas Assembly and a Magnetorquer System. The GN2 reaction control system includes two pressure vessels, valves, regulators and filters, along with 12 attitude control thrusters and two orbit control thrusters. Three magnetorquers with linear dipole moments of 27.5 Am2 complete the set of AOCS actuators.
The µASC Star Tracker Assembly (STR) determines the orientation of the satellite by tracking it relative to the position of the stars. These measurements are used for fine-pointing and on-board control of the satellite. Additionally they are required for the interpretation of measurements made in the satellite reference frame, such as those from the SuperSTAR accelerometer (see in the Satellite Instrument System (SIS) page).
The SCA consists of three temperature controlled CCD star cameras mounted to the accelerometer, along with the respective baffle assemblies. The STR delivers its video frames to the On-Board Computer (OBC), which then computes the attitude quaternions. The OBC also acts as the power and command/control interface to the STR. Once switched on and initialized, the STR proceeds with automatic coarse attitude acquisition and then on to fine attitude derivation.
The Center of Mass Trim (CMT) Assembly consists of six (two per axis) Mass Trim Mechanisms (MTM), associated electronics, and the power and signal harness. Each MTM consists of a trim mass driven on a nut rotor with a stepper motor. The CMT Assembly is used to center the center of gravity (CG) of the satellite at the center of the proof-mass of the accelerometer after CG calibration maneuvers.
The GRACE-FO Science Instrument System (SIS) includes all elements of the microwave inter-satellite ranging system, the GNSS receivers required for orbit determination and occultation measurements, and associated sensors, such as the star tracker and accelerometers. These elements are in general identical to GRACE, but based on up-to-date hardware (H/W) and software (S/W). In addition, a Laser Ranging Interferometer (LRI) will be added to the GRACE-FO mission for the purpose of demonstrating improved inter-satellite ranging accuracy. Additionally, GFZ provides Laser Retroreflectors (LRR) for both satellites which provide independent orbit determination control. The SIS also coordinates the integration activities of all sensors, assuring their compatibility with each other and the satellite. In the following the different SIS components are shortly summarized. Note that the Attitude and Orbit Control System (AOCS) sensors, which include besides the GNSS Receiver Assembly a Coarse Earth Sun Sensor (CESS), an Inertial Measurement Unit (IMU) and a fluxgate magnetometer, are described in the Satellite System (SAT) page.
The GRACE-FO Microwave Instrument (MWI) consists of the GNSS receiver and the K-band ranging (KBR) assembly.
The GNSS Receiver Assembly is part of the Microwave Instrument (MWI) and tracks the GNSS satellite ranging signals for
The GNSS Assembly consists of the main zenith navigation antenna, the rear back-up navigation antenna and a rear occultation antenna through which the GNSS signals are received. These signals are passed to the Signal Processing Unit (SPU), which receives, down-converts and digitizes the GNSS signals. The sampled data are passed to the Instrument Processing Unit (IPU), which extracts and delivers the GNSS phase and pseudo-range measurements. GNSS data are also the basis for the determination of the satellite position and clock estimates for use in on-board operations and for science from this GNSS data.
The K-band Ranging System (KBR) is another part of the MWI and provides precise (within 10 µm) measurements of the distance (and its change) between the two satellites from which the fluctuations in gravity can be determined. The KBR consists of a transmit/receive horn at the front face of the satellite, a wave-guide assembly and a Microwave Assembly (MWA). A reference signal is generated by the Ultra Stable Oscillator (USO), which is then up-converted to 24 and 32 GHz bands by the MWA, and transmitted to the other GRACE-FO satellite through the wave-guide and horn. The horn and the MWA also receive and down-converts the K-Band signals from the other satellite, using the same reference K-Band carrier that was transmitted. The received signals are passed to the SPU for digitization and in turn to the IPU for digital signal processing. The SPU ensures that the KBR signals are sampled at the same epoch as the GNSS tracking signals, ensuring precise time-tagging of the KBR data.
The SuperSTAR Accelerometer measures the non-gravitational accelerations acting on the satellite. These accelerations include air drag, solar radiation pressure, Earth radiation forces, thermal forces and forces created by operating the attitude control activator. The measurements are used to model the evolution of the satellite orbits due to the non-gravitational forces, so that the contributions to the inter-satellite range change due to purely gravitational effects can be correctly discriminated. Accelerometer measurements may also be used, in conjunction with other data and models, to determine upper atmospheric density variations.
The LRI is a joint US/German instrument and serves as a technical demonstration to assess if precision laser interferometry can improve microwave inter-satellite ranging performance for future GRACE-like geodetic missions. Inter-satellite ranging sensitivity is one of the factors that determine the overall performance of GRACE-FO.
The passive Laser Retro-Reflector (LRR) is contributed by GFZ and provides distance measurements of the GRACE-FO satellite orbits relative to the terrestrial ILRS tracking network. Four corner cubes are mounted on the nadir surface of the satellite, which reflect laser-ranging signals transmitted from the ground. The operating characteristics for the LRR are completely determined by the ILRS ground segment.
The GRACE-FO Mission Operations System (MOS) and Ground Data System (GDS) are funded by GFZ (for the first 5 years) and sub-contracted to DLR’s (DeutschesZentrum für Luft- und Raumfahrt) German Space Operation Center (GSOC) in Oberpfaffenhofen. GFZ provides the Operations Mission Manager (OMM), validated flight procedures and the Ny-Ålesund/Spitzbergen (NYA) ground station complex.
The MOS/GDS consists of the people, processes and procedures, facilities, and ground hardware and software required to operate the GRACE-FO Flight Segment (FS). These capabilities are used to monitor and control the satellite, perform initial processing of the telemetry data, and to deliver all data to the Science Data System (SDS) for further processing and generation of science products. The combination of the ground hardware and software is the Ground Data System (GDS).
The GDS is composed of the:
Additionally MOS/GDS will make use of the following stations (if required):
The five distinct GRACE-FO mission phases can be described as follows:
Pre-Launch (Test and Integration)
The Pre-Launch Phase C/D (Design & Development) starts about 3.5 years before launch. During this phase schedules are negotiated and the system is designed and developed. The phase begins with building and integrating subsystems and experiments into the spacecraft. In a process called ATLO (Assembly, Test, and Launch Operations) the spacecraft is assembled, integrated, and tested in a simulated space environment. Additionally, ground systems to support the mission are developed in parallel with the satellites, and are exercised along with the spacecraft during tests.
Launch and Early Operations Phase
The Launch and Early Operations Phase (LEOP) will last up to 15 days. The GRACE-FO satellites will be launched together on a single launch. Once in the desired orbit, the satellites will be simultaneously released from the upper stage of the launch vehicle, separated and start transmitting to the ground. During LEOP the basic satellite functions (incl. MWI-GPS) will be checked. At the end the satellites are insafe, stable orbits using nominal attitude control and nominal uplink and downlink communications are achieved. No anomalies exit that pose a near-term threat to the mission. The satellites will have a nominal separation distance of 220 km ± 50 km.
In-Orbit Checkout Phase
The In-orbit Checkout Phase (IOC) will last until 90 days after launch and will focus on the power-up, evaluation and calibration of the Science instruments.
In the Science Phase, instrument data will be routinely gathered and processed. This phase will continue until the end of the mission, and will include brief interruptions for orbit maneuvers and instrument re-calibrations.
A sub-phase is the Validation Phase which will last 120 days and focus on providing an end-to-end characterization of the Science Instrument and Data Systems.
The Decommissioning Phase is the final phase handling the end of mission. In this phase the satellites will be passivated followed by disposal to atmospheric reentry. Emphasis will be given to limit the generation of orbital debris, to limit the risk to the public and to comply with NASA policy directives as well as U.S. national space policy.
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