The GRACE (Gravity Recovery and Climate Experiment, 2002-2017) and GRACE-FO (Follow-On, since 2018) satellite missions document large-scale mass changes in the Earth system that cannot be detected by any other remote sensing satellites. The aim of the missions is to permanently observe the effects of the diverse and complex feedbacks of human activity on the global water cycle, sea-level rise and the climate system.

To obtain reliable statistics and a better understanding of how anthropogenic climate change and natural climatic cycles interact, uninterrupted data sets of at least three decades are required. Therefore, the US Space Agency NASA along with its partners at the Jet Propulsion Laboratory (JPL) or Goddard Space Flight Center (GSFC) and the German Space Agency at the German Aerospace Centre (DLR), the Max Planck Institute for Gravitational Physics in Hannover (Albert Einstein Institute), and the GFZ German Research Centre for Geosciences are now realizing the third generation of GRACE missions. The successor of GRACE-FO is called GRACE-C (Continuity) and shall be launched end of 2028 again by a Space-X Falcon-9 into a polar orbit at ca. 500 km altitude and 89º inclination. The two identical spacecraft, built again by Airbus Defence and Space using GRACE-FO heritage, will be separated by 100-300 km.

In contrast to GRACE and GRACE-FO the satellite-to-satellite tracking (SST) will now be made exclusively with a Laser Ranging Interferometer (LRI) which has been successfully operated on GRACE-FO as a technology demonstrator. The LRI has the same contributions as on GRACE-FO: The US will provide electronics and the Laser while Germany will contribute the complete optics. The LRI scale needs to be known to ~25 ppb which was derived on GRACE-FO by Microwave Instrument (MWI) and LRI intercomparisons. As no MWI will be available on GRACE-C NASA will provide also a new development called Scale Factor Unit (SFU). Additionally, redundancy is added as much as possible.

The French company ONERA has already built two flight spare accelerometer units for GRACE-FO (SM1/SM2) that will be used on GRACE-C to measure non-gravitational forces on both satellites. The units have been recertified for flight (delivery review held in June 2024) by repeating functional and performance tests. The accelerometer H/W is flight-proven, built, and will be delivered to the GRACE-C spacecraft in mid-2026 for integration.

For precise orbit determination and time tagging a PODRIX GNSS (Global Navigation Satellite System) receiver from Beyond Gravity will be implemented which replaces the JPL furnished GPS (Global Positioning System) within the MWI on GRACE/GRACE-FO. The GNSS receiver will be block redundant, provides triple frequencies (L1, L2, L5; GPS/Galileo) and was already successfully flown on Airbus projects Sentinel-2 and Sentinel-6.

Low shock thrusters of 7 mN built by Advanced Space Technologies (AST) will replace the 10 mN thrusters used on GRACE-FO. These will provide a factor ~10x reduced shock spectrum relative to the GRACE-FO thrusters which will result in a quieter platform for science measurements.

GFZ will again be part of the US/German GRACE-C Science Data System (SDS) with very similar responsibilities as on GRACE and GRACE-FO (see Figure 1).

GFZ will also play again a significant role in the realization of the Mission Operations System (MOS, Figure 2). The primary data downlink station will be again GFZ´s Satellite Receiving Station in Ny-Ålesund (NYA) on Spitsbergen. Uplink and additional downlinks will be made via Weilheim (WHM, Germany), Neustrelitz (NST, Germany), O’Higgins (OHG, Antarctica) and Inuvik (INU, Canada). NASA´s Near Space Network (NSN) ground stations will be used during the Launch and Early Operation Phase (LEOP), contingency support and larger software uploads. The Mission Control Center (MCC) will be again located at DLR/GSOC (German Space Operation Center), Oberpfaffenhofen, Germany. The Raw Data Center (RDC) is the pickup point for the SDS and again located at the DLR/DFD (Deutsches Fernerkundungsdatenzentrum), Neustrelitz, Germany. Funding of the MOS after end of the Commissioning Phase will be provided by GFZ (similar as for GRACE-FO). Therefore, GFZ will also provide the German GRACE-C Mission Operations Manager.

Finally, GFZ will also built Laser Retro-Reflektors (LRR) for both satellites to enable GNSS independent orbit verification.

The GRACE-C mission has successfully passed the Preliminary Design Review in March 2024 and entered building Phase of the satellites and instruments in June 2024. The Critical Design Review is scheduled for May 2025, the launch is targeted for December 2028. It is desired to have a minimum of 6 months of overlap with GRACE-FO for Calibration/Validation purposes.

Project Partners:

• NASA (National Space Agency, Washington, USA)
• JPL (Jet Propulsion Laboratory, Pasadena, USA)
• GSFC (Goddard Space Flight Center, Greenbelt, USA)
• DLR (German Space Agency at the German Aerospace Centre, Bonn)
• DLR/GSOC (DLR´s German Space Operation Center, Oberpfaffenhofen)
• MPG/AEI (Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Hanover)

Project Duration:

• Launch December 2028 with a nominal mission lifetime of 5 years

Funding (German contributions):

• Federal Ministry for Economic Affairs and Climate Action (BMWK)
• Federal Ministry of Education and Research (BMBF)
• Max Planck Institute for Gravitational Physics (Albert Einstein Institute)
• GeoForschungsZentrum (GFZ)