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Satellite Missions Catalogue

GRACE-FO

Feb 12, 2013

NASA

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GFZ

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Gravity, Magnetic and Geodynamic measurements

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Gravity and Magnetic Fields

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Quick facts

Overview

Mission typeEO
AgencyNASA, GFZ
Mission statusOperational (nominal)
Launch date22 May 2018
End of life date28 May 2023
Measurement domainGravity and Magnetic Fields
Measurement categoryGravity, Magnetic and Geodynamic measurements
InstrumentsMWI, LRI
Instrument typeGravity instruments
CEOS EO HandbookSee GRACE-FO summary

GRACE-FO (Gravity Recovery And Climate Experiment - Follow-On) / GFO (GRACE Follow-On)

Spacecraft     Launch    Mission Status     Sensor Complement    References 

The GRACE-FO (a.k.a. GFO) mission is heavily focused on maintaining data continuity from GRACE and minimizing any data gap after GRACE. Since 2009, the GRACE Follow-On mission is under definition/negotiation by the US-German GRACE consortium (NASA/JPL, CSR/UTexas, DLR, GFZ Helmholtz Center Potsdam). Many studies have been conducted on national and international levels by agencies, academia, institutions, and by industry to investigate new observation techniques fora long term strategy of observing the gravity field from space. 1) 2) 3)

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 GSFC (Goddard Space Flight Center). JPL (Jet Propulsion Laboratory) is assigned responsibility for the GRACE-FO project.

The GRACE-FO mission has significant German participation managed by the German Research Center 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 Center for Geosciences). The GRACE-FO mission will be operated by DLR/GSOC.

In the fall of 2012, after more than 10 years of very successful operation in orbit, the US-German GRACE-1 mission has demonstrated in a very impressive way its outstanding capability to monitor mass motions in the Earth system with unprecedented accuracy and temporal resolution. These results have stimulated many novel research activities in hydrology, oceanography, glaciology, geophysics and geodesy, which also indicate that long-term monitoring of such mass motions, possibly with improved spatial and temporal resolution, is a must for further understanding of various phenomena.

GRACE-1 gave many breakthroughs in the understanding of (Ref. 9):

  • Changes in the terrestrial water cycle
  • Melting and growing of glaciers and ice sheets
  • sea level rise and its causes (ice melt, thermal expansion)
  • solid Earth (past glaciation, large Earthquakes)

GRACE-FO mission objectives:

  • The primary objective is to continue the high-resolution monthly global models of Earth's gravity field of the GRACE-1 mission for an expected length of 5 years.
  • The secondary objectives are:
    • demonstrate the effectiveness of a LRI  (Laser Ranging Interferometer) in improving the low-low SST (Satellite-to-Satellite Tracking) measurement performance.
    • continue measurements of GRACE radio occultations for operational provision of e.g. vertical temperature/humidity profiles to weather services

Satellite gravimetry

Satellite gravimetry, that is, measuring spatial and temporal change in the gravity field caused by mass variations from space, provides a unique opportunity to advance mass transport studies and improve our understanding of the Earth system. From the GRACE mission, new fundamental insights into the changing mass distribution have been achieved in the first decade of the 21st century. The results from the GRACE mission have actually revolutionized the field of Earth system research and have established the necessity for future satellite gravity missions. 4)

The large-scale mass distribution in the Earth system is continuously changing. Most of the mass transport is associated with well-monitored atmospheric variability, and with the global water cycle. Through this cycle, the ocean, atmosphere, land, and cryosphere storages of water interact through temporally and spatially variable water mass exchanges. The distribution of water mass in these reservoirs changes at timescales ranging from sub-daily to inter-annual, and decadal, and is strongly related to long-term global change, including sea-level rise, loss of land ice, and extensive droughts and floods. These mass variations may indicate a change in the forcing or the feedback mechanisms that moderate the climate. Water mass variations may therefore be considered a proxy for ongoing climate variations driven by natural and/or anthropogenic causes, which has the potential for impacting society very strongly (Ref. 4).

Gravity is determined by mass. By measuring gravity, GRACE shows how mass is distributed around the planet and how it varies over time. Data from the GRACE satellites is an important tool for studying Earth's ocean, geology, and climate. Mass variations are caused either by redistribution of mass in, on or above the Earth's surface or by geophysical processes in the Earth's interior. The first set of observations of monthly variations of the Earth gravity field was provided by the GRACE-1 twin-satellite mission beginning in March 2002. In 2015, this mission is still providing valuable information to the science community. 5)

Mapping the global gravity field:

  • Static and dynamic components
  • Many applications in geosciences

Techniques:

  • Orbit determination and tracking
  • SST (Satellite-to-Satellite Tracking)

Recent gravity field satellites:

  • CHAMP
  • GRACE
  • GOCE
  • GRAIL (lunar gravity)

In the framework of a cooperation on future EO technologies and missions, NASA and ESA established an Interagency Gravity Science Working Group of experts tasked to define the overall objectives and the observation requirements for a future constellation with better performance than that of a concept elaborated by a single agency – in fact, a performance better than "the sum of the parts" when these were not jointly defined. 6)

Figure 1 outlines the expected performance (in terms of spatial and temporal resolution) of a next generation gravity constellation, with two cases (three and ten day solutions) separately considered, and also indicates the performance of GRACE and GOCE as a reference. For geophysical signals the targeted geoid height accuracy of 1 mm at scales of 500 km and 150 km, respectively, will give (compared to GRACE) increased temporal and spatial performance and so allow resolving various geophysical processes in far more detail.

Studies have been performed in order to find the constellations of two satellite pairs equipped with laser-based distance metrology (of similar but not necessarily equal performance) capable to deliver the best scientific return. This was supported by closed-loop simulations, with realistic inputs from system design aspects. Constellation candidates have been found, with orbits at different inclinations that optimise the return for the different applications whilst "absorbing" in their gravity solutions most of the disturbing effects of daily and subdaily phenomena with a dedicated parameterisation. This would remove the need for de-aliasing based upon external models, as is current practice for GRACE data analysis, and almost entirely remove the "striping" or "striations" seen in single-pair gravity field solutions.

Figure 1: Time variable gravity field sources in terms of spatial resolution and time variability including GRACE sensitivity and goals for future gravity field missions (image credit: Ref. 5)
Figure 1: Time variable gravity field sources in terms of spatial resolution and time variability including GRACE sensitivity and goals for future gravity field missions (image credit: Ref. 5)

GRACE-FO, like GRACE, is designed to measure changes in gravitational pull that result from changes in mass on Earth. More than 99 percent of Earth's mean gravitational pull does not change from one month to the next. That's because it comes from the mass of the solid Earth itself — its surface and interior — and that rarely moves, or moves very slowly. Water, on the other hand, moves continually nearly everywhere: Snow falls, ocean currents flow, ice melts and so on. As the twin GRACE-FO satellites orbit Earth, one closely following the other, the changes in mass below change the distance between the two satellites very slightly. The record of these changes is analyzed to create monthly global maps of changes and redistribution of Earth's mass near the surface. 7)


Spacecraft

In November 2012, Airbus DS (formerly EADS Astrium GmbH, Friedrichshafen) has been commissioned to build two new GRACE Follow-On research satellites for NASA/JPL ( Jet Propulsion Laboratory). The goal of the GRACE Follow-on mission is to continue the extremely accurate measurement data collection of the first twin GRACE satellites, which have been in orbit since March 17, 2002. A launch of the mission is planned for August 2017, the minimum mission life is 5 years. 8)

JPL leads the development of the GFO satellite system 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 satellite system consists of the following subsystems where most are available with main and redundant units.

TT&C (Telemetry, Tracking & Control): The 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 subsystem. The telecommand function of the satellite is designed according to the ESA CCSDS (Consultative Committee for Space Data Systems) packet telecommand standard tailored for G-PUS (GRACE-FO Packet Utilization Standard) with adaptations mutually agreed with DLR/GSOC (German Space Operation Center). The satellites support the command and control capabilities of the MOS (Mission Operations System) by means of:

  • HPC1 (High Priority Commands of priority 1), which are directly handled by the telecommand decoder and by-passes all OBC (On-Board Computer) software
  • Normal telecommands, which will be processed by the OBC on-board software

 

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 HPC1 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 OBPs (On-Board command Packets). The OBPs are further distributed according to their indicated functionality.

Power subsystem:  The power subsystem is responsible for the 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 GaAs (Gallium Arsenide) 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.

TCS (Thermal Control Subsystem): The TCS 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.

OBC (On-Board Computer) subsystem: The OBC subsystem provides processor and software resources, as well as necessary I/O capabilities for AOCS (Attitude and Orbit Control System), power subsystem and TCS operations, including necessary fault detection, isolation and recovery operations.

AOCS (Attitude and Orbit Control Subsystem):  The AOCS consists of sensors, actuators and software to:

  • Provide adequate knowledge of satellite attitude during all phases of the mission
  • Generate on-board error signals to accurately maintain satellite attitude
  • Provide necessary orbital control to satisfy the GRACE-FO mission requirements

The sensors include a CESS (Coarse Earth Sun Sensor), an IMU (Inertial Measurement Unit) and a fluxgate magnetometer, as well as the STR (Star Tracker Assembly) and GNSS receiver.

 

The CESS provides for omnidirectional, 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 with an accuracy of ~5º.

 

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 measure 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 CGA (Cold Gas Assembly) and a magnetorquer system. The GN2 (Gaseous Nitrogen) 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.

µSTR (micro Star Tracker Assembly): The µSTR 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.

The SCA (Star Camera Assembly) 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 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.

CMT (Center of Mass Trim) assembly: The CMT assembly consists of six (two per axis) MTMs (Mass Trim Mechanisms), 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 CG (Center of Gravity) of the satellite at the center of the proof-mass of the accelerometer after CG calibration maneuvers.

Figure 2: Artist's view of the GRACE Follow-on mission (image credit: Airbus DS)
Figure 2: Artist's view of the GRACE Follow-on mission (image credit: Airbus DS)
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Figure 3: General accommodation of the GRACE-FO instruments on the Flexbus platform (image credit: GRACE-FO consortium)

• MDR (Mission Definition Review) and SSR (System Requirements Review) passed in July 2012.

• Formal transition to Phase A occurred in January 2012 9)

The two GRACE-FO satellites with a size of around 3 m x 3 m x 0.8 m and a mass of ~600 kg, will orbit Earth in a co-planar polar orbit, spaced 220 km apart at an altitude of ~490 km.

Grace-FO_Auto31
Figure 4: In 2015, the build phase of the GRACE-FO climate satellites has begun with the delivery of the highly stable carbon fiber composite structure with a mass of ~200 kg in Friedrichshafen, Germany (image credit: Airbus DS, Mathias Pikelj)
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Figure 5 : Photo of the first of the GRACE-FO research satellites, being prepared for transport in an Airbus cleanroom in Friedrichshafen (image credit: Airbus DS GmbH, A. Ruttloff)

Some project development status dates

April 30, 2018: A pair of new spacecraft that will observe our planet's ever-changing water cycle, ice sheets and crust is in final preparations for a California launch no earlier than Saturday, 19 May. The GRACE-FO mission, a partnership between NASA and the German Research Center for Geosciences (GFZ), will take over where the first GRACE mission left off when it completed its 15-year mission in 2017.10)

GRACE-FO will continue monitoring monthly changes in the distribution of mass within and among Earth's atmosphere, oceans, land and ice sheets, as well as within the solid Earth itself. These data will provide unique insights into Earth's changing climate, Earth system processes and even the impacts of some human activities, and will have far-reaching benefits to society, such as improving water resource management.

Grace-FO_Auto2F
Figure 6: At the Harris facility at Vandenberg Air Force Base in California, one of the twin GRACE-FO satellites is integrated with the multi-satellite dispenser structure that will be used to deploy the satellites during launch on the SpaceX Falcon-9 launch vehicle (image credit: Airbus DS)

December 12, 2017: The twin GRACE-FO satellites have now set off on their journey to Vandenberg Air Force Base in California. Together with test equipment totalling around 45 tons, the gravity research satellites built and developed by Airbus in Friedrichshafen (Germany) were loaded overnight onto an air freighter at Munich Airport and flown to the USA this morning. GRACE-FO is a joint project between NASA/JPL (Jet Propulsion Laboratory) based in Pasadena, California, and the German Research Center for Geosciences (GFZ) in Potsdam. 11) 12)

A team of 15 Airbus space engineers and technicians are currently awaiting the arrival of the satellites in California. The plan is to assemble all test systems on site and check the GRACE-FO spacecraft before the Christmas break in readiness for the launch campaign in 2018.

Grace-FO_Auto2E
Figure 7: Photo of the GRACE-FO satellites being loaded into the airplane at Munich Airport (image credit: Airbus DS)

November 10, 2017: After a successful year-long test campaign by Airbus at IABG in Ottobrunn near Munich, the twin GRACE-FO (Gravity Recovery and Climate Experiment Follow-On) satellites will soon travel to their VAFB launch site in California. 13)

During testing, the gravity-measuring satellites were subjected to conditions similar to those they will experience during launch and in low Earth orbit. Both satellites, each weighing 600 kg, will be flown to the Vandenberg Air Force Base launch site in California in December to begin final launch preparations.

May 2, 2017: The two GRACE-FO satellites, developed at Airbus DS Friedrichshafen (Germany) for NASA/JPL, were 'given an earful' during recent acoustic tests. The sound impact that builds up during rocket launches was simulated in an echo chamber with a volume of around 1,400 m3 at IABG in Ottobrunn near Munich. 14)

In four test cycles, the satellites in their flight position were subjected to a sound impact of nearly 140 decibels (dB). In comparison, a pneumatic drill produces 100 dB and the human pain threshold is 130 dB. Both satellites passed the tests with flying colors.

NASA/JPL in partnership with the GFZ (German Research Center for Geosciences) Potsdam , will send both GRACE-FO research satellites into a polar orbit at an altitude of around 500 km and at a distance of 220 km apart.

Grace-FO_Auto2D
Figure 8: Preparations for the acoustic noise test for the two GRACE-FO satellites, which Airbus constructed for NASA/JPL (image credit: Airbus DS GmbH, Mathias Pikelj)

October 28, 2016: Airbus DS is reporting that the company has finished construction of the first of the two GRACE-FO satellites in Friedrichshafen, Germany (Figure 5). The satellite will now be transferred to Ottobrunn near Munich for several months of operational testing in the IABG (Industrieanlagen-Betriebsgesellschaft mbH) test center. — The second GRACE-FO satellite will be ready for testing in about four weeks. 15)

July 2, 2015: BATC (Ball Aerospace & Technologies Corporation) delivered the "laser frequency stabilization reference flight units" for GRACE-FO to NASA/JPL. Ball's flight units constitute a subsystem that is part of the high-precision LRI (Laser Ranging Interferometer), a secondary payload aboard the spacecraft.

Jan. 20-24, 2014: The project PDR (Preliminary Design Review) was conducted. 16)

Feb. 11, 2013: GFZ signs MoU and provides launcher - Today the Board of the GFZ (German Research Center for Geosciences) has signed the Memorandum of Understanding with NASA Administrator Charles Bolden Jr. to mutually realize the follow-on mission of GRACE (Gravity Recovery and Climate Experiment). This agreement solidifies the joint long-term planning and realization of this unique tandem satellite mission, which will, after having successfully passed the PDR (Preliminary Design Review) in January, now enter it's building phase to be launched in August 2017. 17)

Germany does not only provide the launcher but also optical elements of the LRI (Laser Ranging Interferometer) assembly. The distance measurement between the current dual GRACE satellites which has been performed so far with an accuracy of about two thousands of a millimeter using microwave signals. This shall be improved for GRACE-FO by this new technology by a factor of up to 20. This is important as the prime signal to monitor gravity field variations is the change in separation between both satellites. The LRI design is provided by the Max-Planck-Institute for Gravitational Physics (Albert-Einstein-Institute), the flight hardware will be delivered by STI (SpaceTech GmbH), the GFZ project partner).

Besides NASA/JPL (Jet Propulsion Laboratory) and the University of Texas, GFZ is again responsible for science data processing. Funding of the German contributions is jointly provided by the German Ministry of Education and Research, the German Ministry of Economics and Technology and the Helmholtz Foundation. The German Aerospace Center (DLR) provides in-kind contributions for the LRI. Besides this, GFZ provides Laser Retro-Reflectors for both satellites and funds the mission operations for 5 years. Mission operations will be conducted, as for GRACE, by GSOC (German Space Operation Center) of DLR in Oberpfaffenhofen.

September 3, 2012: Formal transition to Phase B.

Germany will contribute to the GRACE Follow-on mission the following services/developments:

  • Launch provision (a Rockot launch is planned as for GRACE)
  • Major contribution to the German/US LRI instrument
  • Mission operations and ground system
  • Science analysis.

Astrium uses a 3rd generation Flexbus for the GRACE-FO mission (Ref. 9). Each of the GRACE-FO satellites measures approximately 3 m x 2 m x 0.8 m and has a mass of around 580 kg.

Throughout the five-year mission, these measurements will be used to generate an updated model of the Earth's gravitational field every 30 days. In addition, every day each satellite will create up to 200 profiles of temperature distribution and water vapor content in the atmosphere and ionosphere.

Launch: The GRACE-FO twin spacecraft, a joint US/German mission, was launched on 22 May 2018 (19:47:58 UTC) from VAFB, CA, USA. GRACE-FO lifted off on a SpaceX-Falcon-9 flight as a rideshare mission with 5 Iridium NEXT communication satellites. 18)

April 9, 2018: GRACE-FO is officially confirmed on a SpaceX-Falcon-9 flight as a rideshare mission with 5 Iridium NEXT satellites, according to a joint announcement of Iridium, NASA and the GFZ (German Research for Geosciences). This unique "rideshare" launch will first deploy the twin GRACE-FO spacecraft, after which the Falcon 9 second stage will continue onward to the deployment orbit for the five Iridium NEXT satellites. Upon completion, the Iridium-6/GRACE-FO mission will increase the total number of Iridium NEXT satellites in space to 55, leaving just two launches, of 10 satellites each, remaining. Iridium NEXT satellites are scheduled to begin shipping to the launch site this week while the twin GRACE-FO spacecraft are already onsite at the VAFB Harris Corporation facility, and have been stacked, attached to their dispenser and are preparing for transfer to the SpaceX launch complex. 19)

On January 31, 2017, Iridium Communications Inc. announced that it has contracted with SpaceX for an eighth Falcon 9 launch. Along for the ride are the twin-satellites of the NASA/GFZ GRACE-FO (Gravity Recovery and Climate Experiment Follow-On) mission, which will be deployed into a separate low-Earth orbit, marking the first rideshare deal for Iridium. An agreement of this kind is economical for all parties, and affords Iridium the ability to launch five additional satellites for its next-generation global satellite network. The rideshare is anticipated to launch out of VAFB (Vandenberg Air Force Base) in California by early 2018. 20) 21)

This rideshare represents a material savings from other supplemental launch options due to the efficiency of sharing the rocket with GRACE-FO, and the incremental cost during the Iridium NEXT construction period is immaterial when considering the avoidance of unspent amounts contemplated under the Kosmotras program. It also affords Iridium the opportunity to rearrange its launch and satellite drifting plan and launch these five satellites directly into their operational orbital plane while increasing the number of planned in-orbit spares by three satellites. Further, this development allows Iridium to complete the whole operational constellation at a faster rate than it would have with seven launches. Iridium will still consider launching satellites with Kosmotras once approvals are available.

"This is a very smart way to get additional Iridium NEXT satellites into orbit," said Matt Desch, chief executive officer at Iridium. "This launch provides added resiliency to our network for not much more than we had planned originally to launch 72 satellites, including two with Kosmotras."

The GRACE-FO mission was initially booked with the launch provider ISC (International Space Company) Kosmotras on the Russian-Ukrainian Dnepr rocket with a launch from the Baikonur Cosmodrome launch station in Kazakhstan. GFZ arranged the launch under a contract with Moscow-based ISC Kosmotras. However, the Dnepr has not launched since March 2015 because of difficulties winning Russian government approval for additional launches of the Ukrainian-built rocket. The program's future is in doubt in the wake of Russia's annexation of Crimea and military activity in eastern Ukraine. - Russian officials have said they plan to discontinue Dnepr launches.

Iridium has also been a victim of Dnepr delays. The company originally planned to launch its first two next-generation satellites on a Dnepr to test them in orbit before beginning large-scale deployment of the constellation on Falcon- 9 launches. Iridium ultimately decided to start with the Falcon 9, successfully launching its first 10 satellites Jan. 14, 2017. 22)

GFZ (German Research Center for Geosciences) in Potsdam is managing the German involvement in the mission. Managers of GFZ switched the GRACE-FO mission to a Falcon-9 rocket launch. DLR (German Research Center) is funding the launch and is partnering with NASA on the mission. Some important factors in selecting a launch vehicle are reliability, cost, availability, and capability of lifting the satellite to its desired orbit.

Table 1: Some background of the GRACE-FO launch provider switch

Orbit: A circular co-planar orbit (non-repeat ground track), initial altitude of 490 ±10 km, inclination = 89.0±0.06º, eccentricity < 0.0025, spacecraft separation: ~220 km.

After separation from the launch vehicle, the two satellites are in slightly differently sized orbits that cause a drift apart with a nominal drift rate of about 200 km/day for several days. This drift has to be stopped and reversed to bring them into the required formation of 220 km apart. The operations of the intersatellite ranging instruments — MWI and LRI — require pointing information based on TLEs (Two-Line Elements) of both spacecraft, which are provided via command from the ground. To improve this TLE-based pointing information onboard each satellite a GPS-based frame correction is applied for K-band pointing. For LRI operations this corrected pointing still is not accurate enough. Thus, the offset between GPS-based and TLE-based pointing is computed and modelled on the ground using a parametric fit method. The computed fit parameters are uploaded to the satellites by ground command together with each TLE set to compute a more accurate pointing using the TLEs and fit data. 23)

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